Sunday, 23 October 2016

Rabbit of Caerbannog

Subsequent to my previous use of American Fuzzy Lop (AFL) on the NetSurf bitmap image library I applied it to the gif library which, after fixing the test runner, failed to produce any crashes but did result in a better test corpus improving coverage above 90%

I then turned my attention to the SVG processing library. This was different to the bitmap libraries in that it required parsing a much lower density text format and performing operations on the resulting tree representation.

The test program for the SVG library needed some improvement but is very basic in operation. It takes the test SVG, parses it using libsvgtiny and then uses the parsed output to write out an imagemagick mvg file.

The libsvg processing uses the NetSurf DOM library which in turn uses an expat binding to parse the SVG XML text. To process this with AFL required instrumenting not only the XVG library but the DOM library. I did not initially understand this and my first run resulted in a "map coverage" indicating an issue. Helpfully the AFL docs do cover this so it was straightforward to rectify.

Once the test program was written and environment set up an AFL run was started and left to run. The next day I was somewhat alarmed to discover the fuzzer had made almost no progress and was running very slowly. I asked for help on the AFL mailing list and got a polite and helpful response, basically I needed to RTFM.

I must thank the members of the AFL mailing list for being so helpful and tolerating someone who ought to know better asking  dumb questions.

After reading the fine manual I understood I needed to ensure all my test cases were as small as possible and further that the fuzzer needed a dictionary as a hint to the file format because the text file was of such low data density compared to binary formats.

Rabbit of Caerbannog. Death awaits you with pointy teeth
I crafted an SVG dictionary based on the XML one, ensured all the seed SVG files were as small as possible and tried again. The immediate result was thousands of crashes, nothing like being savaged by a rabbit to cause a surprise.

Not being in possession of the appropriate holy hand grenade I resorted instead to GDB and electric fence. Unlike the bitmap library crashes memory bounds issues simply did not feature in the crashes.Instead they mainly centered around actual logic errors when constructing and traversing the data structures.

For example Daniel Silverstone fixed an interesting bug where the XML parser binding would try and go "above" the root node in the tree if the source closed more tags than it opened which resulted in wild pointers and NULL references.

I found and squashed several others including dealing with SVG which has no valid root element and division by zero errors when things like colour gradients have no points.

I find it interesting that the type and texture of the crashes completely changed between the SVG and binary formats. Perhaps it is just the nature of the textural formats that causes this although it might be due to the techniques used to parse the formats.

Once all the immediately reproducible crashes were dealt with I performed a longer run. I used my monster system as previously described and ran the fuzzer for a whole week.

Summary stats

       Fuzzers alive : 10
      Total run time : 68 days, 7 hours
         Total execs : 9268 million
    Cumulative speed : 15698 execs/sec
       Pending paths : 0 faves, 2501 total
  Pending per fuzzer : 0 faves, 250 total (on average)
       Crashes found : 9 locally unique

After burning almost seventy days of processor time AFL found me another nine crashes and possibly more importantly a test corpus that generates over 90% coverage.

A useful tool that AFL provides is afl-cmin. This reduces the number of test files in a corpus to only those that are required to exercise all the code paths reached by the test set. In this case it reduced the number of files from 8242 to 2612

afl-cmin -i queue_all/ -o queue_cmin -- test_decode_svg @@ 1.0 /dev/null
corpus minimization tool for afl-fuzz by 

[+] OK, 1447 tuples recorded.
[*] Obtaining traces for input files in 'queue_all/'...
    Processing file 8242/8242...
[*] Sorting trace sets (this may take a while)...
[+] Found 23812 unique tuples across 8242 files.
[*] Finding best candidates for each tuple...
    Processing file 8242/8242...
[*] Sorting candidate list (be patient)...
[*] Processing candidates and writing output files...
    Processing tuple 23812/23812...
[+] Narrowed down to 2612 files, saved in 'queue_cmin'.

Additionally the actual information within the test files can be minimised with the afl-tmin tool. This must be run on each file individually and can take a relatively long time. Fortunately with GNU parallel one can run many of these jobs simultaneously which merely required another three days of CPU time to process. The resulting test corpus weighs in at a svelte 15 Megabytes or so against the 25 Megabytes before minimisation.

The result is yet another NetSurf library significantly improved by the use of AFL both from finding and squashing crashing bugs and from having a greatly improved test corpus to allow future library changes with a high confidence there will not be any regressions.

Tuesday, 11 October 2016

The pine stays green in winter... wisdom in hardship.

In December 2015 I saw the kickstarter for the Pine64. The project seemed to have a viable hardware design and after my experience with the hikey I decided it could not be a great deal worse.

Pine64 board in my case design
The system I acquired comprises of:
  • Quad core Allwinner A64 processor clocked at 1.2GHz 
  • 2 Gigabytes of DDR3 memory
  • Gigabit Ethernet
  • two 480Mbit USB 2.0 ports
  • HDMI type A
  • micro SD card for storage.
Hardware based kickstarter projects are susceptible to several issues and the usual suspects occurred causing delays:
  • Inability to scale, several thousand backers instead of the hundred they were aiming for
  • Issues with production
  • Issues with shipping
My personal view is that PINE 64 inc. handled it pretty well, much better than several other projects I have backed and as my Norman Douglas quotation suggests I think they have gained some wisdom from this.

I received my hardware at the beginning of April only a couple of months after their initial estimated shipping date which as these things go is not a huge delay. I understand some people who had slightly more complex orders were just receiving their orders in late June which is perhaps unfortunate but still well within kickstarter project norms.

As an aside: I fear that many people simply misunderstand the crowdfunding model for hardware projects and fail to understand that they are not buying a finished product, on the other side of the debate I think many projects need to learn expectation management much better than they do. Hyping the product to get interest is obviously the point of the crowdfunding platform, but over promising and under delivering always causes unhappy customers.

Pine64 board dimensions
Despite the delays in production and shipping the information available for the board was (and sadly remains) inadequate. As usual I wanted to case my board and there were no useful dimension drawings so I had to make my own from direct measurements together with a STL 3D model.

Also a mental sigh for "yet another poor form factor decision" so another special case size and design. After putting together a design and fabricating with the laser cutter I moved on to the software.

Once more this is where, once again, the story turns bleak. We find a very pretty website but no obvious link to the software (hint scroll to the bottom and find the "support" wiki link) once you find the wiki you will eventually discover that the provided software is either an Android 5.1.1 image (which failed to start on my board) or relies on some random guy from the forums who has put together his own OS images using a hacked up Allwinner Board Support Package (BSP) kernel.

