[PATCH 5.0 138/139] bpf: add map_lookup_elem_sys_only for lookups from syscall side

[Greg Kroah-Hartman]

From: Daniel Borkmann <daniel@xxxxxxxxxxxxx>

commit c6110222c6f49ea68169f353565eb865488a8619 upstream.

Add a callback map_lookup_elem_sys_only() that map implementations
could use over map_lookup_elem() from system call side in case the
map implementation needs to handle the latter differently than from
the BPF data path. If map_lookup_elem_sys_only() is set, this will
be preferred pick for map lookups out of user space. This hook is
used in a follow-up fix for LRU map, but once development window
opens, we can convert other map types from map_lookup_elem() (here,
the one called upon BPF_MAP_LOOKUP_ELEM cmd is meant) over to use
the callback to simplify and clean up the latter.

Signed-off-by: Daniel Borkmann <daniel@xxxxxxxxxxxxx>
Acked-by: Martin KaFai Lau <kafai@xxxxxx>
Signed-off-by: Alexei Starovoitov <ast@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 include/linux/bpf.h  |    1 +
 kernel/bpf/syscall.c |    5 ++++-
 2 files changed, 5 insertions(+), 1 deletion(-)

--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -35,6 +35,7 @@ struct bpf_map_ops {
 	void (*map_free)(struct bpf_map *map);
 	int (*map_get_next_key)(struct bpf_map *map, void *key, void *next_key);
 	void (*map_release_uref)(struct bpf_map *map);
+	void *(*map_lookup_elem_sys_only)(struct bpf_map *map, void *key);
 
 	/* funcs callable from userspace and from eBPF programs */
 	void *(*map_lookup_elem)(struct bpf_map *map, void *key);
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c
@@ -738,7 +738,10 @@ static int map_lookup_elem(union bpf_att
 		err = map->ops->map_peek_elem(map, value);
 	} else {
 		rcu_read_lock();
-		ptr = map->ops->map_lookup_elem(map, key);
+		if (map->ops->map_lookup_elem_sys_only)
+			ptr = map->ops->map_lookup_elem_sys_only(map, key);
+		else
+			ptr = map->ops->map_lookup_elem(map, key);
 		if (IS_ERR(ptr)) {
 			err = PTR_ERR(ptr);
 		} else if (!ptr) {