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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | STANDARDS | HISTORY | NOTES | EXAMPLES | SEE ALSO |
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mount_setattr(2) System Calls Manual mount_setattr(2)
mount_setattr - change properties of a mount or mount tree
Standard C library (libc, -lc)
#include <linux/fcntl.h> /* Definition of AT_* constants */
#include <linux/mount.h> /* Definition of MOUNT_ATTR_* constants */
#include <sys/syscall.h> /* Definition of SYS_* constants */
#include <unistd.h>
int syscall(SYS_mount_setattr, int dirfd, const char *pathname,
unsigned int flags, struct mount_attr *attr, size_t size);
Note: glibc provides no wrapper for mount_setattr(),
necessitating the use of syscall(2).
The mount_setattr() system call changes the mount properties of a
mount or an entire mount tree. If pathname is a relative
pathname, then it is interpreted relative to the directory
referred to by the file descriptor dirfd. If dirfd is the
special value AT_FDCWD, then pathname is interpreted relative to
the current working directory of the calling process. If
pathname is the empty string and AT_EMPTY_PATH is specified in
flags, then the mount properties of the mount identified by dirfd
are changed. (See openat(2) for an explanation of why the dirfd
argument is useful.)
The mount_setattr() system call uses an extensible structure
(struct mount_attr) to allow for future extensions. Any non-flag
extensions to mount_setattr() will be implemented as new fields
appended to the this structure, with a zero value in a new field
resulting in the kernel behaving as though that extension field
was not present. Therefore, the caller must zero-fill this
structure on initialization. See the "Extensibility" subsection
under NOTES for more details.
The size argument should usually be specified as sizeof(struct
mount_attr). However, if the caller is using a kernel that
supports an extended struct mount_attr, but the caller does not
intend to make use of these features, it is possible to pass the
size of an earlier version of the structure together with the
extended structure. This allows the kernel to not copy later
parts of the structure that aren't used anyway. With each
extension that changes the size of struct mount_attr, the kernel
will expose a definition of the form MOUNT_ATTR_SIZE_VERnumber.
For example, the macro for the size of the initial version of
struct mount_attr is MOUNT_ATTR_SIZE_VER0.
The flags argument can be used to alter the pathname resolution
behavior. The supported values are:
AT_EMPTY_PATH
If pathname is the empty string, change the mount
properties on dirfd itself.
AT_RECURSIVE
Change the mount properties of the entire mount tree.
AT_SYMLINK_NOFOLLOW
Don't follow trailing symbolic links.
AT_NO_AUTOMOUNT
Don't trigger automounts.
The attr argument of mount_setattr() is a structure of the
following form:
struct mount_attr {
__u64 attr_set; /* Mount properties to set */
__u64 attr_clr; /* Mount properties to clear */
__u64 propagation; /* Mount propagation type */
__u64 userns_fd; /* User namespace file descriptor */
};
The attr_set and attr_clr members are used to specify the mount
properties that are supposed to be set or cleared for a mount or
mount tree. Flags set in attr_set enable a property on a mount
or mount tree, and flags set in attr_clr remove a property from a
mount or mount tree.
When changing mount properties, the kernel will first clear the
flags specified in the attr_clr field, and then set the flags
specified in the attr_set field. For example, these settings:
struct mount_attr attr = {
.attr_clr = MOUNT_ATTR_NOEXEC | MOUNT_ATTR_NODEV,
.attr_set = MOUNT_ATTR_RDONLY | MOUNT_ATTR_NOSUID,
};
are equivalent to the following steps:
unsigned int current_mnt_flags = mnt->mnt_flags;
/*
* Clear all flags set in .attr_clr,
* clearing MOUNT_ATTR_NOEXEC and MOUNT_ATTR_NODEV.
*/
current_mnt_flags &= ~attr->attr_clr;
/*
* Now set all flags set in .attr_set,
* applying MOUNT_ATTR_RDONLY and MOUNT_ATTR_NOSUID.