Now please do not misunderstand me, I think the work by Simon Eisenmann (longsleep) to get a working kernel and Lenny Raposo to get viable OS images is outstanding and useful. I just feel that Allwinner and vendors like Pine64 Inc. should have provided something much, much better than they have. Even the efforts to get mainline support for this hardware are all completely volunteer community efforts and are are making slow progress as a result.

Assuming I wanted to run a useful OS on this hardware and not just use it as a modern work of art I installed a basic Debian arm64 using Lenny Raposo's pine64 pro site downloads. I was going to use the system for compiling and builds so used the "Debian Base" image to get a minimal setup. After generating unique ssh keys, renaming the default user and checking all the passwords and permissions I convinced myself the system was reasonably trustworthy.

The standard Debian Jessie OS runs as expected with few surprises. The main concern I have is that there are a number of unpackaged scripts installed (prefixed with pine64_) which perform several operations from reporting system health (using sysfs entries) to upgrading the kernel and bootloader.

While I understand these scripts have been provided for the novice users to reduce support burden, doing even more of the vendors job, I would much rather have had proper packages for these scripts, kernel and bootloader which apt could manage. This would have reduced image creation to a simple debootstrap giving much greater confidence in the images provenance.

The 3.10 based kernel is three years old at the time of writing and lacks a great number of features for the aarch64 ARM processors introduced since release. However I was pleasantly surprised at kvm apparently being available.

# dmesg|grep -i kvm
[    7.592896] kvm [1]: Using HYP init bounce page @b87c4000
[    7.593361] kvm [1]: interrupt-controller@1c84000 IRQ25
[    7.593778] kvm [1]: timer IRQ27
[    7.593801] kvm [1]: Hyp mode initialized successfully

I installed the libvirt packages (and hence all their dependencies like qemu) and created a bridge ready for the virtual machines.

I needed access to storage for the host disc images and while I could have gone the route of using USB attached SATA as with the hikey I decided to try and use network attached storage instead. Initially I investigated iSCSI but it seems the Linux target (iSCSI uses initiator for client and target for server) support is either old, broken or unpackaged.

I turned to network block device (nbd) which is packaged and seems to have reasonable stability out of the box on modern distributions. This appeared to work well, indeed over the gigabit Ethernet interface I managed to get a sustained 40 megabytes a second read and write rate in basic testing. This is better performance than a USB 2.0 attached SSD on the hikey

I fired up the guest and perhaps I should have known better than to expect a 3.10 vendor kernel to cope. The immediate hard crashes despite tuning many variables convinced me that virtualisation was not viable with this kernel.

So abandoning that approach I attempted to run the CI workload directly on the system. To my dismay this also proved problematic. The processor has the bad habit of throttling due to thermal issues (despite a substantial heatsink) and because the storage is network attached throttling the CPU also massively impacts I/O.

The limitations meant that the workload caused the system to move between high performance and almost no progress on a roughly ten second cycle. This caused a simple NetSurf recompile CI job to take over fifteen minutes. For comparison the same task takes the armhf builder (CubieTruck) four minutes and a 64 bit x86 build which takes around a minute.

If the workload is tuned to a single core which does not trip thermal throttling the build took seven minutes. which is almost identical to the existing single core virtual machine instance running on the hikey.

In conclusion the Pine64 is an interesting bit of hardware with fatally flawed software offering. Without Simon and Lenny providing their builds to the community the device would be practically useless rather than just performing poorly. There appears to have been no progress whatsoever on the software offering from Pine64 in the six months since I received the device and no prospect of mainline Allwinner support for the SoC either.

Effectively I have spent around 50usd (40 for the board and 10 for the enclosure) on a failed experiment. Perhaps in the future the software will improve sufficiently for it to become useful but I do not hold out much hope that this will come from Pine64 themselves.

Saturday, 1 October 2016

Paul Hollywood and the pistoris stone

There has been a great deal of comment among my friends recently about a particularly British cookery program called "The Great British Bake Off". There has been some controversy as the program is moving from the BBC to a commercial broadcaster.

Part of this discussion comes from all the presenters, excepting Paul Hollywood, declining to sign with the new broadcaster and partly because of speculation the BBC might continue with a similar format show with a new name.

Rob Kendrick provided the start to this conversation by passing on a satirical link suggesting Samuel L Jackson might host "cakes on a plane"

This caused a large number of suggestions for alternate names which I will be reporting but Rob Kendrick, Vivek Das Mohapatra, Colin Watson, Jonathan McDowell, Oki Kuma, Dan Alderman, Dagfinn Ilmari MannsÃ¥ke, Lesley Mitchell and Daniel Silverstone are the ones to blame.

  • Strictly come baking
  • Stars and their pies
  • Baking with the stars
  • Bake/Off.
  • Blind Cake
  • Cake or no cake?
  • The cake is a lie
  • Bake That.
  • Bake Me On
  • Bake On Me
  • Bakin' Stevens.
  • The Winner Bakes It All
  • Bakerloo
  • Bake Five
  • Every breath you bake
  • Every bread you bake
  • Unbake my heart
  • Knead and let prove
  • Bake me up before you go-go
  • I want to bake free
  • Another bake bites the dust
  • Cinnamon whorl is not enough
  • The pie who loved me
  • The yeast you can do.
  • Total collapse of the tart
  • Bake and deliver
  • You Gotta Bake
  • Bake's Seven
  • Natural Born Bakers
  • Bake It Or Leaven It
  • Driving the last pikelet
  • Pie crust on the dancefloor
  • Tomorrow never pies
  • Murder on the pie crust
  • The pie who came in from the cold.
  • You only bake twice (Every body has to make one sweet and one savoury dish).

So that is our list, anyone else got better ideas?

Monday, 26 September 2016

I'll huff, and I'll puff, and I'll blow your house in

Sometimes it really helps to have a different view on a problem and after my recent writings on my Public Suffix List (PSL) library I was fortunate to receive a suggestion from my friend Enrico Zini.

I had asked for suggestions on reducing the size of the library further and Enrico simply suggested Huffman coding. This was a technique I had learned about long ago in connection with data compression and the intervening years had made all the details fuzzy which explains why it had not immediately sprung to mind.

A small subset of the Public Suffix List as stored within libnspslHuffman coding named for David A. Huffman is an algorithm that enables a representation of data which is very efficient. In a normal array of characters every character takes the same eight bits to represent which is the best we can do when any of the 256 values possible is equally likely. If your data is not evenly distributed this is not the case for example if the data was english text then the value is fifteen times more likely to be that for e than k.

every step of huffman encoding tree build for the example string tableSo if we have some data with a non uniform distribution of probabilities we need a way the data be encoded with fewer bits for the common values and more bits for the rarer values. To be efficient we would need some way of having variable length representations without storing the length separately. The term for this data representation is a prefix code and there are several ways to generate them.

