2020-10-29 22:41:21

by Ian Rogers

Subject: [PATCH v2] 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.

Suggested-by: Andrii Nakryiko <[email protected]>,
Signed-off-by: Ian Rogers <[email protected]>
---
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

2020-10-29 22:42:55

by Andrii Nakryiko

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

On Thu, Oct 29, 2020 at 3:38 PM Ian Rogers <[email protected]> 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 <[email protected]>,
> Signed-off-by: Ian Rogers <[email protected]>
> ---

Looks good. Thanks and sorry for unnecessary iterations.

Acked-by: Andrii Nakryiko <[email protected]>

> 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
>

2020-10-29 23:00:49

by Song Liu

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

> On Oct 29, 2020, at 3:38 PM, Andrii Nakryiko <[email protected]> wrote:
>
> On Thu, Oct 29, 2020 at 3:38 PM Ian Rogers <[email protected]> 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 <[email protected]>,
>> Signed-off-by: Ian Rogers <[email protected]>
>> ---
>
> Looks good. Thanks and sorry for unnecessary iterations.
>
> Acked-by: Andrii Nakryiko <[email protected]>

Acked-by: Song Liu <[email protected]>