[PATCH bpf-next] selftests/bpf: don't call fsopen() as privileged user

[Matteo Croce <technoboy85@gmail.com>]

From: Matteo Croce <teknoraver@meta.com>

In the BPF token example, the fsopen() syscall is called as privileged
user. This is unneeded because fsopen() can be called also as
unprivileged user from the user namespace.
As the `fs_fd` file descriptor which was sent back and fort is still the
same, keep it open instead of cloning and closing it twice via SCM_RIGHTS.

cfr. https://github.com/systemd/systemd/pull/36134

Signed-off-by: Matteo Croce <teknoraver@meta.com>
---
 .../testing/selftests/bpf/prog_tests/token.c  | 41 ++++++++++---------
 1 file changed, 21 insertions(+), 20 deletions(-)

diff --git a/tools/testing/selftests/bpf/prog_tests/token.c b/tools/testing/selftests/bpf/prog_tests/token.c
index f9392df23f8a..cfc032b910c4 100644
--- a/tools/testing/selftests/bpf/prog_tests/token.c
+++ b/tools/testing/selftests/bpf/prog_tests/token.c
@@ -115,7 +115,7 @@ static int create_bpffs_fd(void)
 
 static int materialize_bpffs_fd(int fs_fd, struct bpffs_opts *opts)
 {
-	int mnt_fd, err;
+	int err;
 
 	/* set up token delegation mount options */
 	err = set_delegate_mask(fs_fd, "delegate_cmds", opts->cmds, opts->cmds_str);
@@ -136,12 +136,7 @@ static int materialize_bpffs_fd(int fs_fd, struct bpffs_opts *opts)
 	if (err < 0)
 		return -errno;
 
-	/* create O_PATH fd for detached mount */
-	mnt_fd = sys_fsmount(fs_fd, 0, 0);
-	if (err < 0)
-		return -errno;
-
-	return mnt_fd;
+	return 0;
 }
 
 /* send FD over Unix domain (AF_UNIX) socket */
@@ -287,6 +282,7 @@ static void child(int sock_fd, struct bpffs_opts *opts, child_callback_fn callba
 {
 	int mnt_fd = -1, fs_fd = -1, err = 0, bpffs_fd = -1, token_fd = -1;
 	struct token_lsm *lsm_skel = NULL;
+	char one;
 
 	/* load and attach LSM "policy" before we go into unpriv userns */
 	lsm_skel = token_lsm__open_and_load();
@@ -333,13 +329,19 @@ static void child(int sock_fd, struct bpffs_opts *opts, child_callback_fn callba
 	err = sendfd(sock_fd, fs_fd);
 	if (!ASSERT_OK(err, "send_fs_fd"))
 		goto cleanup;
-	zclose(fs_fd);
+
+	/* wait that the parent reads the fd, does the fsconfig() calls
+	 * and send us a signal that it is done
+	 */
+	err = read(sock_fd, &one, sizeof(one));
+	if (!ASSERT_GE(err, 0, "read_one"))
+		goto cleanup;
 
 	/* avoid mucking around with mount namespaces and mounting at
-	 * well-known path, just get detach-mounted BPF FS fd back from parent
+	 * well-known path, just create O_PATH fd for detached mount
 	 */
-	err = recvfd(sock_fd, &mnt_fd);
-	if (!ASSERT_OK(err, "recv_mnt_fd"))
+	mnt_fd = sys_fsmount(fs_fd, 0, 0);
+	if (!ASSERT_OK_FD(mnt_fd, "mnt_fd"))
 		goto cleanup;
 
 	/* try to fspick() BPF FS and try to add some delegation options */
@@ -429,24 +431,24 @@ static int wait_for_pid(pid_t pid)
 
 static void parent(int child_pid, struct bpffs_opts *bpffs_opts, int sock_fd)
 {
-	int fs_fd = -1, mnt_fd = -1, token_fd = -1, err;
+	int fs_fd = -1, token_fd = -1, err;
+	char one = 1;
 
 	err = recvfd(sock_fd, &fs_fd);
 	if (!ASSERT_OK(err, "recv_bpffs_fd"))
 		goto cleanup;
 
-	mnt_fd = materialize_bpffs_fd(fs_fd, bpffs_opts);
-	if (!ASSERT_GE(mnt_fd, 0, "materialize_bpffs_fd")) {
+	err = materialize_bpffs_fd(fs_fd, bpffs_opts);
+	if (!ASSERT_GE(err, 0, "materialize_bpffs_fd")) {
 		err = -EINVAL;
 		goto cleanup;
 	}
-	zclose(fs_fd);
 
-	/* pass BPF FS context object to parent */
-	err = sendfd(sock_fd, mnt_fd);
-	if (!ASSERT_OK(err, "send_mnt_fd"))
+	/* notify the child that we did the fsconfig() calls and it can proceed. */
+	err = write(sock_fd, &one, sizeof(one));
+	if (!ASSERT_EQ(err, sizeof(one), "send_one"))
 		goto cleanup;
-	zclose(mnt_fd);
+	zclose(fs_fd);
 
 	/* receive BPF token FD back from child for some extra tests */
 	err = recvfd(sock_fd, &token_fd);
@@ -459,7 +461,6 @@ static void parent(int child_pid, struct bpffs_opts *bpffs_opts, int sock_fd)
 cleanup:
 	zclose(sock_fd);
 	zclose(fs_fd);
-	zclose(mnt_fd);
 	zclose(token_fd);
 
 	if (child_pid > 0)
-- 
2.50.0