Such is the influence of Huffman on the area of prefix codes they are often called Huffman codes even if they were not created using his algorithm. One can dream of becoming immortalised like this, to join the ranks of those whose names are given to units or whole ideas in a field must be immensely rewarding, however given Huffman invented his algorithm and proved it to be optimal to answer a question on a term paper in his early twenties I fear I may already be a bit too late.

The algorithm itself is relatively straightforward. First a frequency analysis is performed, a fancy way of saying count how many of each character is in the input data. Next a binary tree is created by using a priority queue initialised with the nodes sorted by frequency.

The resulting huffman tree and the binary representation of the input symbols
The two least frequent items count is summed together and a node placed in the tree with the two original entries as child nodes. This step is repeated until a single node exists with a count value equal to the length of the input.

To encode data once simply walks the tree outputting a 0 for a left node or 1 for right node until reaching the original value. This generates a mapping of values to bit output, the input is then simply converted value by value to the bit output. To decode the data the data is used bit by bit to walk the tree to arrive at values.

If we perform this algorithm on the example string table *!asiabvcomcoopitamazonawsarsaves-the-whalescomputebasilicata we can reduce the 488 bits (61 * 8 bit characters) to 282 bits or 40% reduction. Obviously in a real application the huffman tree would need to be stored which would probably exceed this saving but for larger data sets it is probable this technique would yield excellent results on this kind of data.

Once I proved this to myself I implemented the encoder within the existing conversion program. Although my perl encoder is not very efficient it can process the entire PSL string table (around six thousand labels using 40KB or so) in less than a second, so unless the table grows massively an inelegant approach will suffice.

The resulting bits were packed into 32bit values to improve decode performance (most systems prefer to deal with larger memory fetches less frequently) and resulted in 18KB of output or 47% of the original size. This is a great improvement in size and means the statically linked test program is now 59KB and is actually smaller than the gzipped source data.

$ ls -alh test_nspsl
-rwxr-xr-x 1 vince vince 59K Sep 25 23:58 test_nspsl
$ ls -al public_suffix_list.dat.gz 
-rw-r--r-- 1 vince vince 62K Sep  1 08:52 public_suffix_list.dat.gz

To be clear the statically linked program can determine if a domain is in the PSL with no additional heap allocations and includes the entire PSL ordered tree, the domain label string table and the huffman decode table to read it.

An unexpected side effect is that because the decode loop is small it sits in the processor cache. This appears to cause the string comparison function huffcasecmp() (which is not locale dependant because we know the data will be limited ASCII) performance to be close to using strcasecmp() indeed on ARM32 systems there is a very modest improvement in performance.

I think this is as much work as I am willing to put into this library but I am pleased to have achieved a result which is on par with the best of breed (libpsl still has a data representation 20KB smaller than libnspsl but requires additional libraries for additional functionality) and I got to (re)learn an important algorithm too.

Tuesday, 20 September 2016

If I see an ending, I can work backward.

Now while I am sure Arthur Miller was referring to writing a play when he said those words they have an oddly appropriate resonance for my topic.

In the early nineties Lou Montulli applied the idea of magic cookies to HTTP to make the web stateful, I imagine he had no idea of the issues he was going to introduce for the future. Like most of the web technology it was a solution to an immediate problem which it has never been possible to subsequently improve.

Chocolate chip cookie are much tastier than HTTP cookiesThe HTTP cookie is simply a way for a website to identify a connecting browser session so that state can be kept between retrieving pages. Due to shortcomings in the design of cookies and implementation details in browsers this has lead to a selection of unwanted side effects. The specific issue that I am talking about here is the supercookie where the super prefix in this context has similar connotations as to when applied to the word villain.

Whenever the browser requests a resource (web page, image, etc.) the server may return a cookie along with the resource that your browser remembers. The cookie has a domain name associated with it and when your browser requests additional resources if the cookie domain matches the requested resources domain name the cookie is sent along with the request.

As an example the first time you visit a page on you might receive a cookie with the domain so next time you visit a page on your browser will send the cookie along. Indeed it will also send it along for any page on

A supercookies is simply one where instead of being limited to one sub-domain ( the cookie is set for a top level domain (foo.invalid) so visiting any such domain (I used the invalid name in my examples but one could substitute com or your web browser gives out the cookie. Hackers would love to be able to set up such cookies and potentially control and hijack many sites at a time.

This problem was noted early on and browsers were not allowed to set cookie domains with fewer than two parts so example.invalid or were allowed but invalid or com on their own were not. This works fine for top level domains like .com, .org and .mil but not for countries where the domain registrar had rules about second levels like the uk domain (uk domains must have a second level like

NetSurf cookie manager showing a supercookieThere is no way to generate the correct set of top level domains with an algorithm so a database is required and is called the Public Suffix List (PSL). This database is a simple text formatted list with wildcard and inversion syntax and is at time of writing around 180Kb of text including comments which compresses down to 60Kb or so with deflate.

A few years ago with ICANN allowing the great expansion of top level domains the existing NetSurf supercookie handling was found to be wanting and I decided to implement a solution using the PSL. At this point in time the database was only 100Kb source or 40Kb compressed.

I started by looking at limited existing libraries. In fact only the regdom library was adequate but used 150Kb of heap to load the pre-processed list. This would have had the drawback of increasing NetSurf heap usage significantly (we still have users on 8Mb systems). Because of this and the need to run PHP script to generate the pre-processed input it was decided the library was not suitable.

Lacking other choices I came up with my own implementation which used a perl script to construct a tree of domains from the PSL in a static array with the label strings in a separate table. At the time my implementation added 70Kb of read only data which I thought reasonable and allowed for direct lookup of answers from the database.

This solution still required a pre-processing step to generate the C source code but perl is much more readily available, is a language already used by our tooling and we could always simply ship the generated file. As long as the generated file was updated at release time as we already do for our fallback SSL certificate root set this would be acceptable.

wireshark session shown NetSurf sending a supercookie to
I put the solution into NetSurf, was pleased no-one seemed to notice and moved on to other issues. Recently while fixing a completely unrelated issue in the display of session cookies in the management interface and I realised I had some test supercookies present in the display. After the initial "thats odd" I realised with horror there might be a deeper issue.

It quickly became evident the PSL generation was broken and had been for a long time, even worse somewhere along the line the "redundant" empty generated source file had been removed and the ancient fallback code path was all that had been used.

This issue had escalated somewhat from a trivial display problem. I took a moment to asses the situation a bit more broadly and came to the conclusion there were a number of interconnected causes, centered around the lack of automated testing, which could be solved by extracting the PSL handling into a "support" library.

