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diff --git a/man/man2/mount_setattr.2 b/man/man2/mount_setattr.2 new file mode 100644 index 000000000..f4bbc088b --- /dev/null +++ b/man/man2/mount_setattr.2 @@ -0,0 +1,1067 @@ +.\" Copyright (c) 2021 by Christian Brauner <christian.brauner@ubuntu.com> +.\" +.\" SPDX-License-Identifier: Linux-man-pages-copyleft +.\" +.TH mount_setattr 2 (date) "Linux man-pages (unreleased)" +.SH NAME +mount_setattr \- change properties of a mount or mount tree +.SH LIBRARY +Standard C library +.RI ( libc ", " \-lc ) +.SH SYNOPSIS +.nf +.BR "#include <linux/fcntl.h>" " /* Definition of " AT_* " constants */" +.BR "#include <linux/mount.h>" " /* Definition of " MOUNT_ATTR_* " constants */" +.BR "#include <sys/syscall.h>" " /* Definition of " SYS_* " constants */" +.B #include <unistd.h> +.P +.BI "int syscall(SYS_mount_setattr, int " dirfd ", const char *" pathname , +.BI " unsigned int " flags ", struct mount_attr *" attr \ +", size_t " size ); +.fi +.P +.IR Note : +glibc provides no wrapper for +.BR mount_setattr (), +necessitating the use of +.BR syscall (2). +.SH DESCRIPTION +The +.BR mount_setattr () +system call changes the mount properties of a mount or an entire mount tree. +If +.I pathname +is a relative pathname, +then it is interpreted relative to +the directory referred to by the file descriptor +.IR dirfd . +If +.I dirfd +is the special value +.BR AT_FDCWD , +then +.I pathname +is interpreted relative to +the current working directory of the calling process. +If +.I pathname +is the empty string and +.B AT_EMPTY_PATH +is specified in +.IR flags , +then the mount properties of the mount identified by +.I dirfd +are changed. +(See +.BR openat (2) +for an explanation of why the +.I dirfd +argument is useful.) +.P +The +.BR mount_setattr () +system call uses an extensible structure +.RI ( "struct mount_attr" ) +to allow for future extensions. +Any non-flag extensions to +.BR 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 +.I must +zero-fill this structure on initialization. +See the "Extensibility" subsection under +.B NOTES +for more details. +.P +The +.I size +argument should usually be specified as +.IR "sizeof(struct mount_attr)" . +However, if the caller is using a kernel that supports an extended +.IR "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 +.IR "struct mount_attr" , +the kernel will expose a definition of the form +.BI MOUNT_ATTR_SIZE_VER number\c +\&. +For example, the macro for the size of the initial version of +.I struct mount_attr +is +.BR MOUNT_ATTR_SIZE_VER0 . +.P +The +.I flags +argument can be used to alter the pathname resolution behavior. +The supported values are: +.TP +.B AT_EMPTY_PATH +If +.I pathname +is the empty string, +change the mount properties on +.I dirfd +itself. +.TP +.B AT_RECURSIVE +Change the mount properties of the entire mount tree. +.TP +.B AT_SYMLINK_NOFOLLOW +Don't follow trailing symbolic links. +.TP +.B AT_NO_AUTOMOUNT +Don't trigger automounts. +.P +The +.I attr +argument of +.BR mount_setattr () +is a structure of the following form: +.P +.in +4n +.EX +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 */ +}; +.EE +.in +.P +The +.I attr_set +and +.I 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 +.I attr_set +enable a property on a mount or mount tree, +and flags set in +.I attr_clr +remove a property from a mount or mount tree. +.P +When changing mount properties, +the kernel will first clear the flags specified +in the +.I attr_clr +field, +and then set the flags specified in the +.I attr_set +field. +For example, these settings: +.P +.in +4n +.EX +struct mount_attr attr = { + .attr_clr = MOUNT_ATTR_NOEXEC | MOUNT_ATTR_NODEV, + .attr_set = MOUNT_ATTR_RDONLY | MOUNT_ATTR_NOSUID, +}; +.EE +.in +.P +are equivalent to the following steps: +.P +.in +4n +.EX +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 &= \(tiattr\->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; +.EE +.in +.P +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. +.P +Multiple changes with the same set of flags requested +in +.I attr_clr +and +.I attr_set +are guaranteed to be idempotent after the changes have been applied. +.P +The following mount attributes can be specified in the +.I attr_set +or +.I attr_clr +fields: +.TP +.B MOUNT_ATTR_RDONLY +If set in +.IR attr_set , +makes the mount read-only. +If set in +.IR attr_clr , +removes the read-only setting if set on the mount. +.TP +.B MOUNT_ATTR_NOSUID +If set in +.IR 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 +.IR attr_clr , +clears the set-user-ID, set-group-ID, +and file capability restriction if set on this mount. +.TP +.B MOUNT_ATTR_NODEV +If set in +.IR attr_set , +prevents access to devices on this mount. +If set in +.IR attr_clr , +removes the restriction that prevented accessing devices on this mount. +.TP +.B MOUNT_ATTR_NOEXEC +If set in +.IR attr_set , +prevents executing programs on this mount. +If set in +.IR attr_clr , +removes the restriction that prevented executing programs on this mount. +.TP +.B MOUNT_ATTR_NOSYMFOLLOW +If set in +.IR attr_set , +prevents following symbolic links on this mount. +If set in +.IR attr_clr , +removes the restriction that prevented following symbolic links on this mount. +.TP +.B MOUNT_ATTR_NODIRATIME +If set in +.IR attr_set , +prevents updating access time for directories on this mount. +If set in +.IR attr_clr , +removes the restriction that prevented updating access time for directories. +Note that +.B 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. +.TP +.BR 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 +.BR MOUNT_ATTR__ATIME . +Even though these bits are an enumeration +(in contrast to the other mount flags such as +.BR MOUNT_ATTR_NOEXEC ), +they are nonetheless passed in +.I attr_set +and +.I attr_clr +for consistency with +.BR fsmount (2), +which introduced this behavior. +.IP +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 +.IR attr_set , +but must also include +.B MOUNT_ATTR__ATIME +in the +.I attr_clr +field. +The kernel will verify that +.B MOUNT_ATTR__ATIME +isn't partially set in +.I attr_clr +(i.e., either all bits in the +.B MOUNT_ATTR__ATIME +bit field are either set or clear), and that +.I attr_set +doesn't have any access-time bits set if +.B MOUNT_ATTR__ATIME +isn't set in +.IR attr_clr . +.RS +.TP +.B 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). +.IP +To enable this access-time setting on a mount or mount tree, +.B MOUNT_ATTR_RELATIME +must be set in +.I attr_set +and +.B MOUNT_ATTR__ATIME +must be set in the +.I attr_clr +field. +.TP +.B MOUNT_ATTR_NOATIME +Do not update access times for (all types of) files on this mount. +.IP +To enable this access-time setting on a mount or mount tree, +.B MOUNT_ATTR_NOATIME +must be set in +.I attr_set +and +.B MOUNT_ATTR__ATIME +must be set in the +.I attr_clr +field. +.TP +.B MOUNT_ATTR_STRICTATIME +Always update the last access time (atime) +when files are accessed on this mount. +.IP +To enable this access-time setting on a mount or mount tree, +.B MOUNT_ATTR_STRICTATIME +must be set in +.I attr_set +and +.B MOUNT_ATTR__ATIME +must be set in the +.I attr_clr +field. +.RE +.TP +.B MOUNT_ATTR_IDMAP +If set in +.IR attr_set , +creates an ID-mapped mount. +The ID mapping is taken from the user namespace specified in +.I userns_fd +and attached to the mount. +.IP +Since it is not supported to +change the ID mapping of a mount after it has been ID mapped, +it is invalid to specify +.B MOUNT_ATTR_IDMAP +in +.IR attr_clr . +.IP +For further details, see the subsection "ID-mapped mounts" under NOTES. +.P +The +.I 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: +.TP +.B MS_PRIVATE +Turn all mounts into private mounts. +.TP +.B MS_SHARED +Turn all mounts into shared mounts. +.TP +.B MS_SLAVE +Turn all mounts into dependent mounts. +.TP +.B MS_UNBINDABLE +Turn all mounts into unbindable mounts. +.P +For further details on the above propagation types, see +.BR mount_namespaces (7). +.SH RETURN VALUE +On success, +.BR mount_setattr () +returns zero. +On error, +\-1 is returned and +.I errno +is set to indicate the cause of the error. +.SH ERRORS +.TP +.B EBADF +.I pathname +is relative