Merge patch series "fs: listmount()/statmount() fix and sample program"

Jeff Layton <jlayton@kernel.org> says:

We had some recent queries internally asking how to use the new
statmount() and listmount() interfaces. I was doing some other work in
this area, so I whipped up this tool.

My hope is that this will represent something of a "rosetta stone" for
how to translate between mountinfo and statmount(), and an example for
other people looking to use the new interfaces.

It may also be possible to use this as the basis for a listmount() and
statmount() testcase. We can call this program, and compare its output
to the mountinfo file.

The second patch adds security mount options to the existing mnt_opts in
the statmount() interface, which I think is the final missing piece
here. The alternative to doing that would be to add a new string field
for that, but I'm not sure that's worthwhile.

* patches from https://lore.kernel.org/r/20241115-statmount-v2-0-cd29aeff9cbb@kernel.org:
  fs: prepend statmount.mnt_opts string with security_sb_mnt_opts()
  samples: add a mountinfo program to demonstrate statmount()/listmount()

Link: https://lore.kernel.org/r/20241115-statmount-v2-0-cd29aeff9cbb@kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>

fs/namespace.c

@@ -5038,6 +5038,10 @@ static int statmount_mnt_opts(struct kstatmount *s, struct seq_file *seq)
 	if (sb->s_op->show_options) {
 		size_t start = seq->count;
 
+		err = security_sb_show_options(seq, sb);
+		if (err)
+			return err;
+
 		err = sb->s_op->show_options(seq, mnt->mnt_root);
 		if (err)
 			return err;

samples/vfs/.gitignore

@@ -1,3 +1,4 @@
 # SPDX-License-Identifier: GPL-2.0-only
 /test-fsmount
 /test-statx
+/mountinfo

samples/vfs/Makefile

@@ -1,4 +1,4 @@
 # SPDX-License-Identifier: GPL-2.0-only
-userprogs-always-y += test-fsmount test-statx
+userprogs-always-y += test-fsmount test-statx mountinfo
 
