map_update_elem is used to create relay files and bind them with the
relay channel, which is created with BPF_MAP_CREATE. This allows users
to set a custom directory name. It must be used with key=NULL and
flag=0.

Here is an example:

struct {
__uint(type, BPF_MAP_TYPE_RELAY);
__uint(max_entries, 4096);
} my_relay SEC(".maps");
...
char dir_name[] = "relay_test";
bpf_map_update_elem(map_fd, NULL, dir_name, 0);

Then, directory `/sys/kerenl/debug/relay_test` will be created, which
includes files of my_relay0...my_relay[#cpu]. Each represents a per-cpu
buffer with size 8 * 4096 B (there are 8 subbufs by default, each with
size 4096B).

Signed-off-by: Philo Lu <redacted>
---
 kernel/bpf/relaymap.c | 32 +++++++++++++++++++++++++++++++-
 1 file changed, 31 insertions(+), 1 deletion(-)

diff --git a/kernel/bpf/relaymap.c b/kernel/bpf/relaymap.c
index d0adc7f67758..588c8de0a4bd 100644
--- a/kernel/bpf/relaymap.c
+++ b/kernel/bpf/relaymap.c

@@ -117,7 +117,37 @@ static void *relay_map_lookup_elem(struct bpf_map *map, void *key)
 static long relay_map_update_elem(struct bpf_map *map, void *key, void *value,
 				   u64 flags)
 {
-	return -EOPNOTSUPP;
+	struct bpf_relay_map *rmap;
+	struct dentry *parent;
+	int err;
+
+	if (unlikely(flags))
+		return -EINVAL;
+
+	if (unlikely(key))
+		return -EINVAL;
+
+	rmap = container_of(map, struct bpf_relay_map, map);
+
+	/* The directory already exists */
+	if (rmap->relay_chan->has_base_filename)
+		return -EEXIST;
+
+	/* Setup relay files. Note that the directory name passed as value should
+	 * not be longer than map->value_size, including the '\0' at the end.
+	 */
+	((char *)value)[map->value_size - 1] = '\0';
+	parent = debugfs_create_dir(value, NULL);
+	if (IS_ERR_OR_NULL(parent))
+		return PTR_ERR(parent);
+
+	err = relay_late_setup_files(rmap->relay_chan, map->name, parent);
+	if (err) {
+		debugfs_remove_recursive(parent);
+		return err;
+	}