NetSurf has several of these support libraries which could be used separately to the main browser project but are principally oriented towards it. These libraries are shipped and built in releases alongside the main browser codebase and mainly serve to make API more obvious and modular. In this case my main aim was to have the functionality segregated into a separate module which could be tested, updated and monitored directly by our CI system meaning the embarrassing failure I had found can never occur again.

Before creating my own library I did consider a library called libpsl had been created since I wrote my original implementation. Initially I was very interested in using this library given it managed a data representation within a mere 32Kb.

Unfortunately the library integrates a great deal of IDN and punycode handling which was not required in this use case. NetSurf already has to handle IDN and punycode translations and uses punycode encoded domain names internally only translating to unicode representations for display so duplicating this functionality using other libraries requires a great deal of resource above the raw data representation.

I put the library together based on the existing code generator Perl program and integrated the test set that comes along with the PSL. I was a little alarmed to discover that the PSL had almost doubled in size since the implementation was originally written and now the trivial test program of the library was weighing in at a hefty 120Kb.

This stemmed from two main causes:
  1. there were now many more domain label strings to be stored
  2. there now being many, many more nodes in the tree.
To address the first cause the length of the domain label strings was moved into the unused padding space within each tree node removing a byte from each domain label saving 6Kb. Next it occurred to me that while building the domain label string table that if the label to be added already existed as a substring within the table it could be elided.

The domain labels were sorted from longest to shortest and added in order searching for substring matches as the table was built this saved another 6Kb. I am sure there are ways to reduce this further I have missed (if you see them let me know!) but a 25% saving (47Kb to 35Kb) was a good start.

The second cause was a little harder to address. The structure representing nodes in the tree I started with was at first look reasonable.

struct pnode {
    uint16_t label_index; /* index into string table of label */
    uint16_t label_length; /* length of label */
    uint16_t child_node_index; /* index of first child node */
    uint16_t child_node_count; /* number of child nodes */

I examined the generated table and observed that the majority of nodes were leaf nodes (had no children) which makes sense given the type of data being represented. By allowing two types of node one for labels and a second for the child node information this would halve the node size in most cases and requiring only a modest change to the tree traversal code.

The only issue with this would be that a way to indicate a node has child information. It was realised that the domain labels can have a maximum length of 63 characters meaning their length can be represented in six bits so a uint16_t was excessive. The space was split into two uint8_t parts one for the length and one for a flag to indicate child data node followed.

union pnode {
    struct {
        uint16_t index; /* index into string table of label */
        uint8_t length; /* length of label */
        uint8_t has_children; /* the next table entry is a child node */
    } label;
    struct {
        uint16_t node_index; /* index of first child node */
        uint16_t node_count; /* number of child nodes */
    } child;

static const union pnode pnodes[8580] = {
    /* root entry */
    { .label = { 0, 0, 1 } }, { .child = { 2, 1553 } },
    /* entries 2 to 1794 */
    { .label = {37, 2, 1 } }, { .child = { 1795, 6 } },


    /* entries 8577 to 8578 */
    { .label = {31820, 6, 1 } }, { .child = { 8579, 1 } },
    /* entry 8579 */
    { .label = {0, 1, 0 } },


This change reduced the node array size from 63Kb to 33Kb almost a 50% saving. I considered using bitfields to try and reduce the label length and has_children flag into a single byte but such packing will not reduce the length of a node below 32bits because it is unioned with the child structure.

A possibility of using the spare uint8_t derived by bitfield packing to store an additional label node in three other nodes was considered but added a great deal of complexity to node lookup and table construction for saving around 4Kb so was not incorporated.

With the changes incorporated the test program was a much more acceptable 75Kb reasonably close to the size of the compressed source but with the benefits of direct lookup. Integrating the libraries single API call into NetSurf was straightforward and resulted in correct operation when tested.

This episode just reminded me of the dangers of code that can fail silently. It exposed our users to a security problem that we thought had been addressed almost six years ago and squandered the limited resources of the project. Hopefully a lesson we will not have to learn again any time soon. If there is a positive to take away it is that the new implementation is more space efficient, automatically built and importantly tested

Monday, 22 August 2016

Down the rabbit hole

My descent began with a user reporting a bug and I fear I am still on my way down.

Like Alice I headed down the hole.
The bug was simple enough, a windows bitmap file caused NetSurf to crash. Pretty quickly this was tracked down to the libnsbmp library attempting to decode the file. As to why we have a heavily used library for bitmaps? I am afraid they are part of every icon file and many websites still have favicons using that format.

Some time with a hex editor and the file format specification soon showed that the image in question was malformed and had a bad offset header entry. So I was faced with two issues, firstly that the decoder crashed when presented with badly encoded data and secondly that it failed to deal with incorrect header data.

This is typical of bug reports from real users, the obvious issues have already been encountered by the developers and unit tests formed to prevent them, what remains is harder to produce. After a debugging session with Valgrind and electric fence I discovered the crash was actually caused by running off the front of an allocated block due to an incorrect bounds check. Fixing the bounds check was simple enough as was working round the bad header value and after adding a unit test for the issue I almost moved on.


american fuzzy lop are almost as cute as cats
We already used the bitmap test suite of images to check the library decode which was giving us a good 75% or so line coverage (I long ago added coverage testing to our CI system) but I wondered if there was a test set that might increase the coverage and perhaps exercise some more of the bounds checking code. A bit of searching turned up the american fuzzy lop (AFL) projects synthetic corpora of bmp and ico images.

After checking with the AFL authors that the images were usable in our project I added them to our test corpus and discovered a whole heap of trouble. After fixing more bounds checks and signed issues I finally had a library I was pretty sure was solid with over 85% test coverage.

Then I had the idea of actually running AFL on the library. I had been avoiding this because my previous experimentation with other fuzzing utilities had been utter frustration and very poor return on investment of time. Following the quick start guide looked straightforward enough so I thought I would spend a short amount of time and maybe I would learn a useful tool.

I downloaded the AFL source and built it with a simple make which was an encouraging start. The library was compiled in debug mode with AFL instrumentation simply by changing the compiler and linker environment variables.

