netdev - [PATCH iproute2 master] bpf: Fix race condition with map pinning

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Message-Id: <20190919220733.31206-1-joe@wand.net.nz>
Date:   Thu, 19 Sep 2019 15:07:33 -0700
From:   Joe Stringer <joe@...d.net.nz>
To:     Stephen Hemminger <stephen@...workplumber.org>
Cc:     netdev@...r.kernel.org, daniel@...earbox.net
Subject: [PATCH iproute2 master] 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@...d.net.nz>
---
 lib/bpf.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/lib/bpf.c b/lib/bpf.c
index f64b58c3bb19..23eb8952cc28 100644
--- 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,
@@ -1674,7 +1676,11 @@ 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) {
+		if (!retried && errno == EEXIST) {
+			retried = true;
+			goto probe;
+		}
 		fprintf(stderr, "Could not pin %s map: %s\n", name,
 			strerror(errno));
 		close(fd);
-- 
2.20.1