but +.I dirfd +is neither +.B AT_FDCWD +nor a valid file descriptor. +.TP +.B EBADF +.I userns_fd +is not a valid file descriptor. +.TP +.B EBUSY +The caller tried to change the mount to +.BR MOUNT_ATTR_RDONLY , +but the mount still holds files open for writing. +.TP +.B EBUSY +The caller tried to create an ID-mapped mount raising +.B MOUNT_ATTR_IDMAP +and specifying +.I userns_fd +but the mount still holds files open for writing. +.TP +.B EINVAL +The pathname specified via the +.I dirfd +and +.I pathname +arguments to +.BR mount_setattr () +isn't a mount point. +.TP +.B EINVAL +An unsupported value was set in +.IR flags . +.TP +.B EINVAL +An unsupported value was specified in the +.I attr_set +field of +.IR mount_attr . +.TP +.B EINVAL +An unsupported value was specified in the +.I attr_clr +field of +.IR mount_attr . +.TP +.B EINVAL +An unsupported value was specified in the +.I propagation +field of +.IR mount_attr . +.TP +.B EINVAL +More than one of +.BR MS_SHARED , +.BR MS_SLAVE , +.BR MS_PRIVATE , +or +.B MS_UNBINDABLE +was set in the +.I propagation +field of +.IR mount_attr . +.TP +.B EINVAL +An access-time setting was specified in the +.I attr_set +field without +.B MOUNT_ATTR__ATIME +being set in the +.I attr_clr +field. +.TP +.B EINVAL +.B MOUNT_ATTR_IDMAP +was specified in +.IR attr_clr . +.TP +.B EINVAL +A file descriptor value was specified in +.I userns_fd +which exceeds +.BR INT_MAX . +.TP +.B EINVAL +A valid file descriptor value was specified in +.IR userns_fd , +but the file descriptor did not refer to a user namespace. +.TP +.B EINVAL +The underlying filesystem does not support ID-mapped mounts. +.TP +.B 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. +.TP +.B EINVAL +A partial access-time setting was specified in +.I attr_clr +instead of +.B MOUNT_ATTR__ATIME +being set. +.TP +.B EINVAL +The mount is located outside the caller's mount namespace. +.TP +.B EINVAL +The underlying filesystem has been mounted in a mount namespace that is +owned by a noninitial user namespace +.TP +.B ENOENT +A pathname was empty or had a nonexistent component. +.TP +.B ENOMEM +When changing mount propagation to +.BR 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. +.TP +.B ENOSPC +When changing mount propagation to +.BR 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. +.\" Christian Brauner: i.e. someone has somehow managed to +.\" allocate so many peer groups and managed to keep the kernel running +.\" (???) that the ida has ran out of ids +.\" Note that technically further error codes are possible that are +.\" specific to the ID allocation implementation used. +.TP +.B EPERM +One of the mounts had at least one of +.BR MOUNT_ATTR_NOATIME , +.BR MOUNT_ATTR_NODEV , +.BR MOUNT_ATTR_NODIRATIME , +.BR MOUNT_ATTR_NOEXEC , +.BR MOUNT_ATTR_NOSUID , +or +.B MOUNT_ATTR_RDONLY +set and the flag is locked. +Mount attributes become locked on a mount if: +.RS +.IP \[bu] 3 +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. +.IP \[bu] +A new mount and user namespace pair is created. +This happens for example when specifying +.B CLONE_NEWUSER | CLONE_NEWNS +in +.BR unshare (2), +.BR clone (2), +or +.BR 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\[em]in the absence of such locking\[em]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). +.RE +.TP +.B EPERM +A valid file descriptor value was specified in +.IR userns_fd , +but the file descriptor refers to the initial user namespace. +.TP +.B EPERM +An attempt was made to add an ID mapping to a mount that is already ID mapped. +.TP +.B EPERM +The caller does not have +.B CAP_SYS_ADMIN +in the initial user namespace. +.SH STANDARDS +Linux. +.SH HISTORY +Linux 5.12. +.\" commit 7d6beb71da3cc033649d641e1e608713b8220290 +.\" commit 2a1867219c7b27f928e2545782b86daaf9ad50bd +.