[PATCH bpf 1/3] bpf: add map_lookup_elem_sys_only for lookups from syscall side

[Daniel Borkmann]

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 <dan...@iogearbox.net>
Acked-by: Martin KaFai Lau <ka...@fb.com>
---
 include/linux/bpf.h  | 1 +
 kernel/bpf/syscall.c | 5 ++++-
 2 files changed, 5 insertions(+), 1 deletion(-)

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index 59631dd..4fb3aa2 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -36,6 +36,7 @@ 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);
diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index ad3ccf8..cb5440b 100644
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c
@@ -808,7 +808,10 @@ static int map_lookup_elem(union bpf_attr *attr)
                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) {
-- 
2.9.5