$ LD=afl-gcc CC=afl-gcc AFL_HARDEN=1 make VARIANT=debug test
afl-cc 2.32b by <>
afl-cc 2.32b by <>
 COMPILE: src/libnsbmp.c
afl-cc 2.32b by <>
afl-as 2.32b by <>
[+] Instrumented 751 locations (64-bit, hardened mode, ratio 100%).
      AR: build-x86_64-linux-gnu-x86_64-linux-gnu-debug-lib-static/libnsbmp.a
 COMPILE: test/decode_bmp.c
afl-cc 2.32b by <>
afl-as 2.32b by <>
[+] Instrumented 52 locations (64-bit, hardened mode, ratio 100%).
    LINK: build-x86_64-linux-gnu-x86_64-linux-gnu-debug-lib-static/test_decode_bmp
afl-cc 2.32b by <>
 COMPILE: test/decode_ico.c
afl-cc 2.32b by <>
afl-as 2.32b by <>
[+] Instrumented 65 locations (64-bit, hardened mode, ratio 100%).
    LINK: build-x86_64-linux-gnu-x86_64-linux-gnu-debug-lib-static/test_decode_ico
afl-cc 2.32b by <>
Test bitmap decode
Tests:606 Pass:606 Error:0
Test icon decode
Tests:392 Pass:392 Error:0
    TEST: Testing complete

I stuffed the AFL build directory on the end of my PATH, created a directory for the output and ran afl-fuzz

afl-fuzz -i test/bmp -o findings_dir -- ./build-x86_64-linux-gnu-x86_64-linux-gnu-debug-lib-static/test_decode_bmp @@ /dev/null

The result was immediate and not a little worrying, within seconds there were crashes and lots of them! Over the next couple of hours I watched as the unique crash total climbed into the triple digits.

I was forced to abort the run at this point as, despite clear warnings in the AFL documentation of the demands of the tool, my laptop was clearly not cut out to do this kind of work and had become distressingly hot.

AFL has a visualisation tool so you can see what kind of progress it is making which produced a graph that showed just how fast it managed to produce crashes and how much the return plateaus after just a few cycles. Although it was finding a new unique crash every ten minutes or so when aborted.

I dove in to analyse the crashes and it immediately became obvious the main issue was caused when the test tool attempted allocations of absurdly large bitmaps. The browser itself uses a heuristic to determine the maximum image size based on used memory and several other values. I simply applied an upper bound of 48 megabytes per decoded image which fits easily within the fuzzers default heap limit of 50 megabytes.

The main source of "hangs" also came from large allocations so once the test was fixed afl-fuzz was re-run with a timeout parameter set to 100ms. This time after several minutes no crashes and only a single hang were found which came as a great relief, at which point my laptop had a hard shutdown due to thermal event!

Once the laptop cooled down I spooled up a more appropriate system to perform this kind of work a 24way 2.1GHz Xeon system. A Debian Jessie guest vm with 20 processors and 20 gigabytes of memory was created and the build replicated and instrumented.

AFL master node display
To fully utilise this system the next test run would utilise AFL in parallel mode. In this mode there is a single "master" running all the deterministic checks and many "secondary" instances performing random tweaks.

If I have one tiny annoyance with AFL, it is that breeding and feeding a herd of rabbits by hand is annoying and something I would like to see a convenience utility for.

The warren was left overnight with 19 instances and by morning had generated crashes again. This time though the crashes actually appeared to be real failures.

$ afl-whatsup sync_dir/
Summary stats

       Fuzzers alive : 19
      Total run time : 5 days, 12 hours
         Total execs : 214 million
    Cumulative speed : 8317 execs/sec
       Pending paths : 0 faves, 542 total
  Pending per fuzzer : 0 faves, 28 total (on average)
       Crashes found : 554 locally unique

All the crashing test cases are available and a simple file command immediately showed that all the crashing test files had one thing in common the height of the image was -2147483648 This seemingly odd number is actually meaningful to a programmer, it is the largest negative number which can be stored in a 32bit integer (INT32_MIN) I immediately examined the source code that processes the height in the image header.

if ((width <= 0) || (height == 0))          
        return BMP_DATA_ERROR;
if (height < 0) {    
        bmp->reversed = true;   
        height = -height;        

The bug is where the height is made a positive number and results in height being set to 0 after the existing check for zero and results in a crash later in execution. A simple fix was applied and test case added removing the crash and any possible future failure due to this.

Another AFL run has been started and after a few hours has yet to find a crash or non false positive hang so it looks like if there are any more crashes to find they are much harder to uncover.

Main lessons learned are:
  • AFL is an easy to use and immensely powerful and effective tool. State of the art has taken a massive step forward.
  • The test harness is part of the test! make sure it does not behave in a poor manner and cause issues itself.
  • Even a library with extensive test coverage and real world users can benefit from this technique. But it remains to be seen how quickly the rate of return will reduce after the initial fixes.
  • Use the right tool for the job! Ensure you head the warnings in the manual as AFL uses a lot of resources including CPU, disc and memory.
I will of course be debugging any new crashes that occur and perhaps turning my sights to all the projects other unit tested libraries. I will also be investigating the generation of our own custom test corpus from AFL to replace the demo set, this will hopefully increase our unit test coverage even further.

Overall this has been my first successful use of a fuzzing tool and a very positive experience. I would wholeheartedly recommend using AFL to find errors and perhaps even integrate as part of a CI system.

Sunday, 13 March 2016

I changed my mind, Erase and rewind

My recent rack design turned out to simply not be practical. It did not hold all the SBC I needed it to and most troubling accessing connectors was impractical. I was forced to remove the enclosure from the rack and go back to piles of SBC on a shelf.

View of the acrylic being laser cut through the heavily tinted window
This sent me back to the beginning of the design process. The requirement for easy access to connectors had been compromised on in my first solution because I wanted a compact 1U size. This time I returned to my initial toast rack layout but retaining the SBC inside their clip cases.

By facing the connectors downwards and providing basic cable management the design should be much more practical.

My design process is to use the QCAD package to create layered 2D outlines which are then converted from DXF into toolpaths with Lasercut CAM software. The toolpaths are then uploaded to the laser cutter directly from the PC running Lasercut.

Assembled sub rack enclosureDespite the laser cutters being professional grade systems the Lasercut software is a continuous cause of issues for many users, it is the only closed source piece of software in the production process and it has a pretty poor user interface. On this occasion my main issue with it was my design was quite large at 700mm by 400mm which caused the software to crash repeatedly. I broke the design down into two halves and this allowed me to continue.

Once I defeated the software the design was laser cut from 3mm clear extruded acrylic. The assembled is secured with 72 off M3 nuts and bolts. The resulting construction is very strong and probably contains much more material than necessary.

One interesting thing I discovered is that in going from a 1U enclosure holding 5 units to a 2U design holding 11 units I had increased the final weight from 320g to 980g and when all 11 SBC are installed that goes up to a whopping 2300g. Fortunately this is within the mechanical capabilities of the material but it is the heaviest thing I have ever constructed from 3mm acrylic.

bolted into the rack and operatingOnce installed in the rack with all SBC inserted and connected this finally actually works and provides a practical solution. The self is finally clear of SBC and has enough space for all the other systems I need to accommodate for various projects.

As usual the design files are all freely available though I really cannot see anyone else needing to replicate this.

Tuesday, 1 March 2016

Hope is tomorrow's veneer over today's disappointment.