\" commit 9caccd41541a6f7d6279928d9f971f6642c361af +.SH NOTES +.SS 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. +.P +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 +.BR xattr (7) +keys: +.IP \[bu] 3 +.IR security.capability , +whenever filesystem capabilities +are stored or returned in the +.B VFS_CAP_REVISION_3 +format, +which stores a root user ID alongside the capabilities +(see +.BR capabilities (7)). +.IP \[bu] +.I system.posix_acl_access +and +.IR system.posix_acl_default , +whenever user IDs or group IDs are stored in +.B ACL_USER +or +.B ACL_GROUP +entries. +.P +The following conditions must be met in order to create an ID-mapped mount: +.IP \[bu] 3 +The caller must have the +.B CAP_SYS_ADMIN +capability in the user namespace the filesystem was mounted in. +.\" commit bd303368b776eead1c29e6cdda82bde7128b82a7 +.\" Christian Brauner +.\" Note, currently no filesystems mountable in non-initial user namespaces +.\" support ID-mapped mounts. +.IP \[bu] +The underlying filesystem must support ID-mapped mounts. +Currently, the following filesystems support ID-mapped mounts: +.\" fs_flags = FS_ALLOW_IDMAP in kernel sources +.RS +.IP \[bu] 3 +.PD 0 +.BR xfs (5) +(since Linux 5.12) +.IP \[bu] +.BR ext4 (5) +(since Linux 5.12) +.IP \[bu] +.B FAT +(since Linux 5.12) +.IP \[bu] +.BR btrfs (5) +(since Linux 5.15) +.\" commit 5b9b26f5d0b88b74001dcfe4ab8a8f2f4e744112 +.IP \[bu] +.B ntfs3 +(since Linux 5.15) +.\" commit 82cae269cfa953032fbb8980a7d554d60fb00b17 +.IP \[bu] +.B f2fs +(since Linux 5.18) +.\" commit 984fc4e76d63345499f01c0c198a4b44860cf027 +.IP \[bu] +.B erofs +(since Linux 5.19) +.\" commit 6c459b78d4793afbba6d864c466cc5cd2932459d +.IP \[bu] +.B overlayfs +(ID-mapped lower and upper layers supported since Linux 5.19) +.IP \[bu] +.B squashfs +(since Linux 6.2) +.IP \[bu] +.B tmpfs +(since Linux 6.3) +.IP \[bu] +.B cephfs +(since Linux 6.7) +.IP \[bu] +.B hugetlbfs +(since Linux 6.9) +.PD +.RE +.IP \[bu] +The mount must not already be ID-mapped. +This also implies that the ID mapping of a mount cannot be altered. +.IP \[bu] +The mount must not have any writers. +.\" commit 1bbcd277a53e08d619ffeec56c5c9287f2bf42f +.IP \[bu] +The mount must be a detached mount; +that is, +it must have been created by calling +.BR open_tree (2) +with the +.B 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 +.BR move_mount (2).) +.P +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. +.P +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. +.P +Further details on setting up ID mappings can be found in +.BR user_namespaces (7). +.P +In the common case, the user namespace passed in +.I userns_fd +(together with +.B MOUNT_ATTR_IDMAP +in +.IR 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 +.BR systemd-homed.service (8)). +It is also perfectly fine to create a dedicated user namespace +for the sake of ID mapping a mount. +.P +ID-mapped mounts can be useful in the following +and a variety of other scenarios: +.IP \[bu] 3 +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 +.BR 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. +.IP \[bu] +Sharing files or filesystems +from the host with unprivileged containers. +This allows a user to avoid having to change ownership permanently through +.BR chown (2). +.IP \[bu] +ID mapping a container's root filesystem. +Users don't need to change ownership permanently through +.BR chown (2). +Especially for large root filesystems, using +.BR chown (2) +can be prohibitively expensive. +.IP \[bu] +Sharing files or filesystems +between containers with non-overlapping ID mappings. +.IP \[bu] +Implementing discretionary access (DAC) permission checking +for filesystems lacking a concept of ownership. +.IP \[bu] +Efficiently changing ownership on a per-mount basis. +In contrast to +.BR 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 +.BR mount_setattr () +system call will be sufficient to change the ownership of all files. +.IP \[bu] +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 +.BR 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. +.IP \[bu] +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 +.BR chown (2) +system call changes the ownership globally and permanently. +.