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

[Yosry Ahmed]

On Thu, Oct 8, 2026 at 7:03 AM Qiliang Yuan <odys.yuan@xxxxxxxxx> wrote:
>
> 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.

Seems like both of these tests do not mount bpffs, but rely on its
existence. If it's generally expected that bpffs is mounted when the
tests are run, should we drop the mounting code here instead?

Alternatively, should we have a common helper for creating directories
in bpffs that mounts it if it isn't already there?

>
> 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
>