• David Howells's avatar
    statx: Add a system call to make enhanced file info available · a528d35e
    David Howells authored
    Add a system call to make extended file information available, including
    file creation and some attribute flags where available through the
    underlying filesystem.
    The getattr inode operation is altered to take two additional arguments: a
    u32 request_mask and an unsigned int flags that indicate the
    synchronisation mode.  This change is propagated to the vfs_getattr*()
    Functions like vfs_stat() are now inline wrappers around new functions
    vfs_statx() and vfs_statx_fd() to reduce stack usage.
    The idea was initially proposed as a set of xattrs that could be retrieved
    with getxattr(), but the general preference proved to be for a new syscall
    with an extended stat structure.
    A number of requests were gathered for features to be included.  The
    following have been included:
     (1) Make the fields a consistent size on all arches and make them large.
     (2) Spare space, request flags and information flags are provided for
         future expansion.
     (3) Better support for the y2038 problem [Arnd Bergmann] (tv_sec is an
     (4) Creation time: The SMB protocol carries the creation time, which could
         be exported by Samba, which will in turn help CIFS make use of
         FS-Cache as that can be used for coherency data (stx_btime).
         This is also specified in NFSv4 as a recommended attribute and could
         be exported by NFSD [Steve French].
     (5) Lightweight stat: Ask for just those details of interest, and allow a
         netfs (such as NFS) to approximate anything not of interest, possibly
         without going to the server [Trond Myklebust, Ulrich Drepper, Andreas
         Dilger] (AT_STATX_DONT_SYNC).
     (6) Heavyweight stat: Force a netfs to go to the server, even if it thinks
         its cached attributes are up to date [Trond Myklebust]
    And the following have been left out for future extension:
     (7) Data version number: Could be used by userspace NFS servers [Aneesh
         Can also be used to modify fill_post_wcc() in NFSD which retrieves
         i_version directly, but has just called vfs_getattr().  It could get
         it from the kstat struct if it used vfs_xgetattr() instead.
         (There's disagreement on the exact semantics of a single field, since
         not all filesystems do this the same way).
     (8) BSD stat compatibility: Including more fields from the BSD stat such
         as creation time (st_btime) and inode generation number (st_gen)
         [Jeremy Allison, Bernd Schubert].
     (9) Inode generation number: Useful for FUSE and userspace NFS servers
         [Bernd Schubert].
         (This was asked for but later deemed unnecessary with the
         open-by-handle capability available and caused disagreement as to
         whether it's a security hole or not).
    (10) Extra coherency data may be useful in making backups [Andreas Dilger].
         (No particular data were offered, but things like last backup
         timestamp, the data version number and the DOS archive bit would come
         into this category).
    (11) Allow the filesystem to indicate what it can/cannot provide: A
         filesystem can now say it doesn't support a standard stat feature if
         that isn't available, so if, for instance, inode numbers or UIDs don't
         exist or are fabricated locally...
         (This requires a separate system call - I have an fsinfo() call idea
         for this).
    (12) Store a 16-byte volume ID in the superblock that can be returned in
         struct xstat [Steve French].
         (Deferred to fsinfo).
    (13) Include granularity fields in the time data to indicate the
         granularity of each of the times (NFSv4 time_delta) [Steve French].
         (Deferred to fsinfo).
    (14) FS_IOC_GETFLAGS value.  These could be translated to BSD's st_flags.
         Note that the Linux IOC flags are a mess and filesystems such as Ext4
         define flags that aren't in linux/fs.h, so translation in the kernel
         may be a necessity (or, possibly, we provide the filesystem type too).
         (Some attributes are made available in stx_attributes, but the general
         feeling was that the IOC flags were to ext[234]-specific and shouldn't
         be exposed through statx this way).
    (15) Mask of features available on file (eg: ACLs, seclabel) [Brad Boyer,
         Michael Kerrisk].
         (Deferred, probably to fsinfo.  Finding out if there's an ACL or
         seclabal might require extra filesystem operations).
    (16) Femtosecond-resolution timestamps [Dave Chinner].
         (A __reserved field has been left in the statx_timestamp struct for
         this - if there proves to be a need).
    (17) A set multiple attributes syscall to go with this.
    The new system call is:
    	int ret = statx(int dfd,
    			const char *filename,
    			unsigned int flags,
    			unsigned int mask,
    			struct statx *buffer);
    The dfd, filename and flags parameters indicate the file to query, in a
    similar way to fstatat().  There is no equivalent of lstat() as that can be
    emulated with statx() by passing AT_SYMLINK_NOFOLLOW in flags.  There is
    also no equivalent of fstat() as that can be emulated by passing a NULL
    filename to statx() with the fd of interest in dfd.
    Whether or not statx() synchronises the attributes with the backing store
    can be controlled by OR'ing a value into the flags argument (this typically
    only affects network filesystems):
     (1) AT_STATX_SYNC_AS_STAT tells statx() to behave as stat() does in this
     (2) AT_STATX_FORCE_SYNC will require a network filesystem to synchronise
         its attributes with the server - which might require data writeback to
         occur to get the timestamps correct.
