Re: [PATCH v2] elf: handle addition overflow in _dl_find_object_update_1 [BZ #32245]

[Florian Weimer <fweimer@redhat.com>]

* Aurelien Jarno:

> The remaining_to_add variable can be 0 if (current_used + count) wraps,
> This is caught by GCC 14+ on hppa, which determines from there that
> target_seg could be be NULL when remaining_to_add is zero, which in
> turns causes a -Wstringop-overflow warning:
>
>  In file included from ../include/atomic.h:49,
>                   from dl-find_object.c:20:
>  In function '_dlfo_update_init_seg',
>      inlined from '_dl_find_object_update_1' at dl-find_object.c:689:30,
>      inlined from '_dl_find_object_update' at dl-find_object.c:805:13:
>  ../sysdeps/unix/sysv/linux/hppa/atomic-machine.h:44:4: error: '__atomic_store_4' writing 4 bytes into a region of size 0 overflows the destination [-Werror=stringop-overflow=]
>     44 |    __atomic_store_n ((mem), (val), __ATOMIC_RELAXED);                        \
>        |    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>  dl-find_object.c:644:3: note: in expansion of macro 'atomic_store_relaxed'
>    644 |   atomic_store_relaxed (&seg->size, new_seg_size);
>        |   ^~~~~~~~~~~~~~~~~~~~
>  In function '_dl_find_object_update':
>  cc1: note: destination object is likely at address zero
>
> In practice, this is not possible as it represent counts of link maps.
> Link maps have sizes larger than 1 byte, so the sum of any two link map
> counts will always fit within a size_t without wrapping around.
>
> This patch therefore adds a check on remaining_to_add == 0 and tell GCC
> that this can not happen using __builtin_unreachable.
>
> Thanks to Andreas Schwab for the investigation.
>
> Closes: BZ #32245
> Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
> ---
>  elf/dl-find_object.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
>
> diff --git a/elf/dl-find_object.c b/elf/dl-find_object.c
> index 449302eda3..ae18b438d3 100644
> --- a/elf/dl-find_object.c
> +++ b/elf/dl-find_object.c
> @@ -662,6 +662,14 @@ _dl_find_object_update_1 (struct link_map **loaded, size_t count)
>      = _dlfo_loaded_mappings[!active_idx];
>    size_t remaining_to_add = current_used + count;
>  
> +  /* remaining_to_add can be 0 if (current_used + count) wraps, but in practice
> +     this is not possible as it represent counts of link maps.  Link maps have
> +     sizes larger than 1 byte, so the sum of any two link map counts will
> +     always fit within a size_t without wrapping around.  This check ensures
> +     that target_seg is not erroneously considered potentially NULL by GCC. */
> +  if (remaining_to_add == 0)
> +    __builtin_unreachable ();
> +
>    /* Ensure that the new segment chain has enough space.  */
>    {
>      size_t new_allocated

Okay from me as well.

Reviewed-by: Florian Weimer <fweimer@redhat.com>

Thanks,
Florian