Re: [PATCH v3 09/10] stackdepot: Store compressed frames in stack records

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential
issue to consider:
- [High] stackdepot: uninitialized frame in depot_stack_entry() on
  generic arches
--

commit 8ef363f0fde887ad99497bbd2e7b00c5bd8bac35
Author: Matt Fleming <mfleming@cloudflare.com>

stackdepot: Store compressed frames in stack records

This commit stores stack frames in a 32-bit compressed array inside
struct stack_record when all frames of a persistent stack trace can be
compressed by architecture hooks.

> diff --git a/lib/stackdepot.c b/lib/stackdepot.c
> index 8f1aaa0c6d889..f323f1e97e129 100644
> --- a/lib/stackdepot.c
> +++ b/lib/stackdepot.c
[ ... ]
> @@ -419,9 +422,37 @@ static struct stack_record *depot_pop_free(void)
>  	return stack;
>  }
>  
> -static inline size_t depot_stack_record_size(struct stack_record *s, unsigned int nr_entries)
> +/* Check whether every frame of a stack can be stored in 32 bits. */
> +static bool depot_entries_compressible(const unsigned long *entries,
> +				       unsigned int nr_entries)
> +{
> +	u32 payload;
> +
> +	for (unsigned int i = 0; i < nr_entries; i++) {
> +		if (!arch_stack_depot_frame_try_compress(entries[i], &payload))
> +			return false;
> +	}
> +	return true;
> +}
> +
> +static inline unsigned long depot_stack_entry(const struct stack_record *stack,
> +					      unsigned int i)
>  {
> -	const size_t used = flex_array_size(s, entries, nr_entries);
> +	unsigned long frame;
> +
> +	if (!(stack->flags & STACK_RECORD_COMPRESSED))
> +		return stack->entries[i];
> +	arch_stack_depot_frame_decompress(stack->compressed_entries[i], &frame);
> +	return frame;
> +}

[Severity: High]
Does depot_stack_entry() return an uninitialized frame on architectures
using the generic header?

include/asm-generic/stackdepot.h:arch_stack_depot_frame_decompress() {
	/* Generic code never compresses frames, so this hook is unreachable. */
}

Because stack->flags is read at runtime and the generic
arch_stack_depot_frame_decompress() neither writes to *frame nor calls
unreachable(), the compiler cannot tell that the compressed branch is dead.
In depot_stack_equal(), the early return when
!(stack->flags & STACK_RECORD_COMPRESSED) folds the same check inside the
inlined depot_stack_entry() call, leaving frame unconditionally
uninitialized in the loop and triggering -Wuninitialized build failures
under CONFIG_WERROR=y.

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20261113185702.1332171-1-matt@readmodwrite.com?part=9