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Message-ID: <9b0f44041794d2e0bc5c39263aba8b59441a8704.camel@kernel.org>
Subject: Re: [PATCH RFC 0/5] nsfs: iterate through mount namespaces
From: Jeff Layton <jlayton@kernel.org>
To: Christian Brauner <brauner@kernel.org>, linux-fsdevel@vger.kernel.org
Cc: Josef Bacik <josef@toxicpanda.com>, Karel Zak <kzak@redhat.com>, 
 Stephane Graber <stgraber@stgraber.org>, Alexander Mikhalitsyn
 <alexander@mihalicyn.com>
Date: Mon, 22 Jul 2024 10:42:40 -0400
In-Reply-To: <20240719-work-mount-namespace-v1-0-834113cab0d2@kernel.org>
References: <20240719-work-mount-namespace-v1-0-834113cab0d2@kernel.org>
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On Fri, 2024-07-19 at 13:41 +0200, Christian Brauner wrote:
> Hey,
>=20
> Recently, we added the ability to list mounts in other mount
> namespaces
> and the ability to retrieve namespace file descriptors without having
> to
> go through procfs by deriving them from pidfds.
>=20
> This extends nsfs in two ways:
>=20
> (1) Add the ability to retrieve information about a mount namespace
> via
> =C2=A0=C2=A0=C2=A0 NS_MNT_GET_INFO. This will return the mount namespace =
id and the
> =C2=A0=C2=A0=C2=A0 number of mounts currently in the mount namespace. The=
 number of
> =C2=A0=C2=A0=C2=A0 mounts can be used to size the buffer that needs to be=
 used for
> =C2=A0=C2=A0=C2=A0 listmount() and is in general useful without having to=
 actually
> =C2=A0=C2=A0=C2=A0 iterate through all the mounts.
>=20
> =C2=A0=C2=A0=C2=A0 The structure is extensible.
>=20
> (2) Add the ability to iterate through all mount namespaces over
> which
> =C2=A0=C2=A0=C2=A0 the caller holds privilege returning the file descript=
or for the
> =C2=A0=C2=A0=C2=A0 next or previous mount namespace.
>=20
> =C2=A0=C2=A0=C2=A0 To retrieve a mount namespace the caller must be privi=
leged wrt
> to
> =C2=A0=C2=A0=C2=A0 it's owning user namespace. This means that PID 1 on t=
he host can
> =C2=A0=C2=A0=C2=A0 list all mounts in all mount namespaces or that a cont=
ainer can
> list
> =C2=A0=C2=A0=C2=A0 all mounts of its nested containers.
>=20
> =C2=A0=C2=A0=C2=A0 Optionally pass a structure for NS_MNT_GET_INFO with
> =C2=A0=C2=A0=C2=A0 NS_MNT_GET_{PREV,NEXT} to retrieve information about t=
he mount
> =C2=A0=C2=A0=C2=A0 namespace in one go.
>=20
> (1) and (2) can be implemented for other namespace types easily.
>=20
> Together with recent api additions this means one can iterate through
> all mounts in all mount namespaces without ever touching procfs.
