Wrong float4 struct pack on ppc64le and musl-based linux.

[dvarrazzo]

Normally: struct.Struct('!f').pack(1.0).hex() = 3f800000
On musllinux ppc64le, struct.Struct('!f').pack(1.0).hex() = 00000000

Example working manylinux run
Example broken musllinux run

The architecture is the same for both the runs. Reproducible on Python 3.6-3.10.

See psycopg/psycopg#304
Reported to the Alpine project at https://gitlab.alpinelinux.org/alpine/aports/-/issues/13811

[mdickinson]

What does float.__getformat__("float") give on this system?

[mdickinson]

What does float.__getformat__("float") give on this system?

And the reason for asking: if the above is not returning "unknown", then this is the relevant code for converting a double value x to a sequence of four bytes:

cpython/Objects/floatobject.c

Lines 2263 to 2282 in 9d38120

	float y = (float)x;
	int i, incr = 1;
	if (Py_IS_INFINITY(y) && !Py_IS_INFINITY(x))
	goto Overflow;
	unsigned char s[sizeof(float)];
	memcpy(s, &y, sizeof(float));
	if ((float_format == ieee_little_endian_format && !le)
	|| (float_format == ieee_big_endian_format && le)) {
	p += 3;
	incr = -1;
	}
	for (i = 0; i < 4; i++) {
	*p = s[i];
	p += incr;
	}
	return 0;

If you have the facilities to debug on the target machine, that would be the starting point to look at. I'd want to double-check that sizeof(float) really is 4, and that the memcpy is doing the expected thing.

[mdickinson]

One more question: after x = float.fromhex("0x0.23456789abcde"), what does struct.Struct('!f').pack(x) give?

[dvarrazzo]

Requested info at https://github.com/psycopg/psycopg/runs/6607562276?check_suite_focus=true#step:6:437:

  float.__getformat__('float')='IEEE, little-endian'
  struct.Struct('!f').pack(x)=b'\x00\x00\x00\x00'

[mdickinson]

Thanks. The first is what I was expecting (or at least hoping for). That second result is interesting - I was expecting to at least see a subset of the bytes of either the original double value or the converted float.

Staring at the code, I'm having a hard time seeing what could be wrong. Is it possible that there's a compiler or libc bug? Are you in a position to test with a version of Python compiled without the -O3 flag?

[mdickinson]

@tiran Do I recall correctly that you have some familiarity with musl-based Linux distros? If so, any ideas what might be going wrong here?

[tiran]

I'm neither an Alpine expert nor a heavy user of Alpine Linux. All I did was founding a couple of problems with musl libc implementation. By the way, Alpine is still an unsupported platform. Python upstream does not support it because it has known bugs and we lack a stable build bot.

Anyhow I can reproduce the issue with Alpine container image, podman, and qemu-static:

$ podman run -ti --rm --platform linux/ppc64le alpine
/ # apk add python3
/ # python3
Python 3.10.4 (main, Apr 30 2022, 16:49:16) [GCC 11.2.1 20220219] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import struct
>>> struct.Struct('!f').pack(1.0).hex()
'00000000'

The same code works on emulated PPC64 LE Fedora:

$ podman run -ti --rm --platform linux/ppc64le fedora
# python3
Python 3.10.2 (main, Jan 17 2022, 00:00:00) [GCC 11.2.1 20211203 (Red Hat 11.2.1-7)] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import struct
>>> struct.Struct('!f').pack(1.0).hex()
'3f800000'

[tiran]

It looks like a compiler or libc bug on Alpine:

/ # gcc -O0 floatpack.c -o floatpack && ./floatpack 1
1.000000
3f800000
/ # gcc -O1 floatpack.c -o floatpack && ./floatpack 1
1.000000
30303030
/ # gcc -O3 floatpack.c -o floatpack && ./floatpack 1
1.000000
30303030

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main(int argc, char **argv) {
    double d = atof(argv[1]);
    printf("%f\n", d);

    float y = (float)d;
    unsigned char s[sizeof(float)];
    memcpy(s, &y, sizeof(float));
    printf("%02x%02x%02x%02x\n", s[3], s[2], s[1], s[0]);
    return 0;
}

