[LTP] [PATCH v2] mount08.c: Restrict overmounting of ephemeral entities on /proc/<pid>/fd/<nr>

[Wei Gao]

Signed-off-by: Wei Gao <wegao@suse.com>

Add a new test to verify that mount will raise ENOENT if we try to mount
on magic links under /proc/<pid>/fd/<nr>.
Refer to the following kernel commit for more information:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=d80b065bb172

Signed-off-by: Wei Gao <wegao@suse.com>
---
 runtest/syscalls                           |  1 +
 testcases/kernel/syscalls/mount/.gitignore |  1 +
 testcases/kernel/syscalls/mount/mount08.c  | 57 ++++++++++++++++++++++
 3 files changed, 59 insertions(+)
 create mode 100644 testcases/kernel/syscalls/mount/mount08.c

diff --git a/runtest/syscalls b/runtest/syscalls
index ded035ee8..d3abc8b85 100644
--- a/runtest/syscalls
+++ b/runtest/syscalls
@@ -852,6 +852,7 @@ mount04 mount04
 mount05 mount05
 mount06 mount06
 mount07 mount07
+mount08 mount08
 
 mount_setattr01 mount_setattr01
 
diff --git a/testcases/kernel/syscalls/mount/.gitignore b/testcases/kernel/syscalls/mount/.gitignore
index 80885dbf0..3eee5863a 100644
--- a/testcases/kernel/syscalls/mount/.gitignore
+++ b/testcases/kernel/syscalls/mount/.gitignore
@@ -6,3 +6,4 @@
 /mount05
 /mount06
 /mount07
+/mount08
diff --git a/testcases/kernel/syscalls/mount/mount08.c b/testcases/kernel/syscalls/mount/mount08.c
new file mode 100644
index 000000000..1938c5519
--- /dev/null
+++ b/testcases/kernel/syscalls/mount/mount08.c
@@ -0,0 +1,57 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * Copyright (C) 2024 Wei Gao <wegao@suse.com>
+ */
+
+/*\
+ * Verify that mount will raise ENOENT if we try to mount on magic links
+ * under /proc/<pid>/fd/<nr>.
+ */
+
+#include "tst_test.h"
+#include <sys/mount.h>
+#include "tst_safe_file_at.h"
+
+#define MNTPOINT "mntpoint"
+#define FOO MNTPOINT "/foo"
+#define BAR MNTPOINT "/bar"
+
+static void run(void)
+{
+	char path[PATH_MAX];
+	int foo_fd, newfd, proc_fd;
+
+	foo_fd = SAFE_OPEN(FOO, O_RDONLY | O_NONBLOCK, 0640);
+	newfd = SAFE_DUP(foo_fd);
+	SAFE_CLOSE(foo_fd);
+
+	sprintf(path, "/proc/%d/fd/%d", getpid(), newfd);
+
+	proc_fd = SAFE_OPENAT(AT_FDCWD, path, O_PATH | O_NOFOLLOW);
+
+	sprintf(path, "/proc/%d/fd/%d", getpid(), proc_fd);
+
+	TST_EXP_FAIL(
+		mount(BAR, path, "", MS_BIND, 0),
+		ENOENT,
+		"mount() on proc failed expectedly"
+	);
+}
+
+static void setup(void)
+{
+	SAFE_CREAT(FOO, 0777);
+	SAFE_CREAT(BAR, 0777);
+}
+
+static struct tst_test test = {
+	.setup = setup,
+	.test_all = run,
+	.needs_root = 1,
+	.mntpoint = MNTPOINT,
+	.min_kver = "6.12",
+	.tags = (const struct tst_tag[]) {
+		{"linux-git", "d80b065bb172"},
+		{}
+	}
+};
-- 
2.35.3