     (3) AT_STATX_DONT_SYNC will suppress synchronisation with the server in a
         network filesystem.  The resulting values should be considered
    mask is a bitmask indicating the fields in struct statx that are of
    interest to the caller.  The user should set this to STATX_BASIC_STATS to
    get the basic set returned by stat().  It should be noted that asking for
    more information may entail extra I/O operations.
    buffer points to the destination for the data.  This must be 256 bytes in
    The following structures are defined in which to return the main attribute
    	struct statx_timestamp {
    		__s64	tv_sec;
    		__s32	tv_nsec;
    		__s32	__reserved;
    	struct statx {
    		__u32	stx_mask;
    		__u32	stx_blksize;
    		__u64	stx_attributes;
    		__u32	stx_nlink;
    		__u32	stx_uid;
    		__u32	stx_gid;
    		__u16	stx_mode;
    		__u16	__spare0[1];
    		__u64	stx_ino;
    		__u64	stx_size;
    		__u64	stx_blocks;
    		__u64	__spare1[1];
    		struct statx_timestamp	stx_atime;
    		struct statx_timestamp	stx_btime;
    		struct statx_timestamp	stx_ctime;
    		struct statx_timestamp	stx_mtime;
    		__u32	stx_rdev_major;
    		__u32	stx_rdev_minor;
    		__u32	stx_dev_major;
    		__u32	stx_dev_minor;
    		__u64	__spare2[14];
    The defined bits in request_mask and stx_mask are:
    	STATX_TYPE		Want/got stx_mode & S_IFMT
    	STATX_MODE		Want/got stx_mode & ~S_IFMT
    	STATX_NLINK		Want/got stx_nlink
    	STATX_UID		Want/got stx_uid
    	STATX_GID		Want/got stx_gid
    	STATX_ATIME		Want/got stx_atime{,_ns}
    	STATX_MTIME		Want/got stx_mtime{,_ns}
    	STATX_CTIME		Want/got stx_ctime{,_ns}
    	STATX_INO		Want/got stx_ino
    	STATX_SIZE		Want/got stx_size
    	STATX_BLOCKS		Want/got stx_blocks
    	STATX_BASIC_STATS	[The stuff in the normal stat struct]
    	STATX_BTIME		Want/got stx_btime{,_ns}
    	STATX_ALL		[All currently available stuff]
    stx_btime is the file creation time, stx_mask is a bitmask indicating the
    data provided and __spares*[] are where as-yet undefined fields can be
    Time fields are structures with separate seconds and nanoseconds fields
    plus a reserved field in case we want to add even finer resolution.  Note
    that times will be negative if before 1970; in such a case, the nanosecond
    fields will also be negative if not zero.
    The bits defined in the stx_attributes field convey information about a
    file, how it is accessed, where it is and what it does.  The following
    attributes map to FS_*_FL flags and are the same numerical value:
    	STATX_ATTR_COMPRESSED		File is compressed by the fs
    	STATX_ATTR_IMMUTABLE		File is marked immutable
    	STATX_ATTR_APPEND		File is append-only
    	STATX_ATTR_NODUMP		File is not to be dumped
    	STATX_ATTR_ENCRYPTED		File requires key to decrypt in fs
    Within the kernel, the supported flags are listed by:
    [Are any other IOC flags of sufficient general interest to be exposed
    through this interface?]
    New flags include:
    	STATX_ATTR_AUTOMOUNT		Object is an automount trigger
    These are for the use of GUI tools that might want to mark files specially,
    depending on what they are.
    Fields in struct statx come in a number of classes:
     (0) stx_dev_*, stx_blksize.
         These are local system information and are always available.
     (1) stx_mode, stx_nlinks, stx_uid, stx_gid, stx_[amc]time, stx_ino,
         stx_size, stx_blocks.
         These will be returned whether the caller asks for them or not.  The
         corresponding bits in stx_mask will be set to indicate whether they
         actually have valid values.
         If the caller didn't ask for them, then they may be approximated.  For
         example, NFS won't waste any time updating them from the server,
         unless as a byproduct of updating something requested.
         If the values don't actually exist for the underlying object (such as
         UID or GID on a DOS file), then the bit won't be set in the stx_mask,
         even if the caller asked for the value.  In such a case, the returned
         value will be a fabrication.
         Note that there are instances where the type might not be valid, for
         instance Windows reparse points.
     (2) stx_rdev_*.
         This will be set only if stx_mode indicates we're looking at a
         blockdev or a chardev, otherwise will be 0.
     (3) stx_btime.
         Similar to (1), except this will be set to 0 if it doesn't exist.
    The following test program can be used to test the statx system call:
    Just compile and run, passing it paths to the files you want to examine.
    The file is built automatically if CONFIG_SAMPLES is enabled.
    Here's some example output.  Firstly, an NFS directory that crosses to
    another FSID.  Note that the AUTOMOUNT attribute is set because transiting
    this directory will cause d_automount to be invoked by the VFS.
    	[root@andromeda ~]# /tmp/test-statx -A /warthog/data
    	statx(/warthog/data) = 0
    	  Size: 4096            Blocks: 8          IO Block: 1048576  directory
    	Device: 00:26           Inode: 1703937     Links: 125
    	Access: (3777/drwxrwxrwx)  Uid:     0   Gid:  4041
    	Access: 2016-11-24 09:02:12.219699527+0000
    	Modify: 2016-11-17 10:44:36.225653653+0000
    	Change: 2016-11-17 10:44:36.225653653+0000
    	Attributes: 0000000000001000 (-------- -------- -------- -------- -------- -------- ---m---- --------)
    Secondly, the result of automounting on that directory.
    	[root@andromeda ~]# /tmp/test-statx /warthog/data
    	statx(/warthog/data) = 0
    	  Size: 4096            Blocks: 8          IO Block: 1048576  directory
    	Device: 00:27           Inode: 2           Links: 125
    	Access: (3777/drwxrwxrwx)  Uid:     0   Gid:  4041
    	Access: 2016-11-24 09:02:12.219699527+0000
    	Modify: 2016-11-17 10:44:36.225653653+0000
    	Change: 2016-11-17 10:44:36.225653653+0000
    Signed-off-by: 's avatarDavid Howells <dhowells@redhat.com>
    Signed-off-by: 's avatarAl Viro <viro@zeniv.linux.org.uk>