Re: [PATCH -mm v2] stackdepot: fix -Wstringop-overflow warning

[Stephen Rothwell <sfr@canb.auug.org.au>]

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Hi all,

On Thu,  1 Feb 2024 10:04:30 +0100 Marco Elver <elver@google.com> wrote:
>
> Since 113a61863ecb ("Makefile: Enable -Wstringop-overflow globally")
> string overflow checking is enabled by default. Within stackdepot, the
> compiler (GCC 13.2.0) assumes that a multiplication overflow may be
> possible and flex_array_size() can return SIZE_MAX (4294967295 on
> 32-bit), resulting in this warning:
> 
>  In function 'depot_alloc_stack',
>      inlined from 'stack_depot_save_flags' at lib/stackdepot.c:688:4:
>  arch/x86/include/asm/string_32.h:150:25: error: '__builtin_memcpy' specified bound 4294967295 exceeds maximum object size 2147483647 [-Werror=stringop-overflow=]
>    150 | #define memcpy(t, f, n) __builtin_memcpy(t, f, n)
>        |                         ^~~~~~~~~~~~~~~~~~~~~~~~~
>  lib/stackdepot.c:459:9: note: in expansion of macro 'memcpy'
>    459 |         memcpy(stack->entries, entries, flex_array_size(stack, entries, nr_entries));
>        |         ^~~~~~
>  cc1: all warnings being treated as errors
> 
> This is due to depot_alloc_stack() accepting an 'int nr_entries' which
> could be negative without deeper analysis of callers.
> 
> The call to depot_alloc_stack() from stack_depot_save_flags(), however,
> only passes in its nr_entries which is unsigned int. Fix the warning by
> switching depot_alloc_stack()'s nr_entries to also be unsigned.
> 
> Link: https://lore.kernel.org/all/20240201135747.18eca98e@canb.auug.org.au/
> Fixes: d869d3fb362c ("stackdepot: use variable size records for non-evictable entries")
> Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
> Signed-off-by: Marco Elver <elver@google.com>
> ---
> v2:
> * Just switch 'nr_entries' to unsigned int which is already the case
>   elsewhere.
> ---
>  lib/stackdepot.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/lib/stackdepot.c b/lib/stackdepot.c
> index 8f3b2c84ec2d..4a7055a63d9f 100644
> --- a/lib/stackdepot.c
> +++ b/lib/stackdepot.c
> @@ -420,7 +420,7 @@ static inline size_t depot_stack_record_size(struct stack_record *s, unsigned in
>  
>  /* Allocates a new stack in a stack depot pool. */
>  static struct stack_record *
> -depot_alloc_stack(unsigned long *entries, int nr_entries, u32 hash, depot_flags_t flags, void **prealloc)
> +depot_alloc_stack(unsigned long *entries, unsigned int nr_entries, u32 hash, depot_flags_t flags, void **prealloc)
>  {
>  	struct stack_record *stack = NULL;
>  	size_t record_size;
> -- 
> 2.43.0.429.g432eaa2c6b-goog
> 

I have applied this patch to the merge of the mm tree today.

-- 
Cheers,
Stephen Rothwell

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