\" +.SS Extensibility +In order to allow for future extensibility, +.BR mount_setattr () +requires the user-space application to specify the size of the +.I mount_attr +structure that it is passing. +By providing this information, it is possible for +.BR mount_setattr () +to provide both forwards- and backwards-compatibility, with +.I 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 +.BR perf_setattr (2), +.BR perf_event_open (2), +.BR clone3 (2) +and +.BR openat2 (2). +.P +Let +.I usize +be the size of the structure as specified by the user-space application, +and let +.I ksize +be the size of the structure which the kernel supports, +then there are three cases to consider: +.IP \[bu] 3 +If +.I ksize +equals +.IR usize , +then there is no version mismatch and +.I attr +can be used verbatim. +.IP \[bu] +If +.I ksize +is larger than +.IR 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. +.IP \[bu] +If +.I ksize +is smaller than +.IR 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 +.I errno +is set to +.BR E2BIG . +This provides forwards-compatibility. +.P +Because the definition of +.I struct mount_attr +may change in the future +(with new fields being added when system headers are updated), +user-space applications should zero-fill +.I 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: +.P +.in +4n +.EX +struct mount_attr attr = { + .attr_set = MOUNT_ATTR_RDONLY, + .attr_clr = MOUNT_ATTR_NODEV +}; +.EE +.in +.P +Alternatively, the structure can be zero-filled using +.BR memset (3) +or similar functions: +.P +.in +4n +.EX +struct mount_attr attr; +memset(&attr, 0, sizeof(attr)); +attr.attr_set = MOUNT_ATTR_RDONLY; +attr.attr_clr = MOUNT_ATTR_NODEV; +.EE +.in +.P +A user-space application that wishes to determine which extensions the running +kernel supports can do so by conducting a binary search on +.I size +with a structure which has every byte nonzero +(to find the largest value which doesn't produce an error of +.BR E2BIG ). +.SH EXAMPLES +.\" SRC BEGIN (mount_setattr.c) +.EX +/* + * 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, \[aq]a\[aq]}, + {"recursive", no_argument, NULL, \[aq]b\[aq]}, + {"read\-only", no_argument, NULL, \[aq]c\[aq]}, + {"block\-setid", no_argument, NULL, \[aq]d\[aq]}, + {"block\-devices", no_argument, NULL, \[aq]e\[aq]}, + {"block\-exec", no_argument, NULL, \[aq]f\[aq]}, + {"no\-access\-time", no_argument, NULL, \[aq]g\[aq]}, + { 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 \[aq]a\[aq]: + fd_userns = open(optarg, O_RDONLY | O_CLOEXEC); + if (fd_userns == \-1) + err(EXIT_FAILURE, "open(%s)", optarg); + break; + case \[aq]b\[aq]: + recursive = true; + break; + case \[aq]c\[aq]: + attr\->attr_set |= MOUNT_ATTR_RDONLY; + break; + case \[aq]d\[aq]: + attr\->attr_set |= MOUNT_ATTR_NOSUID; + break; + case \[aq]e\[aq]: + attr\->attr_set |= MOUNT_ATTR_NODEV; + break; + case \[aq]f\[aq]: + attr\->attr_set |= MOUNT_ATTR_NOEXEC; + break; + case \[aq]g\[aq]: + 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 \[aq]dirfd\[aq] argument ensures that + open_tree() fails if \[aq]source\[aq] is not an absolute pathname. */ +.\" Christian Brauner +.\" When writing programs I like to never use relative paths with AT_FDCWD +.\" because. Because making assumptions about the current working directory +.\" of the calling process is just too easy to get wrong; especially when +.\" pivot_root() or chroot() are in play. +.\" My absolut preference (joke intended) is to open a well-known starting +.\" point with an absolute path to get a dirfd and then scope all future +.\" operations beneath that dirfd. This already works with old-style +.\" openat() and _very_ cautious programming but openat2() and its +.\" resolve-flag space have made this **chef's kiss**. +.\" If I can't operate based on a well-known dirfd I use absolute paths +.\" with a -EBADF dirfd passed to *at() functions. +\& + 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 \[aq]to_dirfd\[aq] argument ensures that + open_tree() fails if \[aq]target\[aq] 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); +} +.EE +.\" SRC END +.SH SEE ALSO +.BR newgidmap (1), +.BR newuidmap (1), +.BR clone (2), +.BR mount (2), +.BR unshare (2), +.BR proc (5), +.BR capabilities (7), +.BR mount_namespaces (7), +.BR user_namespaces (7), +.BR xattr (7) |