> Here's
> a sample program list_all_mounts_everywhere.c:
>=20
> =C2=A0 // SPDX-License-Identifier: GPL-2.0-or-later
>=20
> =C2=A0 #define _GNU_SOURCE
> =C2=A0 #include <asm/unistd.h>
> =C2=A0 #include <assert.h>
> =C2=A0 #include <errno.h>
> =C2=A0 #include <fcntl.h>
> =C2=A0 #include <getopt.h>
> =C2=A0 #include <linux/stat.h>
> =C2=A0 #include <sched.h>
> =C2=A0 #include <stddef.h>
> =C2=A0 #include <stdint.h>
> =C2=A0 #include <stdio.h>
> =C2=A0 #include <stdlib.h>
> =C2=A0 #include <string.h>
> =C2=A0 #include <sys/ioctl.h>
> =C2=A0 #include <sys/param.h>
> =C2=A0 #include <sys/pidfd.h>
> =C2=A0 #include <sys/stat.h>
> =C2=A0 #include <sys/statfs.h>
>=20
> =C2=A0 #define die_errno(format,
> ...)=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=
=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=
=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 \
> =C2=A0=C2=A0	do
> {=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=
=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=
=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
 \
> =C2=A0=C2=A0		fprintf(stderr, "%m | %s: %d: %s: " format "\n",
> __FILE__, \
> =C2=A0=C2=A0			__LINE__, __func__,
> ##__VA_ARGS__);=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=
=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 \
> =C2=A0=C2=A0		exit(EXIT_FAILURE);=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=
=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=
=A0=C2=A0
> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 \
> =C2=A0=C2=A0	} while (0)
>=20
> =C2=A0 /* Get the id for a mount namespace */
> =C2=A0 #define NS_GET_MNTNS_ID		_IO(0xb7, 0x5)
> =C2=A0 /* Get next mount namespace. */
>=20
> =C2=A0 struct mnt_ns_info {
> =C2=A0=C2=A0	__u32 size;
> =C2=A0=C2=A0	__u32 nr_mounts;
> =C2=A0=C2=A0	__u64 mnt_ns_id;
> =C2=A0 };
>=20
> =C2=A0 #define MNT_NS_INFO_SIZE_VER0 16 /* size of first published struct
> */
>=20
> =C2=A0 /* Get information about namespace. */
> =C2=A0 #define NS_MNT_GET_INFO		_IOR(0xb7, 10, struct
> mnt_ns_info)
> =C2=A0 /* Get next namespace. */
> =C2=A0 #define NS_MNT_GET_NEXT		_IOR(0xb7, 11, struct
> mnt_ns_info)
> =C2=A0 /* Get previous namespace. */
> =C2=A0 #define NS_MNT_GET_PREV		_IOR(0xb7, 12, struct
> mnt_ns_info)
>=20
> =C2=A0 #define PIDFD_GET_MNT_NAMESPACE _IO(0xFF, 3)
>=20
> =C2=A0 #define STATX_MNT_ID_UNIQUE	0x00004000U	/* Want/got extended
> stx_mount_id */
>=20
> =C2=A0 #define __NR_listmount 458
> =C2=A0 #define __NR_statmount 457
>=20
> =C2=A0 /*
> =C2=A0=C2=A0 * @mask bits for statmount(2)
> =C2=A0=C2=A0 */
> =C2=A0 #define STATMOUNT_SB_BASIC		0x00000001U=C2=A0=C2=A0=C2=A0=C2=A0 /*=
 Want/got
> sb_... */
> =C2=A0 #define STATMOUNT_MNT_BASIC		0x00000002U	/* Want/got
> mnt_... */
> =C2=A0 #define STATMOUNT_PROPAGATE_FROM	0x00000004U	/* Want/got
> propagate_from */
> =C2=A0 #define STATMOUNT_MNT_ROOT		0x00000008U	/* Want/got
> mnt_root=C2=A0 */
> =C2=A0 #define STATMOUNT_MNT_POINT		0x00000010U	/* Want/got
