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vfs: syscall: Add fsmount() to create a mount for a superblock

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Provide a system call by which a filesystem opened with fsopen() and
configured by a series of fsconfig() calls can have a detached mount object
created for it.  This mount object can then be attached to the VFS mount
hierarchy using move_mount() by passing the returned file descriptor as the
from directory fd.

The system call looks like:

	int mfd = fsmount(int fsfd, unsigned int flags,
			  unsigned int attr_flags);

where fsfd is the file descriptor returned by fsopen().  flags can be 0 or
FSMOUNT_CLOEXEC.  attr_flags is a bitwise-OR of the following flags:

	MOUNT_ATTR_RDONLY	Mount read-only
	MOUNT_ATTR_NOSUID	Ignore suid and sgid bits
	MOUNT_ATTR_NODEV	Disallow access to device special files
	MOUNT_ATTR_NOEXEC	Disallow program execution
	MOUNT_ATTR__ATIME	Setting on how atime should be updated
	MOUNT_ATTR_RELATIME	- Update atime relative to mtime/ctime
	MOUNT_ATTR_NOATIME	- Do not update access times
	MOUNT_ATTR_STRICTATIME	- Always perform atime updates
	MOUNT_ATTR_NODIRATIME	Do not update directory access times

In the event that fsmount() fails, it may be possible to get an error
message by calling read() on fsfd.  If no message is available, ENODATA
will be reported.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: linux-api@vger.kernel.org
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
This commit is contained in:
David Howells 2018-11-01 23:36:14 +00:00 • committed by Al Viro
parent ecdab150fd
commit 93766fbd26
5 changed files with 165 additions and 4 deletions
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18
include/uapi/linux/mount.h
View file

@ -91,4 +91,22 @@ enum fsconfig_command {
FSCONFIG_CMD_RECONFIGURE = 7, /* Invoke superblock reconfiguration */
};
/*
* fsmount() flags.
*/
#define FSMOUNT_CLOEXEC 0x00000001
/*
* Mount attributes.
*/
#define MOUNT_ATTR_RDONLY 0x00000001 /* Mount read-only */
#define MOUNT_ATTR_NOSUID 0x00000002 /* Ignore suid and sgid bits */
#define MOUNT_ATTR_NODEV 0x00000004 /* Disallow access to device special files */
#define MOUNT_ATTR_NOEXEC 0x00000008 /* Disallow program execution */
#define MOUNT_ATTR__ATIME 0x00000070 /* Setting on how atime should be updated */
#define MOUNT_ATTR_RELATIME 0x00000000 /* - Update atime relative to mtime/ctime. */
#define MOUNT_ATTR_NOATIME 0x00000010 /* - Do not update access times. */
#define MOUNT_ATTR_STRICTATIME 0x00000020 /* - Always perform atime updates */
#define MOUNT_ATTR_NODIRATIME 0x00000080 /* Do not update directory access times */
#endif /* _UAPI_LINUX_MOUNT_H */