Re: DEFINE_FLEX_test: EXPECTATION FAILED at lib/overflow_kunit.c:1200:

[Nathan Chancellor <nathan@kernel.org>]

Hi Naresh,

+ Kees (it does not look like you own lib/overflow_kunit.c, should that
be updated?)

On Tue, Nov 19, 2024 at 04:17:41PM +0530, Naresh Kamboju wrote:
> The overflow_DEFINE_FLEX_test KUnit test case. This test consistently
> passes when built with GCC-13 but fails when using Clang-19 or
> Clang-nightly.
> 
> Test Case: overflow_DEFINE_FLEX_test
> Compilers: Passing: GCC-13
>            Failing: Clang-19, Clang-nightly
> Observed Behavior: The test failure is reproducible with Clang builds,
>                    while GCC builds produce consistent success.
> 
> This inconsistency suggests a potential issue either in the Clang toolchain
> or in the test implementation that is exposed by Clang's compilation behavior.
> 
> Test log:
> ----------
> <6>[   92.471692]     # castable_to_type_test: 103 castable_to_type()
> tests finished
> <6>[   92.474933]     ok 21 castable_to_type_test
> <3>[   92.476715]     # DEFINE_FLEX_test: EXPECTATION FAILED at
> lib/overflow_kunit.c:1200
> <3>[   92.476715]     Expected
> __builtin_dynamic_object_size(two_but_zero, 0) == expected_raw_size,
> but
> <3>[   92.476715]         __builtin_dynamic_object_size(two_but_zero,
> 0) == 12 (0xc)
> <3>[   92.476715]         expected_raw_size == 8 (0x8)
> <6>[   92.480178]     not ok 22 DEFINE_FLEX_test
> <6>[   92.483020] # overflow: pass:21 fail:1 skip:0 total:22

I can reproduce this with Clang 19.1.3 on 6.12, so it does not appear to
be a recent problem.

  $ printf 'CONFIG_%s=y\n' KUNIT OVERFLOW_KUNIT_TEST >kernel/configs/overflow_kunit.config

  $ make -skj"$(nproc)" ARCH=arm64 LLVM=1 mrproper {def,hardening.,overflow_kunit.}config Image.gz

  $ boot-qemu.py ...
  [    0.000000] Linux version 6.12.0 (nathan@thelio-3990X) (ClangBuiltLinux clang version 19.1.3 (https://github.com/llvm/llvm-project.git ab51eccf88f5321e7c60591c5546b254b6afab99), ClangBuiltLinux LLD 19.1.3 (https://github.com/llvm/llvm-project.git ab51eccf88f5321e7c60591c5546b254b6afab99)) #1 SMP PREEMPT Tue Nov 19 07:28:39 MST 2024
  ...
  [    4.184764]     # DEFINE_FLEX_test: EXPECTATION FAILED at lib/overflow_kunit.c:1200
  [    4.184764]     Expected __builtin_dynamic_object_size(two_but_zero, 0) == expected_raw_size, but
  [    4.184764]         __builtin_dynamic_object_size(two_but_zero, 0) == 12 (0xc)
  [    4.184764]         expected_raw_size == 8 (0x8)
  [    4.190023]     not ok 22 DEFINE_FLEX_test
  [    4.206181] # overflow: pass:21 fail:1 skip:0 total:22
  [    4.208635] # Totals: pass:21 fail:1 skip:0 total:22
  [    4.212218] not ok 1 overflow
  ...

I do not really understand how __builtin_dynamic_object_size() can
return 12 for two_but_zero with __counted_by() because DEFINE_RAW_FLEX()
does not initialize the counter so it should be zero... Kees? I guess
maybe something changed on the LLVM side, I will see if I can bisect
later (all the boxes are tied up with other compilations at the moment).

Cheers,
Nathan