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libbpf: Fix race when pinning maps in parallel

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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
This commit is contained in:
Martynas Pumputis
2021-07-26 17:20:01 +02:00
committed by Andrii Nakryiko
parent 7c25b1d569
commit 3a0fc666ef
1 changed files with 14 additions and 1 deletions
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15
src/libbpf.c
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@@ -4657,10 +4657,13 @@ bpf_object__create_maps(struct bpf_object *obj)
char *cp, errmsg[STRERR_BUFSIZE]; char *cp, errmsg[STRERR_BUFSIZE];
unsigned int i, j; unsigned int i, j;
int err; int err;
bool retried;
for (i = 0; i < obj->nr_maps; i++) { for (i = 0; i < obj->nr_maps; i++) {
map = &obj->maps[i]; map = &obj->maps[i];
retried = false;
retry:
if (map->pin_path) { if (map->pin_path) {
err = bpf_object__reuse_map(map); err = bpf_object__reuse_map(map);
if (err) { if (err) {
@@ -4668,6 +4671,12 @@ bpf_object__create_maps(struct bpf_object *obj)
map->name); map->name);
goto err_out; 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) { if (map->fd >= 0) {
@@ -4701,9 +4710,13 @@ bpf_object__create_maps(struct bpf_object *obj)
if (map->pin_path && !map->pinned) { if (map->pin_path && !map->pinned) {
err = bpf_map__pin(map, NULL); err = bpf_map__pin(map, NULL);
if (err) { 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", pr_warn("map '%s': failed to auto-pin at '%s': %d\n",
map->name, map->pin_path, err); map->name, map->pin_path, err);
zclose(map->fd);
goto err_out; goto err_out;
} }
} }