Re: [Bug] Failing kunit test on ARCH=arm and LLVM=1

[Kees Cook <kees@kernel.org>]

On Fri, Jun 07, 2024 at 07:33:29AM -0700, Nathan Chancellor wrote:
> Hi Christian,
> 
> On Fri, Jun 07, 2024 at 02:37:19PM +0200, Christian Schrefl wrote:
> > Greetings,
> > 
> > when trying to port Rust to ARM I noticed that the DEFINE_FLEX_test
> > kunit test in lib/overflow_kunit.c:1188 fails when combining LLVM=1
> > and ARCH=arm.
> > 
> > I have reproduced this on v6.10-rc1 and next-20240606.
> > 
> > Here is the clang/llvm version I'm using:
> > clang version 18.1.6 (Fedora 18.1.6-3.fc40)
> > Target: x86_64-redhat-linux-gnu
> > Thread model: posix
> > InstalledDir: /usr/bin
> > Configuration file: /etc/clang/x86_64-redhat-linux-gnu-clang.cfg
> > 
> > I have not looked closer at the failure so I'm unsure if this is a
> > problem with LLVM or if the test case is to speciffic.
> 
> Thanks a lot for the report! I can reproduce this with tip of tree LLVM
> as well.
> 
> $ echo 'CONFIG_KUNIT=y
> CONFIG_OVERFLOW_KUNIT_TEST=y
> CONFIG_RUNTIME_KERNEL_TESTING_MENU=y' >kernel/configs/repro.config
> 
> $ make -skj"$(nproc)" ARCH=arm LLVM=1 {def,repro.}config zImage
> 
> $ boot-qemu.py -a arm -k .
> ...
> [    0.000000] Linux version 6.10.0-rc2-00235-g8a92980606e3 (nathan@thelio-3990X) (ClangBuiltLinux clang version 19.0.0git (https://github.com/llvm/llvm-project e635520be888335dd59874038d33e60cca3a7143), ClangBuiltLinux LLD 19.0.0) #1 SMP Fri Jun  7 06:12:02 MST 2024
> ...
> [    1.832472]     # castable_to_type_test: 75 castable_to_type() tests finished
> [    1.833483]     ok 21 castable_to_type_test
> [    1.834122]     # DEFINE_FLEX_test: EXPECTATION FAILED at lib/overflow_kunit.c:1188
> [    1.834122]     Expected __builtin_dynamic_object_size(two, 0) == sizeof(struct foo) + sizeof(s16) + sizeof(s16), but
> [    1.834122]         __builtin_dynamic_object_size(two, 0) == 8 (0x8)
> [    1.834122]         sizeof(struct foo) + sizeof(s16) + sizeof(s16) == 12 (0xc)
> [    1.834746]     not ok 22 DEFINE_FLEX_test
> ...
> 
> I don't see the same failure with GCC 13.2.0. This test fails when
> building for arm64 and x86_64 as well, so it does not appear to be
> architecture specific.
> 
> I think I see what is going on here. Looking at the documentation for
> DEFINE_RAW_FLEX(), it states "Define an on-stack instance of structure
> with a trailing flexible array member, when it does not have a
> __counted_by annotation." but commit d8e45f2929b9 ("overflow: Change
> DEFINE_FLEX to take __counted_by member") defined 'struct foo' with
> __counted_by on it. __counted_by informs __builtin_dynamic_object_size()
> about the size of the flexible array. With DEFINE_FLEX_RAW, the counter
> is zero, so the size of array in 'struct foo' is zero, meaning this test
> is incorrect when built with a compiler that supports __counted_by,
> which is just Clang 18+ right now (it should land in GCC 15 if I
> understand correctly).
> 
> I see two potential solutions that work for me.
> 
> One would be to stop using DEFINE_RAW_FLEX() and match the other uses
> (but I assume testing DEFINE_RAW_FLEX() was intentional):
> 
> diff --git a/lib/overflow_kunit.c b/lib/overflow_kunit.c
> index 4ef31b0bb74d..883670adf0cc 100644
> --- a/lib/overflow_kunit.c
> +++ b/lib/overflow_kunit.c
> @@ -1180,7 +1180,7 @@ struct foo {
>  
>  static void DEFINE_FLEX_test(struct kunit *test)
>  {
> -	DEFINE_RAW_FLEX(struct foo, two, array, 2);
> +	DEFINE_FLEX(struct foo, two, array, counter, 2);
>  	DEFINE_FLEX(struct foo, eight, array, counter, 8);
>  	DEFINE_FLEX(struct foo, empty, array, counter, 0);
>  
> 
> The other would be making the size of the array conditional on not
> having __counted_by support (which is admittedly ugly):
> 
> diff --git a/lib/overflow_kunit.c b/lib/overflow_kunit.c
> index 4ef31b0bb74d..7eed0890e25f 100644
> --- a/lib/overflow_kunit.c
> +++ b/lib/overflow_kunit.c
> @@ -1185,7 +1185,11 @@ static void DEFINE_FLEX_test(struct kunit *test)
>  	DEFINE_FLEX(struct foo, empty, array, counter, 0);
>  
>  	KUNIT_EXPECT_EQ(test, __struct_size(two),
> -			sizeof(struct foo) + sizeof(s16) + sizeof(s16));
> +			sizeof(struct foo)
> +#if !__has_attribute(__counted_by__)
> +			+ sizeof(s16) + sizeof(s16)
> +#endif
> +			);
>  	KUNIT_EXPECT_EQ(test, __struct_size(eight), 24);
>  	KUNIT_EXPECT_EQ(test, __struct_size(empty), sizeof(struct foo));
>  }
> 
> Kees, am I missing anything here?

Thanks for analyzing this! I've sent a patch for this now. It's similar
to what you've suggested here, but I wanted to break out the
non-counted_by usage as well, which is how DEFINE_RAW_FLEX() is supposed
to be used, but it's good to capture the expected behavior of RAW with
counted_by too.

-- 
Kees Cook