*/
current_mnt_flags |= attr->attr_set;
mnt->mnt_flags = current_mnt_flags;
As a result of this change, the mount or mount tree (a) is read-
only; (b) blocks the execution of set-user-ID and set-group-ID
programs; (c) allows execution of programs; and (d) allows access
to devices.
Multiple changes with the same set of flags requested in attr_clr
and attr_set are guaranteed to be idempotent after the changes
have been applied.
The following mount attributes can be specified in the attr_set
or attr_clr fields:
MOUNT_ATTR_RDONLY
If set in attr_set, makes the mount read-only. If set in
attr_clr, removes the read-only setting if set on the
mount.
MOUNT_ATTR_NOSUID
If set in attr_set, causes the mount not to honor the set-
user-ID and set-group-ID mode bits and file capabilities
when executing programs. If set in attr_clr, clears the
set-user-ID, set-group-ID, and file capability restriction
if set on this mount.
MOUNT_ATTR_NODEV
If set in attr_set, prevents access to devices on this
mount. If set in attr_clr, removes the restriction that
prevented accessing devices on this mount.
MOUNT_ATTR_NOEXEC
If set in attr_set, prevents executing programs on this
mount. If set in attr_clr, removes the restriction that
prevented executing programs on this mount.
MOUNT_ATTR_NOSYMFOLLOW
If set in attr_set, prevents following symbolic links on
this mount. If set in attr_clr, removes the restriction
that prevented following symbolic links on this mount.
MOUNT_ATTR_NODIRATIME
If set in attr_set, prevents updating access time for
directories on this mount. If set in attr_clr, removes
the restriction that prevented updating access time for
directories. Note that MOUNT_ATTR_NODIRATIME can be
combined with other access-time settings and is implied by
the noatime setting. All other access-time settings are
mutually exclusive.
MOUNT_ATTR__ATIME - changing access-time settings
The access-time values listed below are an enumeration
that includes the value zero, expressed in the bits
defined by the mask MOUNT_ATTR__ATIME. Even though these
bits are an enumeration (in contrast to the other mount
flags such as MOUNT_ATTR_NOEXEC), they are nonetheless
passed in attr_set and attr_clr for consistency with
fsmount(2), which introduced this behavior.
Note that, since the access-time values are an enumeration
rather than bit values, a caller wanting to transition to
a different access-time setting cannot simply specify the
access-time setting in attr_set, but must also include
MOUNT_ATTR__ATIME in the attr_clr field. The kernel will
verify that MOUNT_ATTR__ATIME isn't partially set in
attr_clr (i.e., either all bits in the MOUNT_ATTR__ATIME
bit field are either set or clear), and that attr_set
doesn't have any access-time bits set if MOUNT_ATTR__ATIME
isn't set in attr_clr.
MOUNT_ATTR_RELATIME
When a file is accessed via this mount, update the
file's last access time (atime) only if the current
value of atime is less than or equal to the file's
last modification time (mtime) or last status
change time (ctime).
To enable this access-time setting on a mount or
mount tree, MOUNT_ATTR_RELATIME must be set in
attr_set and MOUNT_ATTR__ATIME must be set in the
attr_clr field.
MOUNT_ATTR_NOATIME
Do not update access times for (all types of) files
on this mount.
To enable this access-time setting on a mount or
mount tree, MOUNT_ATTR_NOATIME must be set in
attr_set and MOUNT_ATTR__ATIME must be set in the
attr_clr field.
MOUNT_ATTR_STRICTATIME
Always update the last access time (atime) when
files are accessed on this mount.
To enable this access-time setting on a mount or
mount tree, MOUNT_ATTR_STRICTATIME must be set in
attr_set and MOUNT_ATTR__ATIME must be set in the
attr_clr field.
MOUNT_ATTR_IDMAP
If set in attr_set, creates an ID-mapped mount. The ID
mapping is taken from the user namespace specified in
userns_fd and attached to the mount.
Since it is not supported to change the ID mapping of a
mount after it has been ID mapped, it is invalid to
specify MOUNT_ATTR_IDMAP in attr_clr.
