5.15-stable review patch. If anyone has any objections, please let me know.
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From: Yan Zhai yan@cloudflare.com
[ Upstream commit 5644c6b50ffee0a56c1e01430a8c88e34decb120 ]
The generic_map_lookup_batch currently returns EINTR if it fails with ENOENT and retries several times on bpf_map_copy_value. The next batch would start from the same location, presuming it's a transient issue. This is incorrect if a map can actually have "holes", i.e. "get_next_key" can return a key that does not point to a valid value. At least the array of maps type may contain such holes legitly. Right now these holes show up, generic batch lookup cannot proceed any more. It will always fail with EINTR errors.
Rather, do not retry in generic_map_lookup_batch. If it finds a non existing element, skip to the next key. This simple solution comes with a price that transient errors may not be recovered, and the iteration might cycle back to the first key under parallel deletion. For example, Hou Tao houtao@huaweicloud.com pointed out a following scenario:
For LPM trie map: (1) ->map_get_next_key(map, prev_key, key) returns a valid key
(2) bpf_map_copy_value() return -ENOMENT It means the key must be deleted concurrently.
(3) goto next_key It swaps the prev_key and key
(4) ->map_get_next_key(map, prev_key, key) again prev_key points to a non-existing key, for LPM trie it will treat just like prev_key=NULL case, the returned key will be duplicated.
With the retry logic, the iteration can continue to the key next to the deleted one. But if we directly skip to the next key, the iteration loop would restart from the first key for the lpm_trie type.
However, not all races may be recovered. For example, if current key is deleted after instead of before bpf_map_copy_value, or if the prev_key also gets deleted, then the loop will still restart from the first key for lpm_tire anyway. For generic lookup it might be better to stay simple, i.e. just skip to the next key. To guarantee that the output keys are not duplicated, it is better to implement map type specific batch operations, which can properly lock the trie and synchronize with concurrent mutators.
Fixes: cb4d03ab499d ("bpf: Add generic support for lookup batch op") Closes: https://lore.kernel.org/bpf/Z6JXtA1M5jAZx8xD@debian.debian/ Signed-off-by: Yan Zhai yan@cloudflare.com Acked-by: Hou Tao houtao1@huawei.com Link: https://lore.kernel.org/r/85618439eea75930630685c467ccefeac0942e2b.173917159... Signed-off-by: Alexei Starovoitov ast@kernel.org Signed-off-by: Sasha Levin sashal@kernel.org --- kernel/bpf/syscall.c | 18 +++++------------- 1 file changed, 5 insertions(+), 13 deletions(-)
diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c index 37aa1e319165f..7fce461eae102 100644 --- a/kernel/bpf/syscall.c +++ b/kernel/bpf/syscall.c @@ -1435,8 +1435,6 @@ int generic_map_update_batch(struct bpf_map *map, return err; }
-#define MAP_LOOKUP_RETRIES 3 - int generic_map_lookup_batch(struct bpf_map *map, const union bpf_attr *attr, union bpf_attr __user *uattr) @@ -1446,8 +1444,8 @@ int generic_map_lookup_batch(struct bpf_map *map, void __user *values = u64_to_user_ptr(attr->batch.values); void __user *keys = u64_to_user_ptr(attr->batch.keys); void *buf, *buf_prevkey, *prev_key, *key, *value; - int err, retry = MAP_LOOKUP_RETRIES; u32 value_size, cp, max_count; + int err;
if (attr->batch.elem_flags & ~BPF_F_LOCK) return -EINVAL; @@ -1493,14 +1491,8 @@ int generic_map_lookup_batch(struct bpf_map *map, err = bpf_map_copy_value(map, key, value, attr->batch.elem_flags);
- if (err == -ENOENT) { - if (retry) { - retry--; - continue; - } - err = -EINTR; - break; - } + if (err == -ENOENT) + goto next_key;
if (err) goto free_buf; @@ -1515,12 +1507,12 @@ int generic_map_lookup_batch(struct bpf_map *map, goto free_buf; }
+ cp++; +next_key: if (!prev_key) prev_key = buf_prevkey;
swap(prev_key, key); - retry = MAP_LOOKUP_RETRIES; - cp++; cond_resched(); }