Recently I have been very hopeful about the 96boards Hikey SBC and as Evan Esar predicted I have therefore been very disappointed. I was given a Hikey after a Linaro connect event some time ago by another developer who could not get the system working usefully and this is the tale of what followed.

The Standard Design

This board design was presented as Linaro creating a standard for the 64bit Single Board Computer (SBC) market. So I had expected that a project with such lofty goals would have considered many factors and provided at least as good a solution as the existing 32bit boards.

The lamentable hubris of creating a completely new form factor unfortunately sets a pattern for the whole enterprise. Given the aim towards "makers" I would have accepted that the system would not be a ATX PC size motherboard, but mini/micro/nano and pico ITX have been available for several years.

If opting for a smaller "credit card" form factor why not use one of the common ones that have already been defined by systems such as the Raspberry Pi B+? Instead now every 96Board requires special cases and different expansion boards.

Not content with defining their own form factor the design also uses a 8-18V supply, this is the only SBC I own that is not fed from a 5V supply. I understand that a system might require more current than a micro USB connector can provide, but for example the Banana Pi manages with a DC barrel jack readily capable of delivering 25W which would seem more than enough

The new form factor forced the I/O connectors to be placed differently to other SBC, given the opportunity to concentrate all connectors on one edge (like ATX designs) and avoid the issues where two or three sides are used. The 96board design instead puts connectors on two edges removing any possible benefit this might have given.

The actual I/O connectors specified are rather strange. There is a mandate for HDMI removing the possibility of future display technology changes. The odd USB arrangement of two single sockets instead of a stacked seems to be an attempt to keep height down but the expansion headers and CPU heatsink mean this is largely moot.

The biggest issue though is mandating WIFI but not Ethernet (even as an option), everything else in the design I could logically understand but this makes no sense. It means the design is not useful for many applications without adding USB devices.

Expansion is presented as a 2mm pitch DIL socket for "low speed" signals and a high density connector for "high speed" signals. The positioning and arrangement of these connectors proffered an opportunity to improve upon previous SBC designs which was not taken. The use of 2mm pitch and low voltage signals instead of the more traditional 2.54mm pitch 3.3v signals means that most maker type applications will need adapting from the popular Raspberry Pi and Arduino style designs.

In summary the design appears to have been a Linaro project to favour one of their members which took a Hisilicon Android phone reference design and put it onto a board with no actual thought beyond getting it done. Then afterwards attempted to turn that into a specification, this has simply not worked as an approach.

My personal opinion is that this specification is fatally flawed and, is a direct cause of, the bizarre situation where the "consumer" specification exists alongside the "enterprise" edition which itself has an option of microATX form factor anyhow!

The Implementation

If we ignore the specification appearing to be nothing more than a codification of the original HiKey design we can look at the HiKey as an implementation.

Initially the board required modifying to add headers to attach a USB to 1.8V LVTTL serial adaptor on the UART0 serial port. Once Andy Simpkins had made this change for me I was able to work through the instructions and attempt to install a bootloader and OS image.

The initial software was essentially HiSilicon vendor code using the Android fastboot system to configure booting. There was no source and the Ubuntu OS images were an obvious afterthought to the Android images. Just getting these images installed required a great deal of effort, repetition and debugging. It was such a dreadful experience this signalled the commencement one of the repeated hiatuses throughout this project, the allure of 64 bit ARM computing has its limits even for me.

When I returned to the project I attempted to use the system from the on-board eMMC but the pre-built binary only kernel and OS image were very limited. Building a replacement kernel , or even modules for the existing one proved fruitless and the system was dreadfully unstable.

I wanted to use the system as a builder for some Open Source projects but the system instability ruled this out. I considered attempting to use virtualisation which would also give better system isolation for builder systems. By using KVM running a modern host kernel and OS as a guest this would also avoid issues with the host systems limitations. At which point I discovered the system had no virtualisation enabled apparently because the bootloader lacked support.

In addition to these software issues there were hardware problems, despite forcing the use of USB for all additional connectivity the USB implementation was dreadful. For a start all USB peripherals have to run at the same speed! One cannot mix full (12Mbit) and high speed (480Mbit) devices which makes adding a USB Ethernet and SATA device challenging when you cannot use a keyboard.

And because I needed more challenges only one of the USB root hubs was functional. In effect this made the console serial port critical as it was the only reliable way to reconfigure the system without a keyboard or network link (and WIFI was not reliable either)

After another long pause in proceedings I decided that I should house all the components together and that perhaps being out on my bench might be the cause of some instability. I purchased a powered Amazon basics USB 2 hub, an Ethernet adaptor and a USB 2 SATA interface in the hope of accessing some larger mass storage.

The USB hub power supply was 12V DC which matched the Hikey requirements so I worked out I could use a single 4A capable supply and run a 3.5inch SATA hard drive too. I designed a laser cut enclosure and mounted all the components. As it turned out I only had a 2.5inch hard drive handy so the enclosure is a little over size. If I were redoing this design I would attempt to make it fit in 1U of height and be mountable in a 19inch rack instead it is an 83mm high (under 2U) box.

A new software release had also become available which purported to use an UEFI bootloader after struggling to install this version unsuccessfully, made somewhat more problematic by the undocumented change from UART0 (unpopulated header) to UART3 on the low speed 2mm pitch header. The system seemed to start the kernel which panicked and hung either booting from eMMC or SD card. Once again the project went on hold after spending tens of hours trying to make progress.

Third time's a charm

As the year rolled to a close I once again was persuaded to look at the hikey, I followed the much improved instructions and installed the shiny new November software release which appears to have been made for the re-release of the Hikey through LeMaker. This time I obtained a Debian "jessie" system that booted from the eMMC.

Having a booted system meant I could finally try and use it. I had decided to try and use the system to host virtual machines used as builders within the NetSurf CI system.

The basic OS uses a mixture of normal Debian packages with some replacements from Linaro repositories. I would have prefered to see more use of Debain packages even if they were from the backports repositories but on the positive side it is good to see the use of Debian instead of Ubuntu.

The kernel is a heavily patched 3.18 built in a predominantly monolithic (without modules) manner with the usual exceptions such as the mali and wifi drivers (both of which appear to be binary blobs). The use of a non-mainline capable SoC means the standard generic distribution kernels cannot be used and unless Linaro choose to distribute a kernel with the feature built in it is necessary to compile your own from sources.

The default install has a linaro user which I renamed to my user and ensured all the ssh keys and passwords on the system were changed. This is an important step when using this pre-supplied images as often a booted system is identical to every other copy.

To access mass storage my only option was via USB, indeed to add any additional expansion that is the only choice. The first issue here is that the USB host support is compiled in so when the host ports are initialised it is not possible to select a speed other than 12MBit. The speed is changed to 480Mbit by using a user space application found in the users home directory (why this is not a tool provided by a package and held in sbin I do not know).

