[PATCH bpf-next 2/2] selftests/bpf: Don't stack a bpffs in cgroup_hierarchical_stats

[Qiliang Yuan]

setup_bpffs() mounts a bpffs on /sys/fs/bpf even when one is already
there, while test_progs runs other tests that pin objects in /sys/fs/bpf
in parallel.

The new, empty bpffs hides the old one until the test ends, so those
tests lose what they pinned or the directories they pin into.
bpftool_maps_access then fails to pin an iterator because
/sys/fs/bpf/test_bpftool_map doesn't exist anymore, and bpftool_metadata
can't find the program it pinned in /sys/fs/bpf/test_metadata.

Mount a bpffs only if /sys/fs/bpf isn't one already.

Fixes: 88886309d2e8 ("selftests/bpf: add a selftest for cgroup hierarchical stats collection")
Signed-off-by: Qiliang Yuan <odys.yuan@xxxxxxxxx>
---
 .../bpf/prog_tests/cgroup_hierarchical_stats.c        | 19 +++++++++++++++----
 1 file changed, 15 insertions(+), 4 deletions(-)

diff --git a/tools/testing/selftests/bpf/prog_tests/cgroup_hierarchical_stats.c b/tools/testing/selftests/bpf/prog_tests/cgroup_hierarchical_stats.c
index a93b16ed629e9..8e0b67df700bf 100644
--- a/tools/testing/selftests/bpf/prog_tests/cgroup_hierarchical_stats.c
+++ b/tools/testing/selftests/bpf/prog_tests/cgroup_hierarchical_stats.c
@@ -23,8 +23,10 @@
 #include <asm-generic/errno.h>
 #include <errno.h>
 #include <sys/types.h>
+#include <linux/magic.h>
 #include <sys/mount.h>
 #include <sys/stat.h>
+#include <sys/vfs.h>
 #include <unistd.h>
 
 #include <test_progs.h>
@@ -88,13 +90,22 @@ static int read_from_file(const char *path, char *buf, size_t size)
 /* mounts bpffs and mkdir for reading stats, returns 0 on success. */
 static int setup_bpffs(void)
 {
+	struct statfs st;
 	int err;
 
-	/* Mount bpffs */
-	err = mount("bpf", BPFFS_ROOT, "bpf", 0, NULL);
-	mounted_bpffs = !err;
-	if (ASSERT_FALSE(err && errno != EBUSY, "mount"))
+	/*
+	 * Mount bpffs unless it is already there. Mounting another one on top
+	 * would hide what other tests running in parallel pin in it.
+	 */
+	err = statfs(BPFFS_ROOT, &st);
+	if (!ASSERT_OK(err, "statfs " BPFFS_ROOT))
 		return err;
+	if (st.f_type != BPF_FS_MAGIC) {
+		err = mount("bpf", BPFFS_ROOT, "bpf", 0, NULL);
+		if (!ASSERT_OK(err, "mount"))
+			return err;
+		mounted_bpffs = true;
+	}
 
 	/* Create a directory to contain stat files in bpffs */
 	err = mkdir(BPFFS_ATTACH_COUNTERS, 0755);

-- 
2.43.0