Re: [PATCH] samples: add a mountinfo program to demonstrate statmount()/listmount()

[Christian Brauner <brauner@kernel.org>]

On Tue, Nov 12, 2024 at 04:21:23PM -0500, Jeff Layton wrote:
> Add a new "mountinfo" sample userland program that demonstrates how to
> use statmount() and listmount() to get at the same info that
> /proc/pid/mountinfo provides.
> 
> The output of the program tries to mimic the mountinfo procfile
> contents. With the -p flag, it can be pointed at an arbitrary pid to
> print out info about its mount namespace. With the -r flag it will
> attempt to walk all of the namespaces under the pid's mount namespace
> and dump out mount info from all of them.
> 
> Signed-off-by: Jeff Layton <jlayton@kernel.org>
> ---
> 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 statmount()
> testcase. We can call this program, and compare its output to the
> mountinfo file.
> ---

Yes, that's great. Thanks for doing that!

>  samples/vfs/.gitignore  |   1 +
>  samples/vfs/Makefile    |   2 +-
>  samples/vfs/mountinfo.c | 271 ++++++++++++++++++++++++++++++++++++++++++++++++
>  3 files changed, 273 insertions(+), 1 deletion(-)
> 
> diff --git a/samples/vfs/.gitignore b/samples/vfs/.gitignore
> index 79212d91285bca72b0ff85f28aaccd2e803ac092..33a03cffe072fe2466c9df30ad47e9c58b0eea7c 100644
> --- a/samples/vfs/.gitignore
> +++ b/samples/vfs/.gitignore
> @@ -1,3 +1,4 @@
>  # SPDX-License-Identifier: GPL-2.0-only
>  /test-fsmount
>  /test-statx
> +/mountinfo
> diff --git a/samples/vfs/Makefile b/samples/vfs/Makefile
> index 6377a678134acf0d682151d751d2f5042dbf5e0a..fb9bb33fdc751556e806aa897f0dbd48f7e3a4d8 100644
> --- a/samples/vfs/Makefile
> +++ b/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
> diff --git a/samples/vfs/mountinfo.c b/samples/vfs/mountinfo.c
> new file mode 100644
> index 0000000000000000000000000000000000000000..7f430835dd7fb1a3d87d98cad00c619dbb5c6f70
> --- /dev/null
> +++ b/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] + 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;
> +}
> 
> ---
> base-commit: 3def14eb3d2e9d452623af64e315605150292aa8
> change-id: 20241112-statmount-77808bc02302
> 
> Best regards,
> -- 
> Jeff Layton <jlayton@kernel.org>
>