bpf: add map_lookup_elem_sys_only for lookups from syscall side

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@iogearbox.net>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

1 files changed, 4 insertions, 1 deletions

diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index ad3ccf82f31d..cb5440b02e82 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) {