/ # gcc -v
Using built-in specs.
COLLECT_GCC=/usr/bin/gcc
COLLECT_LTO_WRAPPER=/usr/libexec/gcc/powerpc64le-alpine-linux-musl/11.2.1/lto-wrapper
Target: powerpc64le-alpine-linux-musl
Configured with: /home/buildozer/aports/main/gcc/src/gcc-11.2.1_git20220219/configure --prefix=/usr --mandir=/usr/share/man --infodir=/usr/share/info --build=powerpc64le-alpine-linux-musl --host=powerpc64le-alpine-linux-musl --target=powerpc64le-alpine-linux-musl --with-pkgversion='Alpine 11.2.1_git20220219' --enable-checking=release --disable-fixed-point --disable-libstdcxx-pch --disable-multilib --disable-nls --disable-werror --disable-symvers --enable-__cxa_atexit --enable-default-pie --enable-default-ssp --enable-cloog-backend --enable-languages=c,c++,objc,go,fortran,ada,jit --with-abi=elfv2 --enable-secureplt --enable-decimal-float=no --enable-targets=powerpcle-linux --disable-libquadmath --disable-libssp --disable-libmpx --disable-libmudflap --disable-libsanitizer --enable-shared --enable-threads --enable-tls --enable-host-shared --with-system-zlib --with-linker-hash-style=gnu
Thread model: posix
Supported LTO compression algorithms: zlib
gcc version 11.2.1 20220219 (Alpine 11.2.1_git20220219) 

[mdickinson]

Many thanks, @tiran. Looks like there's not a lot CPython can or should do here, then.

@dvarrazzo Okay to close here? (It may be worth copying @tiran's reproducer to the Alpine bug report, though.)

[dvarrazzo]

@mdickinson that's ok for me, thank you for helping to restrict the cause of the problem. Yes, I will update Alpine folks about the developments here.

As far as psycopg is concerned, we are ok: we have a way to detect the issue and a workaround.

Maybe you can incentivize Alpine linux to provide a solution by adding a failing test in the CPython test suite?

[mdickinson]

adding a failing test in the CPython test suite

Yes, we should definitely have a test; I'd be a bit surprised (and disturbed) if we're not exercising this code in the existing test suite. I'll check.

[tiran]

Maybe you can incentivize Alpine linux to provide a solution by adding a failing test in the CPython test suite?

Python's struct tests are failing on Alpine PPC64LE. This starts to look like a QA problem on Alpine's side. We really need some engineers from Alpine and musl to address the musl libc bugs from #90548 and then stable buildbots that run Python builds + tests on platforms supported by Alpine.

/ # apk add python3 python3-tests
/ # python3 -m test -w test_struct
0:00:00 load avg: 2.00 Run tests sequentially
0:00:00 load avg: 2.00 [1/1] test_struct
test test_struct failed -- multiple errors occurred; run in verbose mode for details
test_struct failed (3 failures)

== Tests result: FAILURE ==

1 test failed:
    test_struct
0:00:08 load avg: 2.36
0:00:08 load avg: 2.36 Re-running failed tests in verbose mode
0:00:08 load avg: 2.36 Re-running test_struct in verbose mode (matching: test_705836, test_new_features, test_transitiveness)
test_705836 (test.test_struct.StructTest) ... FAIL
test_new_features (test.test_struct.StructTest) ... FAIL
test_transitiveness (test.test_struct.StructTest) ... FAIL

======================================================================
FAIL: test_705836 (test.test_struct.StructTest)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/usr/lib/python3.10/test/test_struct.py", line 367, in test_705836
    self.assertEqual(base, unpacked)
AssertionError: 1 != 0.0

======================================================================
FAIL: test_new_features (test.test_struct.StructTest)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/usr/lib/python3.10/test/test_struct.py", line 129, in test_new_features
    self.assertEqual(res, exp)
AssertionError: b'\x00\x00\x00\x00' != b'@\x00\x00\x00'

======================================================================
FAIL: test_transitiveness (test.test_struct.StructTest)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/usr/lib/python3.10/test/test_struct.py", line 78, in test_transitiveness
    self.assertEqual(int(100 * fp), int(100 * f))
AssertionError: 0 != 314

----------------------------------------------------------------------
Ran 3 tests in 0.026s

FAILED (failures=3)
test test_struct failed
1 test failed again:
    test_struct

== Tests result: FAILURE then FAILURE ==

1 test failed:
    test_struct

1 re-run test:
    test_struct

Total duration: 8.6 sec
Tests result: FAILURE then FAILURE

/ # python3 -m test.pythoninfo
Python debug information
========================

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