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

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Thu, Oct 29, 2020 at 9:11 AM Ian Rogers <irogers@google.com> 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 when running with
> address sanitizer.
>
> A variant of this patch was posted previously as:
> https://lore.kernel.org/lkml/20200508063954.256593-1-irogers@google.com/
>
> Signed-off-by: Ian Rogers <irogers@google.com>
> ---
>  tools/lib/bpf/hashmap.h | 14 ++++++++++++++
>  1 file changed, 14 insertions(+)
>
> diff --git a/tools/lib/bpf/hashmap.h b/tools/lib/bpf/hashmap.h
> index d9b385fe808c..27d0556527d3 100644
> --- a/tools/lib/bpf/hashmap.h
> +++ b/tools/lib/bpf/hashmap.h
> @@ -12,9 +12,23 @@
>  #include <stddef.h>
>  #include <limits.h>
>
> +#ifdef __has_feature
> +#define HAVE_FEATURE(f) __has_feature(f)
> +#else
> +#define HAVE_FEATURE(f) 0
> +#endif
> +
>  static inline size_t hash_bits(size_t h, int bits)
>  {
>         /* shuffle bits and return requested number of upper bits */
> +#if defined(ADDRESS_SANITIZER) || HAVE_FEATURE(address_sanitizer)
> +       /*
> +        * If the requested bits == 0 avoid undefined behavior from a
> +        * greater-than bit width shift right (aka invalid-shift-exponent).
> +        */
> +       if (bits == 0)
> +               return -1;
> +#endif

Oh, just too much # magic here :(... If we want to prevent hash_bits()
from being called with bits == 0 (despite the result never used),
let's just adjust hashmap__for_each_key_entry and
hashmap__for_each_key_entry_safe macros:

diff --git a/tools/lib/bpf/hashmap.h b/tools/lib/bpf/hashmap.h
index d9b385fe808c..488e0ef236cb 100644
--- a/tools/lib/bpf/hashmap.h
+++ b/tools/lib/bpf/hashmap.h
@@ -174,9 +174,9 @@ 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))

Either way it's a bit ugly and long, but at least we don't have extra
#-driven ugliness.


>  #if (__SIZEOF_SIZE_T__ == __SIZEOF_LONG_LONG__)
>         /* LP64 case */
>         return (h * 11400714819323198485llu) >> (__SIZEOF_LONG_LONG__ * 8 - bits);
> --
> 2.29.1.341.ge80a0c044ae-goog
>