picolibc-testing

Tue, 01 Mar 2022 23:28:58 -0800
tags/picolibc

Testing Picolibc with the glibc tests

Picolibc has a bunch of built-in tests, but more testing is always better, right? I decided to see how hard it would be to run some of the tests provided in the GNU C Library (glibc).

Parallel meson build files

Similar to how Picolibc uses meson build files to avoid modifying the newlib autotools infrastructure, I decided to take the glibc code and write meson build rules that would compile the tests against Picolibc header files and link against Picolibc libraries.

I decided to select a single target for this project so I could focus on getting things building and not worry too much about making it portable. I wanted to pick something that had hardware floating point so that I would have rounding modes and exception support, so I picked the ARM Cortex M7 with hard float ABI:

$ arm-none-eabi-gcc -mcpu=cortex-m7 -mfloat-abi=hard

It should be fairly easy to extend this to other targets, but for now, that's what I've got working. There's a cross-cortex-m7.txt file containing all of the cross compilation details which is used when running meson setup.

All of the Picolibc-specific files live in a new picolibc directory so they are isolated from the main glibc code.

Pre-loading a pile of hacks

Adapt Picolibc to support the Glibc test code required a bunch of random hacks, from providing _unlocked versions of the stdio macros to stubbing out various unsupportable syscalls (like sleep and chdir). Instead of modifying the Glibc code, I created a file called hacks.h which is full of this stuff and used the gcc -include parameter to read that into the compiler before starting compilation on all of the files.

Supporting command line parameters

The glibc tests all support a range of command line parameters, some of which turned out to be quite useful for this work. Picolibc had limited semihosting support for accessing the command line, but that required modifying applications to go fetch the command line using a special semihosting function.

To make this easier, I added a new crt0 variant for picolibc called (oddly) semihost. This extends the existing hosted variant by adding a call to the semihosting library to fetch the current command line and splitting that into words at each space. It doesn't handle any quoting, but it's sufficient for the needs here.

Avoiding glibc headers

The glibc tests use some glibc-specific extensions to the standard POSIX C library, so I needed to include those in the test builds. Headers for those extensions are mixed in with the rest of the POSIX standard headers, which conflict with the Picolibc versions. To work around this, I stuck stub #include files in the picolibc directory which directly include the appropriate headers for the glibc API extensions. This includes things like argp.h and array_length.h. For other headers which weren't actually needed for picolibc, I created empty files.

Adding more POSIX to Picolibc

At this point, code was compiling but failing to find various standard POSIX functions which aren't available in Picolibc. That included some syscalls which could be emulated using semihosting, like gettimeofday and getpagesize. It also included some generally useful additions, like replacing ecvtbuf and fcvtbuf with ecvt_r and fcvt_r. The _r variants provide a target buffer size instead of assuming that it was large enough as the Picolibc buf variants did.

Which tests are working?

So far, I've got some of the tests in malloc, math, misc and stdio-common running.

There are a lot of tests in the malloc directory which cover glibc API extensions or require POSIX syscalls not supported by semihosting. I think I've added all of the tests which should be supported.

For the math tests, I'm testing the standard POSIX math APIs in both float and double forms, except for the Bessel and Gamma functions. Picolibc's versions of those are so bad that they violate some pretty basic assumptions about error bounds built into the glibc test code. Until Picolibc gets better versions of these functions, we'll have to skip testing them this way.

In the misc directory, I've only added tests for ecvt, fcvt, gcvt, dirname and hsearch. I don't think there are any other tests there which should work.

Finally, for stdio-common, almost all of the tests expect a fully functioning file system, which semihosting really can't support. As a result, we're only able to run the printf, scanf and some of the sprintf tests.

All in all, we're running 78 of the glibc test programs, which is a small fraction of the total tests, but I think it's the bulk of the tests which cover APIs that don't depend too much on the underlying POSIX operating system.

Bugs found and fixed in Picolibc

This exercise has resulted in 17 fixes in Picolibc, which can be classified as:

1. Math functions taking sNaN and not raising FE_INVALID and returning qNaN. Almost any operation on an sNaN value is supposed to signal an invalid operand and replace that with a qNaN so that further processing doesn't raise another exception. This was fairly easy to fix, just need to use return x + x; instead of return x;.

2. Math functions failing to set errno. I'd recently restructured the math library to get rid of the separate IEEE version of the functions which didn't set errno and missed a couple of cases that needed to use the errno-setting helper functions.

3. Corner cases in string/float conversion, including the need to perform round-to-even for '%a' conversions in printf and supporting negative decimal values for fcvt. This particular exercise led to replacing the ecvtbuf and fcvtbuf APIs with glibc's ecvt_r and fcvt_r versions as those pass explicit buffer lengths, making overflow prevention far more reliable.

4. A bunch of malloc entry points were not handling failure correctly; allocation values close to the maximum possible size caused numerous numeric wrapping errors with the usual consequences (allocations "succeed", but return a far smaller buffer than requested). Also, realloc was failing to check the return value from malloc before copying the old data, which really isn't a good idea.

5. Tinystdio's POSIX support code was returning an error instead of EOF at end of file.

6. Error bounds for the Picolibc math library aren't great; I had to generate Picolibc-specific ulps files. Most functions are between 0 and 3 ulps, but for some reason, the float version of erfc (ercf) has errors as large as 64 ulps. That needs investigation.

Tests added to Picolibc

With all of the fixes applied to Picolibc, I added some tests to verify them without relying on running the glibc tests, that includes sNaN vs qNaN tests for math functions, testing fopen and mktemp, checking the printf %a rounding support and adding a ecvt/fcvt tests.

I also discovered that the github-based CI system was compiling but not testing when using clang with a riscv target, so that got added in.

Where's the source code?

The Picolibc changes are sitting on the glibc-testing branch. I'll merge them once the current CI pass finishes.

The hacked-up Glibc bits are in a glibc mirror at github in the picolibc project on the picolibc-testing branch. It would be interesting to know what additional tests should be usable in this environment. And, perhaps, finding a way to use this for picolibc CI testing in the future.

Concluding thoughts

Overall, I'm pretty pleased with these results. The bugs in malloc are fairly serious and could easily result in trouble, but the remaining issues are mostly minor and shouldn't have a big impact on applications using Picolibc.

I'll get the changes merged and start thinking about doing another Picolibc release.