Re: [PATCH v2] libbpf hashmap: Fix undefined behavior in hash_bits

[Andrii Nakryiko]

On Thu, Oct 29, 2020 at 3:38 PM Ian Rogers <irogers@xxxxxxxxxx> wrote:
>
> If bits is 0, the case when the map is empty, then the >> is the size of
> the register which is undefined behavior - on x86 it is the same as a
> shift by 0. Fix by handling the 0 case explicitly and guarding calls to
> hash_bits for empty maps in hashmap__for_each_key_entry and
> hashmap__for_each_entry_safe.
>
> Suggested-by: Andrii Nakryiko <andriin@xxxxxx>,
> Signed-off-by: Ian Rogers <irogers@xxxxxxxxxx>
> ---

Looks good. Thanks and sorry for unnecessary iterations.

Acked-by: Andrii Nakryiko <andrii@xxxxxxxxxx>

>  tools/lib/bpf/hashmap.h | 15 +++++++++------
>  1 file changed, 9 insertions(+), 6 deletions(-)
>
> diff --git a/tools/lib/bpf/hashmap.h b/tools/lib/bpf/hashmap.h
> index d9b385fe808c..10a4c4cd13cf 100644
> --- a/tools/lib/bpf/hashmap.h
> +++ b/tools/lib/bpf/hashmap.h
> @@ -15,6 +15,9 @@
>  static inline size_t hash_bits(size_t h, int bits)
>  {
>         /* shuffle bits and return requested number of upper bits */
> +       if (bits == 0)
> +               return 0;
> +
>  #if (__SIZEOF_SIZE_T__ == __SIZEOF_LONG_LONG__)
>         /* LP64 case */
>         return (h * 11400714819323198485llu) >> (__SIZEOF_LONG_LONG__ * 8 - bits);
> @@ -174,17 +177,17 @@ bool hashmap__find(const struct hashmap *map, const void *key, void **value);
>   * @key: key to iterate entries for
>   */
>  #define hashmap__for_each_key_entry(map, cur, _key)                        \
> -       for (cur = ({ size_t bkt = hash_bits(map->hash_fn((_key), map->ctx),\
> -                                            map->cap_bits);                \
> -                    map->buckets ? map->buckets[bkt] : NULL; });           \
> +       for (cur = map->buckets                                             \
> +                    ? map->buckets[hash_bits(map->hash_fn((_key), map->ctx), map->cap_bits)] \
> +                    : NULL;                                                \
>              cur;                                                           \
>              cur = cur->next)                                               \
>                 if (map->equal_fn(cur->key, (_key), map->ctx))
>
>  #define hashmap__for_each_key_entry_safe(map, cur, tmp, _key)              \
> -       for (cur = ({ size_t bkt = hash_bits(map->hash_fn((_key), map->ctx),\
> -                                            map->cap_bits);                \
> -                    cur = map->buckets ? map->buckets[bkt] : NULL; });     \
> +       for (cur = map->buckets                                             \
> +                    ? map->buckets[hash_bits(map->hash_fn((_key), map->ctx), map->cap_bits)] \
> +                    : NULL;                                                \
>              cur && ({ tmp = cur->next; true; });                           \
>              cur = tmp)                                                     \
>                 if (map->equal_fn(cur->key, (_key), map->ctx))
> --
> 2.29.1.341.ge80a0c044ae-goog
>