When the usb_speed tool is run there is a chance that the previously enumerated devices will be rescanned and what was /dev/sda has become /dev/sdb if this happens there is a high probability that the system must be rebooted to prevent random crashes due to the "zombie" device.

Because the speed change operation is unreliable it cannot be reliably placed in the boot sequence so this must be executed by hand on each boot to get access to the mass storage.

NetSurf project already uses a x86_64 virtual host system which runs an LLVM physical volume from which we allocate logical volumes for each VM. I initially hoped to do this with the hikey but as soon as I tried to use the logical volume with a VM the system locked up with nothing shown on console. I did not really try very hard to discover why and instead simply used files on disc for virtual drives which seemed to work.

To provide reliable network access I used a USB attached Ethernet device, this like the mass storage suffered from unreliable enumeration and for similar reasons could not be automated requiring manually using the serial console to start the system.

Once the system was started I needed to install the guest VM. I had hoped I might be able to install locally from Debian install media as I do for x86 using the libvirt tools. After a great deal of trial and error I finally was forced to abandon this approach when I discovered the Linaro kernel is lacking iso9660 support so installing from standard media was not possible.

Instead I used the instructions provided by Leif Lindholm to create a virtual machine image on my PC and copied the result across. These instructions are great except I used version 2.5 of Qemu instead of 2.2 which had no negative effect. I also installed the Debian backports for Jessie to get an up to date 4.3 kernel.

After copying the image to the Hikey I started it by hand from the command line as a four core virtual machine and was successfully able to log in. The guest would operate for up to a day before stopping with output such as

Message from syslogd@ciworker13 at Jan 29 07:45:28 ...
 kernel:[68903.702501] BUG: soft lockup - CPU#0 stuck for 27s! [mv:24089]

Message from syslogd@ciworker13 at Jan 29 07:45:28 ...

 kernel:[68976.958028] BUG: soft lockup - CPU#2 stuck for 74s! [swapper/2:0]

Message from syslogd@ciworker13 at Jan 29 07:47:39 ...
 kernel:[69103.199724] BUG: soft lockup - CPU#3 stuck for 99s! [swapper/3:0]

Message from syslogd@ciworker13 at Jan 29 07:53:21 ...
 kernel:[69140.321145] BUG: soft lockup - CPU#3 stuck for 30s! [rs:main Q:Reg:505]

Message from syslogd@ciworker13 at Jan 29 07:53:21 ...
 kernel:[69192.880804] BUG: soft lockup - CPU#0 stuck for 21s! [jbd2/vda3-8:107]

Message from syslogd@ciworker13 at Jan 29 07:53:21 ...
 kernel:[69444.805235] BUG: soft lockup - CPU#3 stuck for 22s! [swapper/3:0]

Message from syslogd@ciworker13 at Jan 29 07:55:21 ...
 kernel:[69570.177600] BUG: soft lockup - CPU#1 stuck for 112s! [systemd:1]

Timeout, server not responding.

After this output the host system would not respond and had to be power cycled never mind the guest!

Once I changed to single core operation the system would run for some time until the host suffered from the dreaded kernel OOM killer. I was at a loss as to why the oom killer was running as the VM was only allocated half the physical memory (512MB) allowing the host what I presumed to be an adequate amount.

By adding a 512MB swapfile the system was able to push the few hundred kilobytes it wanted to swap and the system was now stable! The swapfile of course has to be started by hand as the external storage is unreliable and unavailable at boot.

I converted the qemu command line to a libvirt config using the virsh tool
virsh domxml-from-native qemu-argv cmdln.args

The converted configuration required manual editing to get a working system but now I have a libvirt based VM guest I can control along with all my other VM using the virt-manager tool.

This system is now stable and has been in production use for a month at time of writing. The one guest VM is a single core 512MB aarch64 system which takes over 1100 seconds (19 minutes) to do what a Banana Pi 2 dual core 1GB memory 32bit native ARM system manages in 300 seconds.

It seems the single core limited memory system with USB SATA attached storage is very, very slow.

I briefly attempted to run the CI system job natively within the host system but within minutes it crashed hard and required a power cycle to retrieve, it had also broken the UEFI boot. I must thank Leif for walking me through recovering the system otherwise I would have needed to start over.


I must stress these conclusions and observations are my own and do not represent anyone else.

My main conclusions are:

  • My experience is of a poorly conceived, designed and implemented product rushed to market before it was ready.
  • It was the first announced 64bit ARM single board computer to market but that lead was squandered with issues around availability, reliability and software.
  • Value for money appears poor. Product is £70 plus an additional £50 for USB hubs, power supplies, USB Ethernet and USB SATA. Other comparable SBC are around the £30 mark and require fewer extras.
  • The limited I/O within the core product yields a heavy reliance on USB
  • The USB system is poorly implemented resulting in large additional administrative burdens.
  • Limited memory of 1Gigabyte reduces the scope for making use of the benefits of 64bit processors.
  • Recent change to UEFI bootloader is very welcome and something I would like to see across all aarch64 platforms. This is the time for there to be a single unified boot method, having to deal with multiple bad copies of bootloaders in the 32bit world was painful, perhaps this mistake can be avoided in the future.
  • The kernel provision is abysmal and nothing like the quality I would expect from Linaro. The non-upstream kernel combined with missing features after almost a year of development is inexplicable.
  • The Pine64 with 2G of memory and the Raspberry Pi 3 with its de-facto standard form factor are both preferable despite their limitations in other areas.
This project has taken almost a year to get to the final state and has been one of the least enjoyable within that time. The only reason I have a running system at the end of it is sheer bloody mindedness because after spending hundreds of hours of my free time I was not prepared to see it all go to waste.

To be fair, the road I travelled is now much smoother and if the application is suited to having a mobile phone without a screen then the Hikey probably works as a solution. For me, however, the Hikey product with the current hardware and software limitations is not something I would recommend in preference to other options.

Sunday, 21 February 2016

Stack 'em, pack 'em and rack 'em.

As you may be aware I have a bit of a problem with Single Board Computers in that I have a lot of them. Keeping them organised has turned into a bit of a problem.

cluttered shelf of SBC
I designed clip cases for many of these systems giving me a higher storage density on my rack shelves and built a power supply to reduce the cabling complexity. These helped but I still ended up with a cluttered shelf full of SBC.

I decided I would make a rack enclosure to hold the SBC, I was limited to material I could easily CNC machine which limited me to acrylic plastics or wood.

laser cutting the design, viewed through heavily tinted filterInitially I started with the idea of housing the individual boards in a toast rack arrangement. This would mean that the enclosure would have to be at least 2U high to fit the boards all the existing cases would have to be discarded. This approach was dropped when the drawbacks of having no flexibility and only being able to fit the units that were selected at design time became apparent (connector cutouts and mounting hole placement.

