MOUNT_SETATTR(2) Linux Programmer's Manual MOUNT_SETATTR(2)

mount_setattr - change properties of a mount or mount tree

#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.
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.

mount_setattr() first appeared in Linux 5.12.

mount_setattr() is Linux-specific.

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 initial user namespace.
  • The filesystem must be mounted in a mount namespace that is owned by the initial user namespace.
  • The underlying filesystem must support ID-mapped mounts. Currently, the xfs(5), ext4(5), and FAT filesystems support ID-mapped mounts with more filesystems being actively worked on.
  • The mount must not already be ID-mapped. This also implies that the ID mapping of a mount cannot be altered.
  • 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.

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 runtime. 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 <errno.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 },
};
#define exit_log(format, ...)  do           \
{                                           \
    fprintf(stderr, format, ##__VA_ARGS__); \
    exit(EXIT_FAILURE);                     \
} while (0)
int
main(int argc, char *argv[])
{
    struct mount_attr *attr = &(struct mount_attr){};
    int fd_userns = -1;
    bool recursive = false;
    int index = 0;
    int ret;
    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)
                exit_log("%m - Failed top open %s\n", 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:
            exit_log("Invalid argument specified");
        }
    }
    if ((argc - optind) < 2)
        exit_log("Missing source or target mount point\n");
    const char *source = argv[optind];
    const char *target = argv[optind + 1];
    /* In the following, -1 as the 'dirfd' argument ensures that
       open_tree() fails if 'source' is not an absolute pathname. */
    int fd_tree = open_tree(-1, source,
                       OPEN_TREE_CLONE | OPEN_TREE_CLOEXEC |
                       AT_EMPTY_PATH | (recursive ? AT_RECURSIVE : 0));
    if (fd_tree == -1)
        exit_log("%m - Failed to open %s\n", 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)
        exit_log("%m - Failed to change mount attributes\n");
    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)
        exit_log("%m - Failed to attach mount to %s\n", 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)

This page is part of release 5.13 of the Linux man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at https://www.kernel.org/doc/man-pages/.
2021-08-27 Linux