For further details, see the subsection "ID-mapped mounts"
under NOTES.
The propagation field is used to specify the propagation type of
the mount or mount tree. This field either has the value zero,
meaning leave the propagation type unchanged, or it has one of
the following values:
MS_PRIVATE
Turn all mounts into private mounts.
MS_SHARED
Turn all mounts into shared mounts.
MS_SLAVE
Turn all mounts into dependent mounts.
MS_UNBINDABLE
Turn all mounts into unbindable mounts.
For further details on the above propagation types, see
mount_namespaces(7).
On success, mount_setattr() returns zero. On error, -1 is
returned and errno is set to indicate the cause of the error.
EBADF pathname is relative but dirfd is neither AT_FDCWD nor a
valid file descriptor.
EBADF userns_fd is not a valid file descriptor.
EBUSY The caller tried to change the mount to MOUNT_ATTR_RDONLY,
but the mount still holds files open for writing.
EBUSY The caller tried to create an ID-mapped mount raising
MOUNT_ATTR_IDMAP and specifying userns_fd but the mount
still holds files open for writing.
EINVAL The pathname specified via the dirfd and pathname
arguments to mount_setattr() isn't a mount point.
EINVAL An unsupported value was set in flags.
EINVAL An unsupported value was specified in the attr_set field
of mount_attr.
EINVAL An unsupported value was specified in the attr_clr field
of mount_attr.
EINVAL An unsupported value was specified in the propagation
field of mount_attr.
EINVAL More than one of MS_SHARED, MS_SLAVE, MS_PRIVATE, or
MS_UNBINDABLE was set in the propagation field of
mount_attr.
EINVAL An access-time setting was specified in the attr_set field
without MOUNT_ATTR__ATIME being set in the attr_clr field.
EINVAL MOUNT_ATTR_IDMAP was specified in attr_clr.
EINVAL A file descriptor value was specified in userns_fd which
exceeds INT_MAX.
EINVAL A valid file descriptor value was specified in userns_fd,
but the file descriptor did not refer to a user namespace.
EINVAL The underlying filesystem does not support ID-mapped
mounts.
EINVAL The mount that is to be ID mapped is not a detached mount;
that is, the mount has not previously been visible in a
mount namespace.
EINVAL A partial access-time setting was specified in attr_clr
instead of MOUNT_ATTR__ATIME being set.
EINVAL The mount is located outside the caller's mount namespace.
EINVAL The underlying filesystem has been mounted in a mount
namespace that is owned by a noninitial user namespace
ENOENT A pathname was empty or had a nonexistent component.
ENOMEM When changing mount propagation to MS_SHARED, a new peer
group ID needs to be allocated for all mounts without a
peer group ID set. This allocation failed because there
was not enough memory to allocate the relevant internal
structures.
ENOSPC When changing mount propagation to MS_SHARED, a new peer
group ID needs to be allocated for all mounts without a
peer group ID set. This allocation failed because the
kernel has run out of IDs.
EPERM One of the mounts had at least one of MOUNT_ATTR_NOATIME,
MOUNT_ATTR_NODEV, MOUNT_ATTR_NODIRATIME,
MOUNT_ATTR_NOEXEC, MOUNT_ATTR_NOSUID, or MOUNT_ATTR_RDONLY
set and the flag is locked. Mount attributes become
locked on a mount if:
• A new mount or mount tree is created causing mount
propagation across user namespaces (i.e., propagation
to a mount namespace owned by a different user
namespace). The kernel will lock the aforementioned
flags to prevent these sensitive properties from being
altered.
• A new mount and user namespace pair is created. This
happens for example when specifying CLONE_NEWUSER |
CLONE_NEWNS in unshare(2), clone(2), or clone3(2). The
aforementioned flags become locked in the new mount
namespace to prevent sensitive mount properties from
being altered. Since the newly created mount namespace
will be owned by the newly created user namespace, a
calling process that is privileged in the new user
namespace would—in the absence of such locking—be able
to alter sensitive mount properties (e.g., to remount a
mount that was marked read-only as read-write in the
new mount namespace).