> mnt_point */
> =C2=A0 #define STATMOUNT_FS_TYPE		0x00000020U	/* Want/got
> fs_type */
> =C2=A0 #define STATMOUNT_MNT_NS_ID=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=
=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 0x00000040U=C2=A0=C2=A0=C2=A0=C2=A0 /* Wa=
nt/got
> mnt_ns_id */
> =C2=A0 #define STATMOUNT_MNT_OPTS=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=
=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 0x00000080U=C2=A0=C2=A0=C2=A0=C2=A0=
 /* Want/got
> mnt_opts */
>=20
> =C2=A0 struct statmount {
> =C2=A0=C2=A0	__u32 size;		/* Total size, including strings */
> =C2=A0=C2=A0	__u32 mnt_opts;
> =C2=A0=C2=A0	__u64 mask;		/* What results were written */
> =C2=A0=C2=A0	__u32 sb_dev_major;	/* Device ID */
> =C2=A0=C2=A0	__u32 sb_dev_minor;
> =C2=A0=C2=A0	__u64 sb_magic;		/* ..._SUPER_MAGIC */
> =C2=A0=C2=A0	__u32 sb_flags;		/*
> SB_{RDONLY,SYNCHRONOUS,DIRSYNC,LAZYTIME} */
> =C2=A0=C2=A0	__u32 fs_type;		/* [str] Filesystem type */
> =C2=A0=C2=A0	__u64 mnt_id;		/* Unique ID of mount */
> =C2=A0=C2=A0	__u64 mnt_parent_id;	/* Unique ID of parent (for root =3D=3D
> mnt_id) */
> =C2=A0=C2=A0	__u32 mnt_id_old;	/* Reused IDs used in
> proc/.../mountinfo */
> =C2=A0=C2=A0	__u32 mnt_parent_id_old;
> =C2=A0=C2=A0	__u64 mnt_attr;		/* MOUNT_ATTR_... */
> =C2=A0=C2=A0	__u64 mnt_propagation;	/*
> MS_{SHARED,SLAVE,PRIVATE,UNBINDABLE} */
> =C2=A0=C2=A0	__u64 mnt_peer_group;	/* ID of shared peer group */
> =C2=A0=C2=A0	__u64 mnt_master;	/* Mount receives propagation from
> this ID */
> =C2=A0=C2=A0	__u64 propagate_from;	/* Propagation from in current
> namespace */
> =C2=A0=C2=A0	__u32 mnt_root;		/* [str] Root of mount
> relative to root of fs */
> =C2=A0=C2=A0	__u32 mnt_point;	/* [str] Mountpoint relative to
> current root */
> =C2=A0=C2=A0	__u64 mnt_ns_id;
> =C2=A0=C2=A0	__u64 __spare2[49];
> =C2=A0=C2=A0	char str[];		/* Variable size part containing
> strings */
> =C2=A0 };
>=20
> =C2=A0 struct mnt_id_req {
> =C2=A0=C2=A0	__u32 size;
> =C2=A0=C2=A0	__u32 spare;
> =C2=A0=C2=A0	__u64 mnt_id;
> =C2=A0=C2=A0	__u64 param;
> =C2=A0=C2=A0	__u64 mnt_ns_id;
> =C2=A0 };
>=20
> =C2=A0 #define MNT_ID_REQ_SIZE_VER1	32 /* sizeof second published struct
> */
>=20
> =C2=A0 #define LSMT_ROOT		0xffffffffffffffff	/* root
> mount */
>=20
> =C2=A0 static int __statmount(__u64 mnt_id, __u64 mnt_ns_id, __u64 mask,
> =C2=A0=C2=A0		=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 struct statmount *stmn=
t, size_t bufsize,
> unsigned int flags)
> =C2=A0 {
> =C2=A0=C2=A0	struct mnt_id_req req =3D {
> =C2=A0=C2=A0		.size =3D MNT_ID_REQ_SIZE_VER1,
> =C2=A0=C2=A0		.mnt_id =3D mnt_id,
> =C2=A0=C2=A0		.param =3D mask,
> =C2=A0=C2=A0		.mnt_ns_id =3D mnt_ns_id,
> =C2=A0=C2=A0	};
>=20
> =C2=A0=C2=A0	return syscall(__NR_statmount, &req, stmnt, bufsize, flags);
> =C2=A0 }
>=20
> =C2=A0 static struct statmount *sys_statmount(__u64 mnt_id, __u64
> mnt_ns_id,
> =C2=A0=C2=A0				=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 __u64 mask, unsigned=