 userccflags += -I usr/include

samples/vfs/mountinfo.c

@@ -0,0 +1,271 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+/*
+ * Use pidfds, nsfds, listmount() and statmount() mimic the
+ * contents of /proc/self/mountinfo.
+ */
+#define _GNU_SOURCE
+#include <stdio.h>
+#include <stdint.h>
+#include <sys/ioctl.h>
+#include <sys/syscall.h>
+#include <linux/pidfd.h>
+#include <linux/mount.h>
+#include <linux/nsfs.h>
+#include <unistd.h>
+#include <alloca.h>
+#include <getopt.h>
+#include <stdlib.h>
+#include <stdbool.h>
+#include <errno.h>
+
+/* max mounts per listmount call */
+#define MAXMOUNTS		1024
+
+/* size of struct statmount (including trailing string buffer) */
+#define STATMOUNT_BUFSIZE	4096
+
+static bool ext_format;
+
+/*
+ * There are no bindings in glibc for listmount() and statmount() (yet),
+ * make our own here.
+ */
+static int statmount(uint64_t mnt_id, uint64_t mnt_ns_id, uint64_t mask,
+			    struct statmount *buf, size_t bufsize,
+			    unsigned int flags)
+{
+	struct mnt_id_req req = {
+		.size = MNT_ID_REQ_SIZE_VER0,
+		.mnt_id = mnt_id,
+		.param = mask,
+	};
+
+	if (mnt_ns_id) {
+		req.size = MNT_ID_REQ_SIZE_VER1;
+		req.mnt_ns_id = mnt_ns_id;
+	}
+
+	return syscall(__NR_statmount, &req, buf, bufsize, flags);
+}
+
+static ssize_t listmount(uint64_t mnt_id, uint64_t mnt_ns_id,
+			 uint64_t last_mnt_id, uint64_t list[], size_t num,
+			 unsigned int flags)
+{
+	struct mnt_id_req req = {
+		.size = MNT_ID_REQ_SIZE_VER0,
+		.mnt_id = mnt_id,
+		.param = last_mnt_id,
+	};
+
+	if (mnt_ns_id) {
+		req.size = MNT_ID_REQ_SIZE_VER1;
+		req.mnt_ns_id = mnt_ns_id;
+	}
+
+	return syscall(__NR_listmount, &req, list, num, flags);
+}
+
+static void show_mnt_attrs(uint64_t flags)
+{
+	printf("%s", flags & MOUNT_ATTR_RDONLY ? "ro" : "rw");
+
+	if (flags & MOUNT_ATTR_NOSUID)
+		printf(",nosuid");
+	if (flags & MOUNT_ATTR_NODEV)
+		printf(",nodev");
+	if (flags & MOUNT_ATTR_NOEXEC)
+		printf(",noexec");
+
+	switch (flags & MOUNT_ATTR__ATIME) {
+	case MOUNT_ATTR_RELATIME:
+		printf(",relatime");
+		break;
+	case MOUNT_ATTR_NOATIME:
+		printf(",noatime");
+		break;
+	case MOUNT_ATTR_STRICTATIME:
+		/* print nothing */
+		break;
+	}
+
+	if (flags & MOUNT_ATTR_NOSYMFOLLOW)
+		printf(",nosymfollow");
+	if (flags & MOUNT_ATTR_IDMAP)
+		printf(",idmapped");
+}
+
+static void show_propagation(struct statmount *sm)
+{
+	if (sm->mnt_propagation & MS_SHARED)
+		printf(" shared:%llu", sm->mnt_peer_group);
+	if (sm->mnt_propagation & MS_SLAVE) {
+		printf(" master:%llu", sm->mnt_master);
+		if (sm->mnt_master)
+			printf(" propagate_from:%llu", sm->propagate_from);
+	}
+	if (sm->mnt_propagation & MS_UNBINDABLE)
+		printf(" unbindable");
+}
+
+static void show_sb_flags(uint64_t flags)
+{
+	printf("%s", flags & MS_RDONLY ? "ro" : "rw");
+	if (flags & MS_SYNCHRONOUS)
+		printf(",sync");
+	if (flags & MS_DIRSYNC)
+		printf(",dirsync");
+	if (flags & MS_MANDLOCK)
+		printf(",mand");
+	if (flags & MS_LAZYTIME)
+		printf(",lazytime");
+}
+
+static int dump_mountinfo(uint64_t mnt_id, uint64_t mnt_ns_id)
+{
+	int ret;
+	struct statmount *buf = alloca(STATMOUNT_BUFSIZE);
+	const uint64_t mask = STATMOUNT_SB_BASIC | STATMOUNT_MNT_BASIC |
+				STATMOUNT_PROPAGATE_FROM | STATMOUNT_FS_TYPE |
+				STATMOUNT_MNT_ROOT | STATMOUNT_MNT_POINT |
+				STATMOUNT_MNT_OPTS | STATMOUNT_FS_SUBTYPE |
