selftests: mount: add nosymfollow tests

Add tests for the new 'nosymfollow' mount option.  We test to make sure
that symlink traversal fails with ELOOP when 'nosymfollow' is set, but
that readlink(2) and realpath(3) still work as expected.  We also verify
that statfs(2) correctly returns ST_NOSYMFOLLOW when we are mounted with
the 'nosymfollow' option.

Signed-off-by: Ross Zwisler <zwisler@google.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

tools/testing/selftests/mount/.gitignore

@@ -1,2 +1,3 @@
 # SPDX-License-Identifier: GPL-2.0-only
 unprivileged-remount-test
+nosymfollow-test

tools/testing/selftests/mount/Makefile

@@ -3,7 +3,7 @@
 CFLAGS = -Wall \
          -O2
 
-TEST_PROGS := run_tests.sh
-TEST_GEN_FILES := unprivileged-remount-test
+TEST_PROGS := run_unprivileged_remount.sh run_nosymfollow.sh
+TEST_GEN_FILES := unprivileged-remount-test nosymfollow-test
 
 include ../lib.mk

tools/testing/selftests/mount/nosymfollow-test.c

@@ -0,0 +1,218 @@
+// SPDX-License-Identifier: GPL-2.0
+#define _GNU_SOURCE
+#include <errno.h>
+#include <fcntl.h>
+#include <limits.h>
+#include <sched.h>
+#include <stdarg.h>
+#include <stdbool.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/mount.h>
+#include <sys/stat.h>
+#include <sys/types.h>
+#include <sys/vfs.h>
+#include <unistd.h>
+
+#ifndef MS_NOSYMFOLLOW
+# define MS_NOSYMFOLLOW 256     /* Do not follow symlinks */
+#endif
+
+#ifndef ST_NOSYMFOLLOW
+# define ST_NOSYMFOLLOW 0x2000  /* Do not follow symlinks */
+#endif
+
+#define DATA "/tmp/data"
+#define LINK "/tmp/symlink"
+#define TMP  "/tmp"
+
+static void die(char *fmt, ...)
+{
+	va_list ap;
+
+	va_start(ap, fmt);
+	vfprintf(stderr, fmt, ap);
+	va_end(ap);
+	exit(EXIT_FAILURE);
+}
+
+static void vmaybe_write_file(bool enoent_ok, char *filename, char *fmt,
+		va_list ap)
+{
+	ssize_t written;
+	char buf[4096];
+	int buf_len;
+	int fd;
+
+	buf_len = vsnprintf(buf, sizeof(buf), fmt, ap);
+	if (buf_len < 0)
+		die("vsnprintf failed: %s\n", strerror(errno));
+
+	if (buf_len >= sizeof(buf))
+		die("vsnprintf output truncated\n");
+
+	fd = open(filename, O_WRONLY);
+	if (fd < 0) {
+		if ((errno == ENOENT) && enoent_ok)
+			return;
+		die("open of %s failed: %s\n", filename, strerror(errno));
+	}
+
+	written = write(fd, buf, buf_len);
+	if (written != buf_len) {
+		if (written >= 0) {
+			die("short write to %s\n", filename);
+		} else {
+			die("write to %s failed: %s\n",
+				filename, strerror(errno));
+		}
+	}
+
+	if (close(fd) != 0)
+		die("close of %s failed: %s\n", filename, strerror(errno));
+}
+
+static void maybe_write_file(char *filename, char *fmt, ...)
+{
+	va_list ap;
+
+	va_start(ap, fmt);
+	vmaybe_write_file(true, filename, fmt, ap);
+	va_end(ap);
+}
+
+static void write_file(char *filename, char *fmt, ...)
+{
+	va_list ap;
+
+	va_start(ap, fmt);
+	vmaybe_write_file(false, filename, fmt, ap);
+	va_end(ap);
+}
+
+static void create_and_enter_ns(void)
