Merge branch 'libbpf-fix-reuse-of-devmap'

Yureka Lilian says:

====================
libbpf: fix reuse of DEVMAP

changes in v3:

- instead of setting BPF_F_RDONLY_PROG on both sides, just
  clear BPF_F_RDONLY_PROG in map_info.map_flags as suggested
  by Andrii Nakryiko

- in the test, use ASSERT_* instead of CHECK
- shorten the test by using open_and_load from the skel
- in the test, drop NULL check before unloading/destroying bpf objs

- start the commit messages with "libbpf" and "selftests/bpf"
  respectively instead of just "bpf"

changes in v2:

- preserve compatibility with older kernels
- add a basic selftest covering the re-use of DEVMAP maps
====================

Link: https://patch.msgid.link/20250814180113.1245565-2-yuka@yuka.dev
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>

3 files changed, 80 insertions, 0 deletions

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index 8f5a81b672e1..fe4fc5438678 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -5093,6 +5093,16 @@ static bool map_is_reuse_compat(const struct bpf_map *map, int map_fd)
 		return false;
 	}
 
+	/*
+	 * bpf_get_map_info_by_fd() for DEVMAP will always return flags with
+	 * BPF_F_RDONLY_PROG set, but it generally is not set at map creation time.
+	 * Thus, ignore the BPF_F_RDONLY_PROG flag in the flags returned from
+	 * bpf_get_map_info_by_fd() when checking for compatibility with an
+	 * existing DEVMAP.
+	 */
+	if (map->def.type == BPF_MAP_TYPE_DEVMAP || map->def.type == BPF_MAP_TYPE_DEVMAP_HASH)
+		map_info.map_flags &= ~BPF_F_RDONLY_PROG;
+
 	return (map_info.type == map->def.type &&
 		map_info.key_size == map->def.key_size &&
 		map_info.value_size == map->def.value_size &&
diff --git a/tools/testing/selftests/bpf/prog_tests/pinning_devmap_reuse.c b/tools/testing/selftests/bpf/prog_tests/pinning_devmap_reuse.c
new file mode 100644
index 000000000000..9ae49b587f3e
--- /dev/null
+++ b/tools/testing/selftests/bpf/prog_tests/pinning_devmap_reuse.c
@@ -0,0 +1,50 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <unistd.h>
+#include <test_progs.h>
+
+
+#include "test_pinning_devmap.skel.h"
+
+void test_pinning_devmap_reuse(void)
+{
+	const char *pinpath1 = "/sys/fs/bpf/pinmap1";
+	const char *pinpath2 = "/sys/fs/bpf/pinmap2";
+	struct test_pinning_devmap *skel1 = NULL, *skel2 = NULL;
+	int err;
+	DECLARE_LIBBPF_OPTS(bpf_object_open_opts, opts);
+
+	/* load the object a first time */
+	skel1 = test_pinning_devmap__open_and_load();
+	if (!ASSERT_OK_PTR(skel1, "skel_load1"))
+		goto out;
+
+	/* load the object a second time, re-using the pinned map */
+	skel2 = test_pinning_devmap__open_and_load();
+	if (!ASSERT_OK_PTR(skel2, "skel_load2"))
+		goto out;
+
+	/* we can close the reference safely without
+	 * the map's refcount falling to 0
+	 */
+	test_pinning_devmap__destroy(skel1);
+	skel1 = NULL;
+
+	/* now, swap the pins */
+	err = renameat2(0, pinpath1, 0, pinpath2, RENAME_EXCHANGE);
+	if (!ASSERT_OK(err, "swap pins"))
+		goto out;
+
+	/* load the object again, this time the re-use should fail */
+	skel1 = test_pinning_devmap__open_and_load();
+	if (!ASSERT_ERR_PTR(skel1, "skel_load3"))
+		goto out;
+
+out:
+	unlink(pinpath1);
+	unlink(pinpath2);
+	test_pinning_devmap__destroy(skel1);
+	test_pinning_devmap__destroy(skel2);
+}
diff --git a/tools/testing/selftests/bpf/progs/test_pinning_devmap.c b/tools/testing/selftests/bpf/progs/test_pinning_devmap.c
new file mode 100644
index 000000000000..c855f8f87eff
--- /dev/null
+++ b/tools/testing/selftests/bpf/progs/test_pinning_devmap.c
@@ -0,0 +1,20 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#include <linux/bpf.h>
+#include <bpf/bpf_helpers.h>
+
+struct {
+	__uint(type, BPF_MAP_TYPE_DEVMAP);
+	__uint(max_entries, 1);
+	__type(key, __u32);
+	__type(value, __u32);
+	__uint(pinning, LIBBPF_PIN_BY_NAME);
+} pinmap1 SEC(".maps");
+
+struct {
+	__uint(type, BPF_MAP_TYPE_DEVMAP);
+	__uint(max_entries, 2);
+	__type(key, __u32);
+	__type(value, __u32);
+	__uint(pinning, LIBBPF_PIN_BY_NAME);
+} pinmap2 SEC(".maps");