Subject: [PATCH AUTOSEL 5.10 059/176] libbpf: Fix race when pinning maps in parallel

From: Sasha Levin <[email-masked]>
Date: Thu, 9 Sep 2021 07:49:21 -0400

From: Martynas Pumputis <[email-masked]>

[ Upstream commit 043c5bb3c4f43670ab4fea0b847373ab42d25f3e ]

When loading in parallel multiple programs which use the same to-be
pinned map, it is possible that two instances of the loader will call
bpf_object__create_maps() at the same time. If the map doesn't exist
when both instances call bpf_object__reuse_map(), then one of the
instances will fail with EEXIST when calling bpf_map__pin().

Fix the race by retrying reusing a map if bpf_map__pin() returns
EEXIST. The fix is similar to the one in iproute2: e4c4685fd6e4 ("bpf:
Fix race condition with map pinning").

Before retrying the pinning, we don't do any special cleaning of an
internal map state. The closer code inspection revealed that it's not
required:

    - bpf_object__create_map(): map->inner_map is destroyed after a
      successful call, map->fd is closed if pinning fails.
    - bpf_object__populate_internal_map(): created map elements is
      destroyed upon close(map->fd).
    - init_map_slots(): slots are freed after their initialization.

Signed-off-by: Martynas Pumputis <[email-masked]>
Signed-off-by: Andrii Nakryiko <[email-masked]>
Link: https://lore.kernel.org/bpf/[email-masked]
Signed-off-by: Sasha Levin <[email-masked]>
---
 tools/lib/bpf/libbpf.c | 15 ++++++++++++++-
 1 file changed, 14 insertions(+), 1 deletion(-)

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index a4e61dd5d8b3..d24531bb69d4 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -4286,10 +4286,13 @@ bpf_object__create_maps(struct bpf_object *obj)
 	char *cp, errmsg[STRERR_BUFSIZE];
 	unsigned int i, j;
 	int err;
+	bool retried;
 
 	for (i = 0; i < obj->nr_maps; i++) {
 		map = &obj->maps[i];
 
+		retried = false;
+retry:
 		if (map->pin_path) {
 			err = bpf_object__reuse_map(map);
 			if (err) {
@@ -4297,6 +4300,12 @@ bpf_object__create_maps(struct bpf_object *obj)
 					map->name);
 				goto err_out;
 			}
+			if (retried && map->fd < 0) {
+				pr_warn("map '%s': cannot find pinned map\n",
+					map->name);
+				err = -ENOENT;
+				goto err_out;
+			}
 		}
 
 		if (map->fd >= 0) {
@@ -4330,9 +4339,13 @@ bpf_object__create_maps(struct bpf_object *obj)
 		if (map->pin_path && !map->pinned) {
 			err = bpf_map__pin(map, NULL);
 			if (err) {
+				zclose(map->fd);
+				if (!retried && err == -EEXIST) {
+					retried = true;
+					goto retry;
+				}
 				pr_warn("map '%s': failed to auto-pin at '%s': %d\n",
 					map->name, map->pin_path, err);
-				zclose(map->fd);
 				goto err_out;
 			}
 		}
-- 
2.30.2