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   mount_setattr    ( 2 )

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Примечание (Note)

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 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.

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 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).