Instead I changed course to try and incorporate the existing cases which already solved the differing connector and mounting placement problem and gave me a uniform size to consider. Once I had this approach the design came pretty quickly. I used a tube girder construction 1U in height to get as much strength as possible from the 3mm acrylic plastic I would use.

laser cut pieces arranged for assembly still with protective film on
The design was simply laser cut from sheet stock and fastened together with M3 nut and bolts. Once I corrected the initial design errors (I managed to get almost every important dimension wrong on the first attempt) the result was a success.

working prototype resting on initial version
The prototype is a variety of colours because makespace ran out of suitably sized clear acrylic stock but the colouring has no effect on the result other than aesthetical. The structure gives a great deal of rigidity and there is no sagging or warping, indeed testing on the prototype got to almost 50Kg loading without a failure (one end clamped and the other end loaded at 350mm distance)

I added some simple rotating latches at the front which keep the modules held in place and allow units to be removed quickly if necessary.

rack slots installed and in use
Overall this project was successful and I can now pack five SBC per U neatly. It does limit me to using systems cased in my "slimline" designs (68x30x97mm) which currently means the Raspberry Pi B+ style and the Orange Pi PC.

Once small drawback is access to I/O and power connectors. These need to be right angled and must be unplugged before unit removal which can be a little fiddly. Perhaps a toast rack design of cases would have given easier connector access but I am happy with this trade off of space for density.

As usual the design files are freely available, perhaps they could be useful as a basis for other laser cut rack enclosure designs.

Tuesday, 26 January 2016

Creativity is allowing yourself to make mistakes. Art is knowing which ones to keep.

It seems Scott Adams insights sometimes reach beyond his disturbingly accurate satire. I have written before about my iterative approach to designing the things I make. such as my attempts at furniture and more recently enclosures for various projects.

Raspberry Pi B case design with the faliures
In the workshop today I had a selection of freshly laser cut.completed cases for several single board computers out on the desk. I was asked by a new member of the space how I was able to produce these with no failures?

I was slightly taken aback at the question and had to explain that the designs I was happily running off on the laser cutter are all the result of mistakes, lots of them. The person I was talking to was surprised when I revealed that I was not simply generating fully formed working designs first time.

We chatted for a while and it became apparent that they had been holding themselves back from actually making something because they were afraid the result would be wrong. I went to my box and retrieved the failures from my most recent case design for a Raspberry Pi model B to put alongside the successful end product to try and encourage them.

I explained that my process was fairly iterative, sure I attempted to get it right first time by reusing existing working solutions as a basis but that when the cost of iterating is relatively small it is sometimes worthwhile to just accept the failures.

For example in this latest enclosure:

  • my first attempt (in the semi opaque plastic) resulted in a correct top and bottom but the height was a couple of mm short and the audio connector cutout was too small
  • second attempt was in clear acrylic and omitting the top and bottom. I stuffed the laser cutter setup and the resulting cutouts would not actually fit together properly.
  • third attempt went together ok but my connector cutouts were 0.5mm high so the board did not sit properly, this case would have been usable but I like to publish refined designs so I fixed all the small issues.
  • Fourth version is pretty much correct and I have tried all three different Raspberry Pi model B boards (mine and the spaces) and they all fit so I am confident I have a design I can now use anytime I want a case for this SBC.
My collection of failed cases
Generally I do not need this many iterations and get it right second time, however experience caused me to use offcuts and scrap material for the initial versions expecting to have issues. The point is that I was willing to make the iterations and not see them as failures.

The person I was talking to could not get past the possibility of having a pile of scrap material, it was wasteful in their view and my expectation to fail was unfathomable. They left with somewhat of a bad view of me and my approach.

I pondered this turn of events for a time and they did have a point in that I have a collection of thirty or so failures from all my various designs most of which is unusable. I then realised I have produced over fifty copies of those designs not just for myself but for other people and published them for anyone else to replicate, so on balance I think I am doing ok on wastage.

The stronger argument  for me personally is that I have made something. I love making things, be that software, electronics or physical designs. It may not always be the best solution but I usually end up with something that works.

That makespace member may not like my approach but in the final reckoning, I have made something, their idea is still just an idea. So Scott I may not be an artist but I am at least creative and that is halfway there.

Thursday, 14 January 2016

Ampere was the Newton of Electricity.

I think Maxwell was probably right, certainly the unit of current Ampere gives his name to has been a concern of mine recently.

Regular readers may have possibly noticed my unhealthy obsession with single board computers. I have recently rehomed all the systems into my rack which threw up a small issue of powering them all. I had been using an ad-hoc selection of USB wall warts and adapters but this ended up needing nine mains sockets and short of purchasing a very expensive PDU for the rack would have needed a lot of space.

Additionally having nine separate convertors from mains AC to low voltage DC was consuming over 60Watts for 20W of load! The majority of these supplies were simply delivering 5V either via micro USB or DC barrel jack.

Initially I considered using a ten port powered USB hub but this seemed expensive as I was not going to use the data connections, it also had a limit of 5W per port and some of my systems could potentially use more power than that so I decided to build my own supply.

PSU module from ebay
A quick look on ebay revealed that a 150W (30A at 5V) switching supply could be had from a UK vendor for £9.99 which seemed about right. An enclosure, fused and switched IEC inlet, ammeter/voltmeter with shunt and suitable cables were acquired for another £15

Top view of the supply all wired up
A little careful drilling and cutting of the enclosure made openings for the inlets, cables and display. These were then wired together with crimped and insulated spade and ring connectors. I wanted this build to be safe and reliable so care was taken to get the neatest layout I could manage with good separation between the low and high voltage cabling.

Completed supply with all twelve outputs wired up
The result is a neat supply with twelve outputs which i can easily extend to eighteen if needed. I was pleasantly surprised to discover that even with twelve SBC connected generating 20W load the power drawn by the supply was 25W or about 80% efficiency instead of the 33% previously achieved.

The inbuilt meter allows me to easily see the load on the supply which so far has not risen above 5A even at peak draw, despite the cubitruck and BananaPi having spinning rust hard drives attached, so there is plenty of room for my SBC addiction to grow (I already pledged for a Pine64).

Supply installed in the rack with some of the SBC connected
Overall I am pleased with how this turned out and while there are no detailed design files for this project it should be easy to follow if you want to repeat it. One note of caution though, this project has mains wiring and while I am confident in my own capabilities dealing with potentially lethal voltages I cannot be responsible for anyone else so caveat emptor!