bpf: introduce BPF_F_LOCK flag

Introduce BPF_F_LOCK flag for map_lookup and map_update syscall commands
and for map_update() helper function.
In all these cases take a lock of existing element (which was provided
in BTF description) before copying (in or out) the rest of map value.

Implementation details that are part of uapi:

Array:
The array map takes the element lock for lookup/update.

Hash:
hash map also takes the lock for lookup/update and tries to avoid the bucket lock.
If old element exists it takes the element lock and updates the element in place.
If element doesn't exist it allocates new one and inserts into hash table
while holding the bucket lock.
In rare case the hashmap has to take both the bucket lock and the element lock
to update old value in place.

Cgroup local storage:
It is similar to array. update in place and lookup are done with lock taken.

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

2 files changed, 3 insertions, 0 deletions

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index 2ae615b48bb8..bd169a7bcc93 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -119,6 +119,8 @@ static inline void copy_map_value(struct bpf_map *map, void *dst, void *src)
 		memcpy(dst, src, map->value_size);
 	}
 }
+void copy_map_value_locked(struct bpf_map *map, void *dst, void *src,
+			   bool lock_src);
 
 struct bpf_offload_dev;
 struct bpf_offloaded_map;
diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index 86f7c438d40f..1777fa0c61e4 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h
@@ -267,6 +267,7 @@ enum bpf_attach_type {
 #define BPF_ANY		0 /* create new element or update existing */
 #define BPF_NOEXIST	1 /* create new element if it didn't exist */
 #define BPF_EXIST	2 /* update existing element */
+#define BPF_F_LOCK	4 /* spin_lock-ed map_lookup/map_update */
 
 /* flags for BPF_MAP_CREATE command */
 #define BPF_F_NO_PREALLOC	(1U << 0)