[PATCH 6/8] elf32_offscn.c: Use atomic_load_acquire for Elf_ScnList fields

[Mark Wielaard]

Hi Aaron,

On Wed, 2026-09-30 at 15:05 -0400, Aaron Merey wrote:
> elfNN_offscn reads Elf_ScnList fields cnt and data[0].shdr while not
> holding any lock.  These fields may be written to concurrently by
> elf_getscn and load_shdr_wrlock.
> 
> Replace elfNN_offscn's plain loads of these fields with
> atomic_load_acquire to avoid data races.  Atomic ops are used instead
> of locking in order to avoid holding elf->lock when calling
> elfNN_getshdr, which may also need elf->lock's wrlock.

This looks correct, same (atomic) checks as done in the other code.

Thanks,

Mark

> Signed-off-by: Aaron Merey <amerey@redhat.com>
> ---
>  libelf/elf32_offscn.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/libelf/elf32_offscn.c b/libelf/elf32_offscn.c
> index 1a9a3b0a..5ddb3b58 100644
> --- a/libelf/elf32_offscn.c
> +++ b/libelf/elf32_offscn.c
> @@ -58,8 +58,9 @@ elfw2(LIBELFBITS,offscn) (Elf *elf, ElfW2(LIBELFBITS,Off) offset)
>  
>    /* If we have not looked at section headers before,
>       we might need to read them in first.  */
> -  if (runp->cnt > 0
> -      && unlikely (runp->data[0].shdr.ELFW(e,LIBELFBITS) == NULL)
> +  if (atomic_load_acquire (&runp->cnt) > 0
> +      && unlikely (atomic_load_acquire (&runp->data[0].shdr.ELFW(e,LIBELFBITS))
> +		   == NULL)
>        && unlikely (elfw2(LIBELFBITS,getshdr) (&runp->data[0]) == NULL))
>      return NULL;
>