bpf: Implement bpf_prog replacement for an active bpf_cgroup_link

Add new operation (LINK_UPDATE), which allows to replace active bpf_prog from
under given bpf_link. Currently this is only supported for bpf_cgroup_link,
but will be extended to other kinds of bpf_links in follow-up patches.

For bpf_cgroup_link, implemented functionality matches existing semantics for
direct bpf_prog attachment (including BPF_F_REPLACE flag). User can either
unconditionally set new bpf_prog regardless of which bpf_prog is currently
active under given bpf_link, or, optionally, can specify expected active
bpf_prog. If active bpf_prog doesn't match expected one, no changes are
performed, old bpf_link stays intact and attached, operation returns
a failure.

cgroup_bpf_replace() operation is resolving race between auto-detachment and
bpf_prog update in the same fashion as it's done for bpf_link detachment,
except in this case update has no way of succeeding because of target cgroup
marked as dying. So in this case error is returned.

Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200330030001.2312810-3-andriin@fb.com

include/uapi/linux/bpf.h

@@ -112,6 +112,7 @@ enum bpf_cmd {
 	BPF_MAP_UPDATE_BATCH,
 	BPF_MAP_DELETE_BATCH,
 	BPF_LINK_CREATE,
+	BPF_LINK_UPDATE,
 };
 
 enum bpf_map_type {
@@ -577,6 +578,17 @@ union bpf_attr {
 		__u32		attach_type;	/* attach type */
 		__u32		flags;		/* extra flags */
 	} link_create;
+
+	struct { /* struct used by BPF_LINK_UPDATE command */
+		__u32		link_fd;	/* link fd */
+		/* new program fd to update link with */
+		__u32		new_prog_fd;
+		__u32		flags;		/* extra flags */
+		/* expected link's program fd; is specified only if
+		 * BPF_F_REPLACE flag is set in flags */
+		__u32		old_prog_fd;
+	} link_update;
+
 } __attribute__((aligned(8)));
 
 /* The description below is an attempt at providing documentation to eBPF