Re: [PATCH 1/2] src/vfs: probe O_TMPFILE support on the base mount in the idmapped tests

On Mon, Jun 15, 2026 at 5:33 PM Christian Brauner <[email protected]> wrote:
>
> openat_tmpfile_supported() in the idmapped mount tests probes O_TMPFILE
> on the idmapped mount (open_tree_fd) as the test's fsuid -- but that
> fsuid is not mapped by the idmapped mount under test, so it really asks
> "can this unmapped caller create a tmpfile through this idmap?" instead
> of "does the filesystem support O_TMPFILE?".
>
> On overlayfs this is a false negative: the backing tmpfile ends up owned
> by an unmapped id, overlayfs opens it with O_NOATIME and may_open()
> returns -EPERM, so the tmpfile sub-tests are wrongly skipped.
>
> O_TMPFILE support is a property of the filesystem, not of the idmapped
> mount or the caller's mapping. Probe info->t_dir1_fd, the base mount
> that the caller owns, exactly as the non-idmapped setgid tests already
> do. The probe then succeeds on every filesystem and the tmpfile
> sub-tests run as the mapped fsuid.
>
> Signed-off-by: Christian Brauner (Amutable) <[email protected]>

Reviewed-by: Amir Goldstein <[email protected]>

> ---
>  src/vfs/idmapped-mounts.c | 12 ++++++------
>  1 file changed, 6 insertions(+), 6 deletions(-)
>
> diff --git a/src/vfs/idmapped-mounts.c b/src/vfs/idmapped-mounts.c
> index ed9992f9..8f8441c9 100644
> --- a/src/vfs/idmapped-mounts.c
> +++ b/src/vfs/idmapped-mounts.c
> @@ -3838,7 +3838,7 @@ int tcore_setgid_create_idmapped(const struct vfstest_info *info)
>                 goto out;
>         }
>
> -       supported = openat_tmpfile_supported(open_tree_fd);
> +       supported = openat_tmpfile_supported(info->t_dir1_fd);
>
>         pid = fork();
>         if (pid < 0) {
> @@ -4014,7 +4014,7 @@ int tcore_setgid_create_idmapped_in_userns(const struct vfstest_info *info)
>                 goto out;
>         }
>
> -       supported = openat_tmpfile_supported(open_tree_fd);
> +       supported = openat_tmpfile_supported(info->t_dir1_fd);
>
>         pid = fork();
>         if (pid < 0) {
> @@ -7733,7 +7733,7 @@ static int setgid_create_umask_idmapped(const struct vfstest_info *info)
>                 goto out;
>         }
>
> -       supported = openat_tmpfile_supported(open_tree_fd);
> +       supported = openat_tmpfile_supported(info->t_dir1_fd);
>
>         pid = fork();
>         if (pid < 0) {
> @@ -7947,7 +7947,7 @@ static int setgid_create_umask_idmapped_in_userns(const struct vfstest_info *inf
>                 goto out;
>         }
>
> -       supported = openat_tmpfile_supported(open_tree_fd);
> +       supported = openat_tmpfile_supported(info->t_dir1_fd);
>
>         /*
>          * Below we verify that setgid inheritance for a newly created file or
> @@ -8181,7 +8181,7 @@ static int setgid_create_acl_idmapped(const struct vfstest_info *info)
>                 goto out;
>         }
>
> -       supported = openat_tmpfile_supported(open_tree_fd);
> +       supported = openat_tmpfile_supported(info->t_dir1_fd);
>
>         pid = fork();
>         if (pid < 0) {
> @@ -8536,7 +8536,7 @@ static int setgid_create_acl_idmapped_in_userns(const struct vfstest_info *info)
>                 goto out;
>         }
>
> -       supported = openat_tmpfile_supported(open_tree_fd);
> +       supported = openat_tmpfile_supported(info->t_dir1_fd);
>
>         /*
>          * Below we verify that setgid inheritance for a newly created file or
>
> --
> 2.47.3
>