RFC Allow setting file birth time with utimensat()

From: Omar Sandoval <[email protected]>

Hi,

Since statx was added in 4.11, userspace has had an interface for reading btime (file creation time), but no way to set it. This RFC patch series adds support for changing btime with utimensat(). Patch 1 adds the VFS infrastructure, patch 2 adds the support to utimensat() with a new flag, and the rest of the patches add filesystem support; I excluded CIFS for now because I don't have a CIFS setup to test it on.

Updating btime is useful for at least a couple of use cases:

- Backup/restore programs (my motivation for this feature is btrfs send)
- File servers which interoperate with operating systems that allow updating file creation time, including Mac OS [1] and Windows [2]

I've also included a man page patch, xfs_io support, and an xfstest.

Thoughts on the implementation or the interface?

Thanks!

1: https://developer.apple.com/library/archive/documentation/System/Conceptual/ManPages_iPhoneOS/man2/setattrlist.2.html
2: https://docs.microsoft.com/en-us/windows/desktop/api/fileapi/nf-fileapi-setfiletime

Omar Sandoval (6):
fs: add btime to struct iattr
fs: add AT_UTIME_BTIME for utimensat()
Btrfs: add support for setting btime
ext4: add support for setting btime
f2fs: add support for setting btime
xfs: add support for setting btime

 fs/attr.c                      |  6 +++
 fs/btrfs/inode.c               |  2 +
 fs/btrfs/super.c               |  4 +-
 fs/ext4/inode.c                | 15 +++++-
 fs/ext4/super.c                |  2 +-
 fs/f2fs/file.c                 | 19 ++++++--
 fs/f2fs/super.c                |  2 +-
 fs/utimes.c                    | 86 +++++++++++++++++++++-------------
 fs/xfs/libxfs/xfs_format.h     |  2 +-
 fs/xfs/libxfs/xfs_log_format.h |  2 +-
 fs/xfs/xfs_iops.c              | 11 ++++-
 fs/xfs/xfs_super.c             |  2 +-
 include/linux/fs.h             |  4 ++
 include/uapi/linux/fcntl.h     |  2 +
 14 files changed, 111 insertions(+), 48 deletions(-)

2.20.1