bpf: Fix race condition with map pinning

If two processes attempt to invoke bpf_map_attach() at the same time,
then they will both create maps, then the first will successfully pin
the map to the filesystem and the second will not pin the map, but will
continue operating with a reference to its own copy of the map. As a
result, the sharing of the same map will be broken from the two programs
that were concurrently loaded via loaders using this library.

Fix this by adding a retry in the case where the pinning fails because
the map already exists on the filesystem. In that case, re-attempt
opening a fd to the map on the filesystem as it shows that another
program already created and pinned a map at that location.

Signed-off-by: Joe Stringer <joe@wand.net.nz>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>

diff --git a/lib/bpf.c b/lib/bpf.c
index 7d2a322ffbaecf1b37856f164d56c5ff9a968061..23cb0d96a85ba94dc83dd1c316d6d0dedc5c4994 100644 (file)
--- a/lib/bpf.c
+++ b/lib/bpf.c
@@ -1625,7 +1625,9 @@ static int bpf_map_attach(const char *name, struct bpf_elf_ctx *ctx,
                          int *have_map_in_map)
 {
        int fd, ifindex, ret, map_inner_fd = 0;
+       bool retried = false;
 
+probe:
        fd = bpf_probe_pinned(name, ctx, map->pinning);
        if (fd > 0) {
                ret = bpf_map_selfcheck_pinned(fd, map, ext,
@@ -1674,10 +1676,14 @@ static int bpf_map_attach(const char *name, struct bpf_elf_ctx *ctx,
        }
 
        ret = bpf_place_pinned(fd, name, ctx, map->pinning);
-       if (ret < 0 && errno != EEXIST) {
+       if (ret < 0) {
+               close(fd);
+               if (!retried && errno == EEXIST) {
+                       retried = true;
+                       goto probe;
+               }
                fprintf(stderr, "Could not pin %s map: %s\n", name,
                        strerror(errno));
-               close(fd);
                return ret;
        }