 int
> flags)
> =C2=A0 {
> =C2=A0=C2=A0	size_t bufsize =3D 1 << 15;
> =C2=A0=C2=A0	struct statmount *stmnt =3D NULL, *tmp =3D NULL;
> =C2=A0=C2=A0	int ret;
>=20
> =C2=A0=C2=A0	for (;;) {
> =C2=A0=C2=A0		tmp =3D realloc(stmnt, bufsize);
> =C2=A0=C2=A0		if (!tmp)
> =C2=A0=C2=A0			goto out;
>=20
> =C2=A0=C2=A0		stmnt =3D tmp;
> =C2=A0=C2=A0		ret =3D __statmount(mnt_id, mnt_ns_id, mask, stmnt,
> bufsize, flags);
> =C2=A0=C2=A0		if (!ret)
> =C2=A0=C2=A0			return stmnt;
>=20
> =C2=A0=C2=A0		if (errno !=3D EOVERFLOW)
> =C2=A0=C2=A0			goto out;
>=20
> =C2=A0=C2=A0		bufsize <<=3D 1;
> =C2=A0=C2=A0		if (bufsize >=3D UINT_MAX / 2)
> =C2=A0=C2=A0			goto out;
>=20
> =C2=A0=C2=A0	}
>=20
> =C2=A0 out:
> =C2=A0=C2=A0	free(stmnt);
> =C2=A0=C2=A0	printf("statmount failed");
> =C2=A0=C2=A0	return NULL;
> =C2=A0 }
>=20
> =C2=A0 static ssize_t sys_listmount(__u64 mnt_id, __u64 last_mnt_id, __u6=
4
> mnt_ns_id,
> =C2=A0=C2=A0			=C2=A0=C2=A0=C2=A0=C2=A0 __u64 list[], size_t num, unsigne=
d int
> flags)
> =C2=A0 {
> =C2=A0=C2=A0	struct mnt_id_req req =3D {
> =C2=A0=C2=A0		.size =3D MNT_ID_REQ_SIZE_VER1,
> =C2=A0=C2=A0		.mnt_id =3D mnt_id,
> =C2=A0=C2=A0		.param =3D last_mnt_id,
> =C2=A0=C2=A0		.mnt_ns_id =3D mnt_ns_id,
> =C2=A0=C2=A0	};
>=20
> =C2=A0=C2=A0	return syscall(__NR_listmount, &req, list, num, flags);
> =C2=A0 }
>=20
> =C2=A0 int main(int argc, char *argv[])
> =C2=A0 {
> =C2=A0 #define LISTMNT_BUFFER 10
> =C2=A0=C2=A0	__u64 list[LISTMNT_BUFFER], last_mnt_id =3D 0;
> =C2=A0=C2=A0	int ret, pidfd, fd_mntns;
> =C2=A0=C2=A0	struct mnt_ns_info info =3D {};
>=20
> =C2=A0=C2=A0	pidfd =3D pidfd_open(getpid(), 0);
> =C2=A0=C2=A0	if (pidfd < 0)
> =C2=A0=C2=A0		die_errno("pidfd_open failed");
>=20
> =C2=A0=C2=A0	fd_mntns =3D ioctl(pidfd, PIDFD_GET_MNT_NAMESPACE, 0);
> =C2=A0=C2=A0	if (fd_mntns < 0)
> =C2=A0=C2=A0		die_errno("ioctl(PIDFD_GET_MNT_NAMESPACE) failed");
>=20
> =C2=A0=C2=A0	ret =3D ioctl(fd_mntns, NS_MNT_GET_INFO, &info);
> =C2=A0=C2=A0	if (ret < 0)
> =C2=A0=C2=A0		die_errno("ioctl(NS_GET_MNTNS_ID) failed");
>=20
> =C2=A0=C2=A0	printf("Listing %u mounts for mount namespace %d:%llu\n",
> info.nr_mounts, fd_mntns, info.mnt_ns_id);
> =C2=A0=C2=A0	for (;;) {
> =C2=A0=C2=A0		ssize_t nr_mounts;
> =C2=A0=C2=A0	next:
> =C2=A0=C2=A0		nr_mounts =3D sys_listmount(LSMT_ROOT, last_mnt_id,
> info.mnt_ns_id, list, LISTMNT_BUFFER, 0);
> =C2=A0=C2=A0		if (nr_mounts <=3D 0) {
> =C2=A0=C2=A0			printf("Finished listing mounts for mount
> namespace %d:%llu\n\n", fd_mntns, info.mnt_ns_id);
> =C2=A0=C2=A0			ret =3D ioctl(fd_mntns, NS_MNT_GET_NEXT, 0);
> =C2=A0=C2=A0			if (ret < 0)
> =C2=A0=C2=A0				die_errno("ioctl(NS_MNT_GET_NEXT)
> failed");
> =C2=A0=C2=A0			close(ret);
> =C2=A0=C2=A0			ret =3D ioctl(fd_mntns, NS_MNT_GET_NEXT,
> &info);
> =C2=A0=C2=A0			if (ret < 0) {
> =C2=A0=C2=A0				if (errno =3D=3D ENOENT) {
> =C2=A0=C2=A0					printf("Finished listing all
> mount namespaces\n");
> =C2=A0=C2=A0					exit(0);
> =C2=A0=C2=A0				}
