statmount(2)

NAME
statmount - get a mount status

LIBRARY
Standard C library (libc, -lc)

SYNOPSIS
#include <linux/mount.h>  /* Definition of STATMOUNT_* constants */
#include <unistd.h>

int syscall(SYS_statmount, struct mnt_id_req *req,
            struct statmount *smbuf, size_t bufsize,
            unsigned long flags);

#include <linux/mount.h>

    struct mnt_id_req {
        __u32  size;        /* sizeof(struct mnt_id_req) */
        union {
            __u32  mnt_ns_fd;  /* fd of mnt_ns to query the mnt_id in */
            __u32  mnt_fd;     /* fd on the mount being queried */
        };
        __u64  mnt_id;      /* The mnt_id being queried */
        __u64  param;       /* ORed combination of the STATMOUNT_ constants */
        __u32  mnt_ns_id;   /* The id of mnt_ns to query the mnt_id in */
    };

struct statmount {
    __u32  size;
    __u32  mnt_opts;
    __u64  mask;
    __u32  sb_dev_major;
    __u32  sb_dev_minor;
    __u64  sb_magic;
    __u32  sb_flags;
    __u32  fs_type;
    __u64  mnt_id;
    __u64  mnt_parent_id;
    __u32  mnt_id_old;
    __u32  mnt_parent_id_old;
    __u64  mnt_attr;
    __u64  mnt_propagation;
    __u64  mnt_peer_group;
    __u64  mnt_master;
    __u64  propagate_from;
    __u32  mnt_root;
    __u32  mnt_point;
    __u64  mnt_ns_id;
    __u32  fs_subtype;
    __u32  sb_source;
    __u32  opt_num;
    __u32  opt_array;
    __u32  opt_sec_num;
    __u32  opt_sec_array;
    __u32  mnt_uidmap_num;
    __u32  mnt_uidmap;
    __u32  mnt_gidmap_num;
    __u32  mnt_gidmap;
    __u64  supported_mask;
    char   str[];
};

Note: glibc provides no wrapper for statmount(), necessitating the use of syscall(2).

DESCRIPTION
To access a mount's status, the caller must have CAP_SYS_ADMIN in the user namespace. In case of accessing a mount in a foreign mount namespace (specified via req.mnt_ns_id or req.mnt_ns_fd), the foreign mount namespace should be child of the current namespace.

This function returns information about a mount, storing it in the buffer pointed to by smbuf. The returned buffer is a struct statmount which is of size bufsize with the fields filled in as described below. flags must either be 0 or STATMOUNT_BY_FD.

(Note that reserved space and padding is omitted.)

  The mnt_id_req structure
req.size is used by the kernel to determine which struct mnt_id_req is being passed in; it should always be set to sizeof(struct mnt_id_req).

req.mnt_ns_fd can be obtained from PIDFD_GET_MNT_NAMESPACE ioctl(2) operation or by opening a file descriptor to /proc/pid/ns/mnt and is used to specify a foreign mount namespace in which to query req.mnt_id (since Linux 6.18).

req.mnt_fd is a file descriptor on a mount. If STATMOUNT_BY_FD flag is specified, req.mnt_id and req.mnt_ns_id are zeroed, the function will return information about the mount the file descriptor is on (since Linux 7.0).

The fd can also be on a mount that has been lazily unmounted (see umount2(2) with MNT_DETACH). In this case, STATMOUNT_MNT_POINT and STATMOUNT_MNT_NS_ID will be unset, since an unmounted mount is no longer a part of the filesystem.

req.mnt_id can be obtained from either statx(2) using STATX_MNT_ID_UNIQUE or from listmount(2) and is used as the identifier to query the status of the desired mount point.

req.param is used to tell the kernel which fields the caller is interested in. It is an ORed combination of the following constants

STATMOUNT_SB_BASIC | /* Want/got sb_* */
STATMOUNT_MNT_BASIC | /* Want/got mnt_* */
STATMOUNT_PROPAGATE_FROM | /* Want/got propagate_from */
STATMOUNT_MNT_ROOT | /* Want/got mnt_root */
STATMOUNT_MNT_POINT | /* Want/got mnt_point */
STATMOUNT_FS_TYPE | /* Want/got fs_type */
STATMOUNT_MNT_NS_ID | /* Want/got mnt_ns_id */
STATMOUNT_MNT_OPTS | /* Want/got mnt_opts */
STATMOUNT_FS_SUBTYPE | /* Want/got fs_subtype */
STATMOUNT_SB_SOURCE | /* Want/got sb_source */
STATMOUNT_OPT_ARRAY | /* Want/got opt_{num,array} */
STATMOUNT_OPT_SEC_ARRAY | /* Want/got opt_sec_{num,array} */
STATMOUNT_MNT_UIDMAP | /* Want/got uidmap{_num,} */
STATMOUNT_MNT_GIDMAP | /* Want/got gidmap{_num,} */
STATMOUNT_SUPPORTED_MASK | /* Want/got supported mask flags */

In general, the kernel does not reject values in req.param other than the above. (For an exception, see EINVAL in errors.) Instead, it simply informs the caller which values are supported by this kernel and filesystem via the statmount.mask field. Therefore, do not simply set req.param to UINT_MAX (all bits set), as one or more bits may, in the future, be used to specify an extension to the buffer.

req.mnt_ns_id can be obtained from NS_GET_MNTNS_ID ioctl(2) operation and is used to specify a foreign mount namespace in which to query req.mnt_id (since Linux 6.11).

  The returned information
The status information for the target mount is returned in the statmount structure pointed to by smbuf.

