[LU-9512] Implement O_TMPFILE for Lustre

Fri, 5 May 2023 17:38:50 +0000

We should implement O_TMPFILE for Lustre. We already have a similar interface for creating volatile files for lfs_migrate(). This should be hooked into the VFS O_TMPFILE mechanism for applications to use.

From the open(2) man page in RHEL8.5:

       O_TMPFILE (since Linux 3.11)
              Create an unnamed temporary regular file.  The pathname argument
              specifies a directory; an unnamed inode will be created in that
              directory's filesystem.  Anything written to the resulting file
              will be lost when the last file descriptor is closed, unless the
              file is given a name.

              O_TMPFILE must be specified with one of O_RDWR or O_WRONLY  and,
              optionally, O_EXCL.  If O_EXCL is not specified, then linkat(2)
              can be used to link the temporary file into the filesystem,
              making it permanent, using code like the following:

                  char path[PATH_MAX];
                  fd = open("/path/to/dir", O_TMPFILE | O_RDWR,
                                            S_IRUSR | S_IWUSR);

                  /* File I/O on 'fd'... */

                  snprintf(path, PATH_MAX, "/proc/self/fd/%d", fd);
                  linkat(AT_FDCWD, path, AT_FDCWD, "/path/for/file",
                         AT_SYMLINK_FOLLOW);

              In this case, the open() mode argument determines the file
              permission mode, as with O_CREAT.

              Specifying O_EXCL in conjunction with O_TMPFILE prevents a temporary
              file from being linked into the filesystem in the above manner.
              (Note that the meaning of O_EXCL in this case is different from
              the meaning of O_EXCL otherwise.)

              There are two main use cases for O_TMPFILE:

              *  Improved tmpfile(3) functionality: race-free creation of
                 temporary files that (1) are automatically deleted when closed;
                 (2) can never be reached via any pathname; (3) are not subject
                 to symlink attacks; and (4) do not require the caller to devise
                 unique names.

              *  Creating a file that is initially invisible, which is then
                 populated with data and adjusted to have appropriate filesystem
                 attributes (fchown(2), fchmod(2), fsetxattr(2), etc.) before
                 being atomically linked into the filesystem in a fully formed
                 state (using linkat(2) as described above).

              O_TMPFILE requires support by the underlying filesystem; only a
              subset of Linux filesystems provide that support.  In the initial
              implementation, support was provided in the ext2, ext3, ext4,
              UDF, Minix, and shmem filesystems.  Support for other filesystems
              has subsequently been added as follows: XFS (Linux 3.15); Btrfs
              (Linux 3.16); F2FS (Linux 3.16); and ubifs (Linux 4.9)

Whamcloud Community JIRA

[LU-9512] Implement O_TMPFILE for Lustre