libbpf, hashmap: Fix undefined behavior in hash_bits

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.

Fixes: e3b924224028 ("libbpf: add resizable non-thread safe internal hashmap")
Suggested-by: Andrii Nakryiko <andriin@fb.com>,
Signed-off-by: Ian Rogers <irogers@google.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Song Liu <songliubraving@fb.com>
Link: https://lore.kernel.org/bpf/20201029223707.494059-1-irogers@google.com

diff --git a/tools/lib/bpf/hashmap.h b/tools/lib/bpf/hashmap.h
index d9b385f..10a4c4c 100644 (file)
--- 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))