EPERM A valid file descriptor value was specified in userns_fd,
but the file descriptor refers to the initial user
namespace.
EPERM An attempt was made to add an ID mapping to a mount that
is already ID mapped.
EPERM The caller does not have CAP_SYS_ADMIN in the initial user
namespace.
Linux.
Linux 5.12.
ID-mapped mounts
Creating an ID-mapped mount makes it possible to change the
ownership of all files located under a mount. Thus, ID-mapped
mounts make it possible to change ownership in a temporary and
localized way. It is a localized change because the ownership
changes are visible only via a specific mount. All other users
and locations where the filesystem is exposed are unaffected. It
is a temporary change because the ownership changes are tied to
the lifetime of the mount.
Whenever callers interact with the filesystem through an ID-
mapped mount, the ID mapping of the mount will be applied to user
and group IDs associated with filesystem objects. This
encompasses the user and group IDs associated with inodes and
also the following xattr(7) keys:
• security.capability, whenever filesystem capabilities are
stored or returned in the VFS_CAP_REVISION_3 format, which
stores a root user ID alongside the capabilities (see
capabilities(7)).
• system.posix_acl_access and system.posix_acl_default, whenever
user IDs or group IDs are stored in ACL_USER or ACL_GROUP
entries.
The following conditions must be met in order to create an ID-
mapped mount:
• The caller must have the CAP_SYS_ADMIN capability in the user
namespace the filesystem was mounted in.
• The underlying filesystem must support ID-mapped mounts.
Currently, the following filesystems support ID-mapped mounts:
• xfs(5) (since Linux 5.12)
• ext4(5) (since Linux 5.12)
• FAT (since Linux 5.12)
• btrfs(5) (since Linux 5.15)
• ntfs3 (since Linux 5.15)
• f2fs (since Linux 5.18)
• erofs (since Linux 5.19)
• overlayfs (ID-mapped lower and upper layers supported since
Linux 5.19)
• The mount must not already be ID-mapped. This also implies
that the ID mapping of a mount cannot be altered.
• The mount must not have any writers.
• The mount must be a detached mount; that is, it must have been
created by calling open_tree(2) with the OPEN_TREE_CLONE flag
and it must not already have been visible in a mount
namespace. (To put things another way: the mount must not
have been attached to the filesystem hierarchy with a system
call such as move_mount(2).)
ID mappings can be created for user IDs, group IDs, and project
IDs. An ID mapping is essentially a mapping of a range of user
or group IDs into another or the same range of user or group IDs.
ID mappings are written to map files as three numbers separated
by white space. The first two numbers specify the starting user
or group ID in each of the two user namespaces. The third number
specifies the range of the ID mapping. For example, a mapping
for user IDs such as "1000 1001 1" would indicate that user ID
1000 in the caller's user namespace is mapped to user ID 1001 in
its ancestor user namespace. Since the map range is 1, only user
ID 1000 is mapped.
It is possible to specify up to 340 ID mappings for each ID
mapping type. If any user IDs or group IDs are not mapped, all
files owned by that unmapped user or group ID will appear as
being owned by the overflow user ID or overflow group ID
respectively.
Further details on setting up ID mappings can be found in
user_namespaces(7).
In the common case, the user namespace passed in userns_fd
(together with MOUNT_ATTR_IDMAP in attr_set) to create an ID-
mapped mount will be the user namespace of a container. In other
scenarios it will be a dedicated user namespace associated with a
user's login session as is the case for portable home directories
in systemd-homed.service(8)). It is also perfectly fine to
create a dedicated user namespace for the sake of ID mapping a
mount.
ID-mapped mounts can be useful in the following and a variety of
other scenarios:
• Sharing files or filesystems between multiple users or
multiple machines, especially in complex scenarios. For
example, ID-mapped mounts are used to implement portable home
directories in systemd-homed.service(8), where they allow
users to move their home directory to an external storage
device and use it on multiple computers where they are
assigned different user IDs and group IDs. This effectively
makes it possible to assign random user IDs and group IDs at
login time.
