Re: [PATCH 1/1] alloc_tag: work around clang-14 issue with __builtin_object_size()

[Kees Cook]

On Sat, Feb 01, 2025 at 12:05:03PM -0800, Suren Baghdasaryan wrote:
> Additional condition in the allocation hooks causes Clang version 14
> (tested on 14.0.6) to treat the allocated object size as unknown at
> compile-time (__builtin_object_size(obj, 1) returns -1) even though
> both branches of that condition yield the same result. Other versions
> of Clang (tested with 13.0.1, 15.0.7, 16.0.6 and 17.0.6) compile the
> same code without issues. Add build-time Clang version check which
> removes this condition and effectively restores the unconditional tag
> store/restore flow when compiled with clang-14.
> 
> Fixes: 07438779313c ("alloc_tag: avoid current->alloc_tag manipulations when profiling is disabled")
> Reported-by: kernel test robot <lkp@xxxxxxxxx>
> Closes: https://lore.kernel.org/oe-kbuild-all/202501310832.kiAeOt2z-lkp@xxxxxxxxx/
> Signed-off-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>
> ---
>  include/linux/alloc_tag.h | 15 ++++++++++++++-
>  1 file changed, 14 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/alloc_tag.h b/include/linux/alloc_tag.h
> index a946e0203e6d..df432c2c3483 100644
> --- a/include/linux/alloc_tag.h
> +++ b/include/linux/alloc_tag.h
> @@ -222,10 +222,23 @@ static inline void alloc_tag_sub(union codetag_ref *ref, size_t bytes) {}
>  
>  #endif /* CONFIG_MEM_ALLOC_PROFILING */
>  
> +/* See https://lore.kernel.org/all/202501310832.kiAeOt2z-lkp@xxxxxxxxx/ */
> +#if defined(CONFIG_CC_IS_CLANG) && CONFIG_CLANG_VERSION >= 140000 && CONFIG_CLANG_VERSION < 150000

FWIW, this could just be "< 150000" -- < 14 doesn't warn because (as
Nathan mentioned to me today) it didn't support the build-time error
attribute, so it wouldn't have warned even if it did trip over it.

> +static inline bool store_current_tag(void)
> +{
> +	return true;
> +}
> +#else
> +static inline bool store_current_tag(void)
> +{
> +	return mem_alloc_profiling_enabled();
> +}
> +#endif
> +
>  #define alloc_hooks_tag(_tag, _do_alloc)				\
>  ({									\
>  	typeof(_do_alloc) _res;						\
> -	if (mem_alloc_profiling_enabled()) {				\
> +	if (store_current_tag()) {					\
>  		struct alloc_tag * __maybe_unused _old;			\
>  		_old = alloc_tag_save(_tag);				\
>  		_res = _do_alloc;					\

I think the work-around is fine, but I'm trying to dig into the root
cause here.

As you found, it fails on the final strtomem_pad:

	strtomem_pad(key->u.kbd.press_str, press, '\0');
	strtomem_pad(key->u.kbd.repeat_str, repeat, '\0');
	strtomem_pad(key->u.kbd.release_str, release, '\0');

(but not the earlier calls??) The destinations are:

		char press_str[sizeof(void *) + sizeof(int)] __nonstring;
		char repeat_str[sizeof(void *) + sizeof(int)] __nonstring;
		char release_str[sizeof(void *) + sizeof(int)] __nonstring;

Random thoughts include "this is the last array in the struct" which might
imply bad compiler behavior about its sizing via __builtin_object_size()
(i.e. trailing array must always be unknown size to deal with
fake flex arrays), but that wasn't fixed until Clang 16 (with
-fstrict-flex-arrays=3), so that it doesn't trip in Clang 15 is odd.

To Kent's comment[1], I believe I was using __builtin_object_size() here
because I have a knee-jerk aversion to sizeof() due to it blowing up on
flexible arrays, but that's not relevant here. ARRAY_SIZE() would work,
but only if type checking to "char *" succeeds, as Kent suggests.

Let me see if making those changes survives testing...

-Kees

[1] https://lore.kernel.org/all/qbmazoiryyqygjk2x6bc7puqvmik7gyitzo3xnryzsodnrrjek@tahia33lvpli/

> 
> base-commit: 60c828cf80c07394762a1edfaff63bea55cc8e45
> -- 
> 2.48.1.362.g079036d154-goog
> 

-- 
Kees Cook