Re: [PATCH v4] dl-load: add memory barrier before updating the next

[Szabolcs Nagy <szabolcs dot nagy at arm dot com>]

On 10/04/17 09:20, Maninder Singh wrote:
> [BZ #21349]: race condition between dl_open and rtld lazy symbol resolve.
> 
> Issue Fix: race condition between add_name_to_object  & _dl_name_match_p.
> One threads calling dlopen which further calls add_name_to_object &
> other thread trying to resolve RTLD_LAZY symbols through
> _dl_runtime_resolve which further calls.
> 
> _dl_name_match_p checks if libname->next is valid, then it assumes
> libname->next->name to be valid. Also add_name_to_object initialized
> name first and then sets valid next pointer.
> 
> This patch avoids any reorder of instruction when next is set before
> name to avoid any race.
> 
> so as suggested by Torvald Riegel <triegel@redhat.com>:
> "Add C11 memory barrier before updating the liblist next and add comments
> for barriers."
> 
> Signed-off-by: Vaneet Narang <v.narang@samsung.com>
> Signed-off-by: Maninder Singh <maninder1.s@samsung.com>
> ---
> v1 -> v2: use C11 atomics rather than direct memory barriers.
> v2 -> v3: use comments for barriers and enter Bugzilla ID.
> v3 -> v4: remove extra parens.
> 

what happened with this patch?
this fixes a known data race.

>  elf/dl-load.c |    5 ++++-
>  elf/dl-misc.c |    5 ++++-
>  2 files changed, 8 insertions(+), 2 deletions(-)
> 
> diff --git a/elf/dl-load.c b/elf/dl-load.c
> index a5318f9..03c6afb 100644
> --- a/elf/dl-load.c
> +++ b/elf/dl-load.c
> @@ -418,7 +418,10 @@ add_name_to_object (struct link_map *l, const char *name)
>    newname->name = memcpy (newname + 1, name, name_len);
>    newname->next = NULL;
>    newname->dont_free = 0;
> -  lastp->next = newname;
> +  /* We need release memory order here because we need to synchronize
> +     with other thread doing _dl_runtime_resolve which calls _dl_name_match_p
> +     to traverse all names added to libname_list */
> +  atomic_store_release (&lastp->next, newname);
>  }
>  
>  /* Standard search directories.  */
> diff --git a/elf/dl-misc.c b/elf/dl-misc.c
> index 1e9a6ee..a26d6f6 100644
> --- a/elf/dl-misc.c
> +++ b/elf/dl-misc.c
> @@ -295,7 +295,10 @@ _dl_name_match_p (const char *name, const struct link_map *map)
>      if (strcmp (name, runp->name) == 0)
>        return 1;
>      else
> -      runp = runp->next;
> +      /* We need to acquire memory order here because we need to synchronize
> +         with other thread calling dlopen and adding new name to libname_list
> +         through add_name_to_object */
> +      runp = atomic_load_acquire (&runp->next);
>  
>    return 0;
>  }
>