+				STATMOUNT_SB_SOURCE;
+
+	ret = statmount(mnt_id, mnt_ns_id, mask, buf, STATMOUNT_BUFSIZE, 0);
+	if (ret < 0) {
+		perror("statmount");
+		return 1;
+	}
+
+	if (ext_format)
+		printf("0x%lx 0x%lx 0x%llx ", mnt_ns_id, mnt_id, buf->mnt_parent_id);
+
+	printf("%u %u %u:%u %s %s ", buf->mnt_id_old, buf->mnt_parent_id_old,
+				   buf->sb_dev_major, buf->sb_dev_minor,
+				   &buf->str[buf->mnt_root],
+				   &buf->str[buf->mnt_point]);
+	show_mnt_attrs(buf->mnt_attr);
+	show_propagation(buf);
+
+	printf(" - %s", &buf->str[buf->fs_type]);
+	if (buf->mask & STATMOUNT_FS_SUBTYPE)
+		printf(".%s", &buf->str[buf->fs_subtype]);
+	if (buf->mask & STATMOUNT_SB_SOURCE)
+		printf(" %s ", &buf->str[buf->sb_source]);
+	else
+		printf(" :none ");
+
+	show_sb_flags(buf->sb_flags);
+	if (buf->mask & STATMOUNT_MNT_OPTS)
+		printf(",%s", &buf->str[buf->mnt_opts]);
+	printf("\n");
+	return 0;
+}
+
+static int dump_mounts(uint64_t mnt_ns_id)
+{
+	uint64_t mntid[MAXMOUNTS];
+	uint64_t last_mnt_id = 0;
+	ssize_t count;
+	int i;
+
+	/*
+	 * Get a list of all mntids in mnt_ns_id. If it returns MAXMOUNTS
+	 * mounts, then go again until we get everything.
+	 */
+	do {
+		count = listmount(LSMT_ROOT, mnt_ns_id, last_mnt_id, mntid, MAXMOUNTS, 0);
+		if (count < 0 || count > MAXMOUNTS) {
+			errno = count < 0 ? errno : count;
+			perror("listmount");
+			return 1;
+		}
+
+		/* Walk the returned mntids and print info about each */
+		for (i = 0; i < count; ++i) {
+			int ret = dump_mountinfo(mntid[i], mnt_ns_id);
+
+			if (ret != 0)
+				return ret;
+		}
+		/* Set up last_mnt_id to pick up where we left off */
+		last_mnt_id = mntid[count - 1];
+	} while (count == MAXMOUNTS);
+	return 0;
+}
+
+static void usage(const char * const prog)
+{
+	printf("Usage:\n");
+	printf("%s [-e] [-p pid] [-r] [-h]\n", prog);
+	printf("    -e: extended format\n");
+	printf("    -h: print usage message\n");
+	printf("    -p: get mount namespace from given pid\n");
+	printf("    -r: recursively print all mounts in all child namespaces\n");
+}
+
+int main(int argc, char * const *argv)
+{
+	struct mnt_ns_info mni = { .size = MNT_NS_INFO_SIZE_VER0 };
+	int pidfd, mntns, ret, opt;
+	pid_t pid = getpid();
+	bool recursive = false;
+
+	while ((opt = getopt(argc, argv, "ehp:r")) != -1) {
+		switch (opt) {
+		case 'e':
+			ext_format = true;
+			break;
+		case 'h':
+			usage(argv[0]);
+			return 0;
+		case 'p':
+			pid = atoi(optarg);
+			break;
+		case 'r':
+			recursive = true;
+			break;
+		}
+	}
+
+	/* Get a pidfd for pid */
+	pidfd = syscall(SYS_pidfd_open, pid, 0);
+	if (pidfd < 0) {
+		perror("pidfd_open");
+		return 1;
+	}
+
+	/* Get the mnt namespace for pidfd */
+	mntns = ioctl(pidfd, PIDFD_GET_MNT_NAMESPACE, NULL);
+	if (mntns < 0) {
+		perror("PIDFD_GET_MNT_NAMESPACE");
+		return 1;
+	}
+	close(pidfd);
+
+	/* get info about mntns. In particular, the mnt_ns_id */
+	ret = ioctl(mntns, NS_MNT_GET_INFO, &mni);
+	if (ret < 0) {
+		perror("NS_MNT_GET_INFO");
+		return 1;
+	}
+
+	do {
+		int ret;
+
+		ret = dump_mounts(mni.mnt_ns_id);
+		if (ret)
+			return ret;
+
+		if (!recursive)
+			break;
+
+		/* get the next mntns (and overwrite the old mount ns info) */
+		ret = ioctl(mntns, NS_MNT_GET_NEXT, &mni);
+		close(mntns);
+		mntns = ret;
+	} while (mntns >= 0);
+
+	return 0;
+}