Re: [PATCH -mm] stackdepot: do not use flex_array_size() in memcpy()

[Marco Elver <elver@google.com>]

On Thu, 1 Feb 2024 at 09:35, Marco Elver <elver@google.com> wrote:
>
> Since 113a61863ecb ("Makefile: Enable -Wstringop-overflow globally")
> string overflow checking is enabled by default. Unfortunately the
> compiler still isn't smart enough to always see that the size will never
> overflow.
>
> Specifically, in stackdepot, we have this before memcpy()'ing a
> stacktrace:
>
>   if (nr_entries > CONFIG_STACKDEPOT_MAX_FRAMES)
>         nr_entries = CONFIG_STACKDEPOT_MAX_FRAMES;
>   ...
>   memcpy(stack->entries, entries, flex_array_size(stack, entries, nr_entries));
>
> Where 'entries' is an array of unsigned long, and STACKDEPOT_MAX_FRAMES
> is 64 by default (configurable up to 256), thus the maximum size in
> bytes (on 32-bit) would be 1024. For some reason the compiler (GCC
> 13.2.0) assumes that an 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
>
> Silence the false positive warning by inlining the multiplication
> ourselves.
>
> 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>
> Cc: Gustavo A. R. Silva <gustavoars@kernel.org>
> Cc: Kees Cook <keescook@chromium.org>
> ---
>  lib/stackdepot.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/lib/stackdepot.c b/lib/stackdepot.c
> index 8f3b2c84ec2d..e6047f58ad62 100644
> --- a/lib/stackdepot.c
> +++ b/lib/stackdepot.c
> @@ -456,7 +456,7 @@ depot_alloc_stack(unsigned long *entries, int nr_entries, u32 hash, depot_flags_

Sigh, switching this 'int nr_entries' to 'unsigned int' also fixes it
- please disregard this patch.