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

Sasha Levin
2021-09-09T12:41:02Z

From: Martynas Pumputis <m@lambda.lt>

[ 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 <m@lambda.lt>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20210726152001.34845-1-m@lambda.lt
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
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