• Sharing files or filesystems from the host with unprivileged
containers. This allows a user to avoid having to change
ownership permanently through chown(2).
• ID mapping a container's root filesystem. Users don't need to
change ownership permanently through chown(2). Especially for
large root filesystems, using chown(2) can be prohibitively
expensive.
• Sharing files or filesystems between containers with non-
overlapping ID mappings.
• Implementing discretionary access (DAC) permission checking
for filesystems lacking a concept of ownership.
• Efficiently changing ownership on a per-mount basis. In
contrast to chown(2), changing ownership of large sets of
files is instantaneous with ID-mapped mounts. This is
especially useful when ownership of an entire root filesystem
of a virtual machine or container is to be changed as
mentioned above. With ID-mapped mounts, a single
mount_setattr() system call will be sufficient to change the
ownership of all files.
• Taking the current ownership into account. ID mappings
specify precisely what a user or group ID is supposed to be
mapped to. This contrasts with the chown(2) system call which
cannot by itself take the current ownership of the files it
changes into account. It simply changes the ownership to the
specified user ID and group ID.
• Locally and temporarily restricted ownership changes. ID-
mapped mounts make it possible to change ownership locally,
restricting the ownership changes to specific mounts, and
temporarily as the ownership changes only apply as long as the
mount exists. By contrast, changing ownership via the
chown(2) system call changes the ownership globally and
permanently.
Extensibility
In order to allow for future extensibility, mount_setattr()
requires the user-space application to specify the size of the
mount_attr structure that it is passing. By providing this
information, it is possible for mount_setattr() to provide both
forwards- and backwards-compatibility, with size acting as an
implicit version number. (Because new extension fields will
always be appended, the structure size will always increase.)
This extensibility design is very similar to other system calls
such as perf_setattr(2), perf_event_open(2), clone3(2) and
openat2(2).
Let usize be the size of the structure as specified by the user-
space application, and let ksize be the size of the structure
which the kernel supports, then there are three cases to
consider:
• If ksize equals usize, then there is no version mismatch and
attr can be used verbatim.
• If ksize is larger than usize, then there are some extension
fields that the kernel supports which the user-space
application is unaware of. Because a zero value in any added
extension field signifies a no-op, the kernel treats all of
the extension fields not provided by the user-space
application as having zero values. This provides backwards-
compatibility.
• If ksize is smaller than usize, then there are some extension
fields which the user-space application is aware of but which
the kernel does not support. Because any extension field must
have its zero values signify a no-op, the kernel can safely
ignore the unsupported extension fields if they are all zero.
If any unsupported extension fields are non-zero, then -1 is
returned and errno is set to E2BIG. This provides forwards-
compatibility.
Because the definition of struct mount_attr may change in the
future (with new fields being added when system headers are
updated), user-space applications should zero-fill struct
mount_attr to ensure that recompiling the program with new
headers will not result in spurious errors at run time. The
simplest way is to use a designated initializer:
struct mount_attr attr = {
.attr_set = MOUNT_ATTR_RDONLY,
.attr_clr = MOUNT_ATTR_NODEV
};
Alternatively, the structure can be zero-filled using memset(3)
or similar functions:
struct mount_attr attr;
memset(&attr, 0, sizeof(attr));
attr.attr_set = MOUNT_ATTR_RDONLY;
attr.attr_clr = MOUNT_ATTR_NODEV;
A user-space application that wishes to determine which
extensions the running kernel supports can do so by conducting a
binary search on size with a structure which has every byte
nonzero (to find the largest value which doesn't produce an error
of E2BIG).
/*
* This program allows the caller to create a new detached mount
* and set various properties on it.