The fields in the statmount structure are:

smbuf.size
    The size of the returned smbuf structure, including any of the strings fields that were filled.

smbuf.mnt_opts (since Linux 6.11)
    The offset to the location in the smbuf.str buffer that contains a comma separated list of mount options, similiar to those in proc_pid_mountinfo(5). It is a null-terminated string.

smbuf.mask
    The ORed combination of STATMOUNT_* flags indicating which fields were filled in and thus valid. The kernel may return fields that weren't requested, and may fail to return fields that were requested, depending on what the backing file system and kernel supports. In either case, req.param will not be equal to mask.

smbuf.sb_dev_major
smbuf.sb_dev_minor
    The device that is mounted at this mount point.

smbuf.sb_magic
    The file system specific super block magic.

smbuf.sb_flags
    The flags that are set on the super block, an ORed combination of SB_RDONLY, SB_SYNCHRONOUS, SB_DIRSYNC, SB_LAZYTIME.

smbuf.fs_type
    The offset to the location in the smbuf.str buffer that contains the string representation of the mounted file system. It is a null-terminated string.

smbuf.mnt_id
    The unique mount ID of the mount.

smbuf.mnt_parent_id
    The unique mount ID of the parent mount point of this mount. If this is the root mount point then smbuf.mnt_id == smbuf.parent_mount_id.

smbuf.mnt_id_old
    This corresponds to the mount ID that is exported by /proc/pid/mountinfo.

smbuf.mnt_parent_id_old
    This corresponds to the parent mount ID that is exported by /proc/pid/mountinfo.

smbuf.mnt_attr
    The MOUNT_ATTR_* flags set on this mount point.

smbuf.mnt_propagation
    The mount propagation flags, which can be one of MS_SHARED, MS_SLAVE, MS_PRIVATE, MS_UNBINDABLE.

smbuf.mnt_peer_group
    The ID of the shared peer group.

smbuf.mnt_master
    The mount point receives its propagation from this mount ID.

smbuf.propagate_from
    The ID from the namespace we propagated from.

smbuf.mnt_root
    The offset to the location in the smbuf.str buffer that contains the string representation of the mount relative to the root of the file system. It is a null-terminated string.

smbuf.mnt_point
    The offset to the location in the smbuf.str buffer that contains the string representation of the mount relative to the current root (ie if you are in a chroot). It is a null-terminated string.

smbuf.mnt_ns_id (since Linux 6.11)
    The unique ID of the mount namespace the mount belongs to.

smbuf.fs_subtype (since Linux 6.13)
    The offset to the location in the smbuf.str buffer that contains the string representation of the file system subtype. This is particularly useful for disambiguating FUSE mounts. It is a null-terminated string.

smbuf.sb_source (since Linux 6.13)
    The offset to the location in the smbuf.str that contains the source of the mount. It is a null-terminated string.

smbuf.opt_num (since Linux 6.13)
    The number of filesystem options set on the mount.

smbuf.opt_array (since Linux 6.13)
    The offset to the location in the smbuf.str buffer that contains file system options separated by null bytes. They can be iterated over with the help of smbuf.opt_num.

smbuf.opt_sec_num (since Linux 6.13)
    The number of security options set on the mount.

smbuf.opt_sec_array (since Linux 6.13)
    The offset to the location in the smbuf.str buffer that contains security options separated by null bytes. They can be iterated over with the help of smbuf.opt_sec_num.

smbuf.mnt_uidmap_num (since Linux 6.15)
    The number of uid mappings applied on the mount. If smbuf.mask has STATMOUNT_MNT_UIDMAP set and this field is 0, then uid mappings applied on the mount cannot be resolved in the user namespace of the caller.

smbuf.mnt_uidmap (since Linux 6.15)
    The offset to the location in the smbuf.str buffer that contains uid mappings as consecutive null-terminated strings. They can be iterated over with the help of smbuf.mnt_uidmap_num. The whole range of uid mappings must be resolvable in the user namespace of the caller.

smbuf.mnt_gidmap_num (since Linux 6.15)
    The number of gid mappings applied on the mount. If smbuf.mask has STATMOUNT_MNT_GIDMAP set and this field is 0, then gid mappings applied on the mount cannot be resolved in the user namespace of the caller.

smbuf.mnt_gidmap (since Linux 6.15)
    The offset to the location in the smbuf.str buffer that contains gid mappings as consecutive null-terminated strings. They can be iterated over with the help of smbuf.mnt_gidmap_num. The whole range of gid mappings must be resolvable in the user namespace of the caller.

smbuf.supported_mask (since Linux 6.15)
    The ORed combination of STATMOUNT_* flags supported by the current kernel.

RETURN VALUE
On success, zero is returned. On error, -1 is returned, and errno is set to indicate the error.

ERRORS

EPERM
    Permission is denied for accessing this mount.

EFAULT
    req or smbuf points to a location outside the process's accessible address space.

EINVAL
    Invalid flag specified in flags.

EINVAL
    req is of insufficient size to be utilized.

EINVAL
    Both req.mnt_ns_id and req.mnt_ns_fd were set.

EINVAL
    req.mnt_id or req.mnt_ns_id was specified alongside req.mnt_fd.

EBADF
    req.mnt_fd is an invalid file descriptor.

E2BIG
    req is too large.

EOVERFLOW
    The size of smbuf is too small to contain either the smbuf.fs_type, smbuf.mnt_root, or smbuf.mnt_point. Allocate a larger buffer and retry the call.

ENOENT
    The specified req.mnt_id doesn't exist.

ENOMEM
    Out of memory (i.e., kernel memory).

STANDARDS
Linux.

SEE ALSO
listmount(2), statx(2)