> =C2=A0=C2=A0				die_errno("ioctl(NS_MNT_GET_NEXT)
> failed");
> =C2=A0=C2=A0			}
> =C2=A0=C2=A0			close(fd_mntns);
> =C2=A0=C2=A0			fd_mntns =3D ret;
> =C2=A0=C2=A0			last_mnt_id =3D 0;
> =C2=A0=C2=A0			printf("Listing %u mounts for mount
> namespace %d:%llu\n", info.nr_mounts, fd_mntns, info.mnt_ns_id);
> =C2=A0=C2=A0			goto next;
> =C2=A0=C2=A0		}
>=20
> =C2=A0=C2=A0		for (size_t cur =3D 0; cur < nr_mounts; cur++) {
> =C2=A0=C2=A0			struct statmount *stmnt;
>=20
> =C2=A0=C2=A0			last_mnt_id =3D list[cur];
>=20
> =C2=A0=C2=A0			stmnt =3D sys_statmount(last_mnt_id,
> info.mnt_ns_id,
> =C2=A0=C2=A0					=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 STATMOUNT_SB_BASIC |
> =C2=A0=C2=A0					=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 STATMOUNT_MNT_BASIC |
> =C2=A0=C2=A0					=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 STATMOUNT_MNT_ROOT |
> =C2=A0=C2=A0					=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 STATMOUNT_MNT_POINT |
> =C2=A0=C2=A0					=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 STATMOUNT_MNT_NS_ID |
> =C2=A0=C2=A0					=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 STATMOUNT_MNT_OPTS |
> =C2=A0=C2=A0					=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 STATMOUNT_FS_TYPE,
> =C2=A0=C2=A0					=C2=A0 0);
> =C2=A0=C2=A0			if (!stmnt) {
> =C2=A0=C2=A0				printf("Failed to statmount(%llu) in
> mount namespace(%llu)\n", last_mnt_id, info.mnt_ns_id);
> =C2=A0=C2=A0				continue;
> =C2=A0=C2=A0			}
>=20
> =C2=A0=C2=A0			printf("mnt_id(%u/%llu) |
> mnt_parent_id(%u/%llu): %s @ %s =3D=3D> %s with options: %s\n",
> =C2=A0=C2=A0			=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 stmnt->mnt_id_old, st=
mnt->mnt_id,
> =C2=A0=C2=A0			=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 stmnt->mnt_parent_id_=
old, stmnt-
> >mnt_parent_id,
> =C2=A0=C2=A0			=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 stmnt->str + stmnt->f=
s_type,
> =C2=A0=C2=A0			=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 stmnt->str + stmnt->m=
nt_root,
> =C2=A0=C2=A0			=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 stmnt->str + stmnt->m=
nt_point,
> =C2=A0=C2=A0			=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 stmnt->str + stmnt->m=
nt_opts);
> =C2=A0=C2=A0			free(stmnt);
> =C2=A0=C2=A0		}
> =C2=A0=C2=A0	}
>=20
> =C2=A0=C2=A0	exit(0);
> =C2=A0 }
>=20
> Thanks!
> Christian
>=20
> Signed-off-by: Christian Brauner <brauner@kernel.org>
> ---
> ---
> base-commit: 720261cfc7329406a50c2a8536e0039b9dd9a4e5
> change-id: 20240705-work-mount-namespace-126b73a11f5c
>=20

This all looks pretty straightforward to me. I do wish that we had
proper libc bindings for this...or maybe even a new userland library?

I just get the feeling that all of this syscall() and ioctl() usage is
eventually going to bite us in the ass. I don't have any concrete
proposal for that however, and we do have some immediate need for this
functionality, so, you can add

Reviewed-by: Jeff Layton <jlayton@kernel.org>