+{
+	uid_t uid = getuid();
+	gid_t gid = getgid();
+
+	if (unshare(CLONE_NEWUSER) != 0)
+		die("unshare(CLONE_NEWUSER) failed: %s\n", strerror(errno));
+
+	maybe_write_file("/proc/self/setgroups", "deny");
+	write_file("/proc/self/uid_map", "0 %d 1", uid);
+	write_file("/proc/self/gid_map", "0 %d 1", gid);
+
+	if (setgid(0) != 0)
+		die("setgid(0) failed %s\n", strerror(errno));
+	if (setuid(0) != 0)
+		die("setuid(0) failed %s\n", strerror(errno));
+
+	if (unshare(CLONE_NEWNS) != 0)
+		die("unshare(CLONE_NEWNS) failed: %s\n", strerror(errno));
+}
+
+static void setup_symlink(void)
+{
+	int data, err;
+
+	data = creat(DATA, O_RDWR);
+	if (data < 0)
+		die("creat failed: %s\n", strerror(errno));
+
+	err = symlink(DATA, LINK);
+	if (err < 0)
+		die("symlink failed: %s\n", strerror(errno));
+
+	if (close(data) != 0)
+		die("close of %s failed: %s\n", DATA, strerror(errno));
+}
+
+static void test_link_traversal(bool nosymfollow)
+{
+	int link;
+
+	link = open(LINK, 0, O_RDWR);
+	if (nosymfollow) {
+		if ((link != -1 || errno != ELOOP)) {
+			die("link traversal unexpected result: %d, %s\n",
+					link, strerror(errno));
+		}
+	} else {
+		if (link < 0)
+			die("link traversal failed: %s\n", strerror(errno));
+
+		if (close(link) != 0)
+			die("close of link failed: %s\n", strerror(errno));
+	}
+}
+
+static void test_readlink(void)
+{
+	char buf[4096];
+	ssize_t ret;
+
+	bzero(buf, sizeof(buf));
+
+	ret = readlink(LINK, buf, sizeof(buf));
+	if (ret < 0)
+		die("readlink failed: %s\n", strerror(errno));
+	if (strcmp(buf, DATA) != 0)
+		die("readlink strcmp failed: '%s' '%s'\n", buf, DATA);
+}
+
+static void test_realpath(void)
+{
+	char *path = realpath(LINK, NULL);
+
+	if (!path)
+		die("realpath failed: %s\n", strerror(errno));
+	if (strcmp(path, DATA) != 0)
+		die("realpath strcmp failed\n");
+
+	free(path);
+}
+
+static void test_statfs(bool nosymfollow)
+{
+	struct statfs buf;
+	int ret;
+
+	ret = statfs(TMP, &buf);
+	if (ret)
+		die("statfs failed: %s\n", strerror(errno));
+
+	if (nosymfollow) {
+		if ((buf.f_flags & ST_NOSYMFOLLOW) == 0)
+			die("ST_NOSYMFOLLOW not set on %s\n", TMP);
+	} else {
+		if ((buf.f_flags & ST_NOSYMFOLLOW) != 0)
+			die("ST_NOSYMFOLLOW set on %s\n", TMP);
+	}
+}
+
+static void run_tests(bool nosymfollow)
+{
+	test_link_traversal(nosymfollow);
+	test_readlink();
+	test_realpath();
+	test_statfs(nosymfollow);
+}
+
+int main(int argc, char **argv)
+{
+	create_and_enter_ns();
+
+	if (mount("testing", TMP, "ramfs", 0, NULL) != 0)
+		die("mount failed: %s\n", strerror(errno));
+
+	setup_symlink();
+	run_tests(false);
+
+	if (mount("testing", TMP, "ramfs", MS_REMOUNT|MS_NOSYMFOLLOW, NULL) != 0)
+		die("remount failed: %s\n", strerror(errno));
+
+	run_tests(true);
+
+	return EXIT_SUCCESS;
+}

tools/testing/selftests/mount/run_nosymfollow.sh

@@ -0,0 +1,4 @@
+#!/bin/bash
+# SPDX-License-Identifier: GPL-2.0
+
+./nosymfollow-test