*/
#define _GNU_SOURCE
#include <err.h>
#include <fcntl.h>
#include <getopt.h>
#include <linux/mount.h>
#include <linux/types.h>
#include <stdbool.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/syscall.h>
#include <unistd.h>
static inline int
mount_setattr(int dirfd, const char *pathname, unsigned int flags,
struct mount_attr *attr, size_t size)
{
return syscall(SYS_mount_setattr, dirfd, pathname, flags,
attr, size);
}
static inline int
open_tree(int dirfd, const char *filename, unsigned int flags)
{
return syscall(SYS_open_tree, dirfd, filename, flags);
}
static inline int
move_mount(int from_dirfd, const char *from_pathname,
int to_dirfd, const char *to_pathname, unsigned int flags)
{
return syscall(SYS_move_mount, from_dirfd, from_pathname,
to_dirfd, to_pathname, flags);
}
static const struct option longopts[] = {
{"map-mount", required_argument, NULL, 'a'},
{"recursive", no_argument, NULL, 'b'},
{"read-only", no_argument, NULL, 'c'},
{"block-setid", no_argument, NULL, 'd'},
{"block-devices", no_argument, NULL, 'e'},
{"block-exec", no_argument, NULL, 'f'},
{"no-access-time", no_argument, NULL, 'g'},
{ NULL, 0, NULL, 0 },
};
int
main(int argc, char *argv[])
{
int fd_userns = -1;
int fd_tree;
int index = 0;
int ret;
bool recursive = false;
const char *source;
const char *target;
struct mount_attr *attr = &(struct mount_attr){};
while ((ret = getopt_long_only(argc, argv, "",
longopts, &index)) != -1) {
switch (ret) {
case 'a':
fd_userns = open(optarg, O_RDONLY | O_CLOEXEC);
if (fd_userns == -1)
err(EXIT_FAILURE, "open(%s)", optarg);
break;
case 'b':
recursive = true;
break;
case 'c':
attr->attr_set |= MOUNT_ATTR_RDONLY;
break;
case 'd':
attr->attr_set |= MOUNT_ATTR_NOSUID;
break;
case 'e':
attr->attr_set |= MOUNT_ATTR_NODEV;
break;
case 'f':
attr->attr_set |= MOUNT_ATTR_NOEXEC;
break;
case 'g':
attr->attr_set |= MOUNT_ATTR_NOATIME;
attr->attr_clr |= MOUNT_ATTR__ATIME;
break;
default:
errx(EXIT_FAILURE, "Invalid argument specified");
}
}
if ((argc - optind) < 2)
errx(EXIT_FAILURE, "Missing source or target mount point");
source = argv[optind];
target = argv[optind + 1];
/* In the following, -1 as the 'dirfd' argument ensures that
open_tree() fails if 'source' is not an absolute pathname. */
fd_tree = open_tree(-1, source,
OPEN_TREE_CLONE | OPEN_TREE_CLOEXEC |
AT_EMPTY_PATH | (recursive ? AT_RECURSIVE : 0));
if (fd_tree == -1)
err(EXIT_FAILURE, "open(%s)", source);
if (fd_userns >= 0) {
attr->attr_set |= MOUNT_ATTR_IDMAP;
attr->userns_fd = fd_userns;
}
ret = mount_setattr(fd_tree, "",
AT_EMPTY_PATH | (recursive ? AT_RECURSIVE : 0),
attr, sizeof(struct mount_attr));
if (ret == -1)
err(EXIT_FAILURE, "mount_setattr");
close(fd_userns);
/* In the following, -1 as the 'to_dirfd' argument ensures that
open_tree() fails if 'target' is not an absolute pathname. */
ret = move_mount(fd_tree, "", -1, target,
MOVE_MOUNT_F_EMPTY_PATH);
if (ret == -1)
err(EXIT_FAILURE, "move_mount() to %s", target);
close(fd_tree);
exit(EXIT_SUCCESS);
}
newgidmap(1), newuidmap(1), clone(2), mount(2), unshare(2),
proc(5), capabilities(7), mount_namespaces(7),
user_namespaces(7), xattr(7)
Linux man-pages (unreleased) (date) mount_setattr(2)
Pages that refer to this page: mount(2), open(2), mount_namespaces(7), mount(8)
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