Re: Naming O_TMPFILE files

On 09/21/2016 01:50 PM, Florian Weimer wrote:

AT_EMPTY_PATH is supposed to be able to give names to files created with
O_TMPFILE unless O_EXCL was specified at creation time.

However, since this commit

commit 11a7b371b64ef39fc5fb1b6f2218eef7c4d035e3
Author: Aneesh Kumar K.V [off-list ref]
Date:   Sat Jan 29 18:43:42 2011 +0530

fs: allow AT_EMPTY_PATH in linkat(), limit that to CAP_DAC_READ_SEARCH

linkat bails out early with AT_EMPTY_PATH and !CAP_DAC_READ_SEARCH,
never looking at O_EXCL.

The /proc/self/fd kludge works for unprivileged users, but only if
*both* paths use AT_FDCWD.  It fails if the first path uses a real
descriptor for /proc/self/fd, or if the second path uses a real
descriptor for the current directory, or both.  For privileged users,
only AT_EMPTY_PATH case with an AT_FDCWD target works.

The attached test program prints under a non-privileged user:

error: linkat (fd, "", AT_FDCWD, out_name, AT_EMPTY_PATH):
  No such file or directory
error: linkat (fd, "", current_fd, out_name, AT_EMPTY_PATH):
  No such file or directory
success: linkat (AT_FDCWD, proc_name, AT_FDCWD, out_name,
AT_SYMLINK_FOLLOW)
error: linkat (AT_FDCWD, proc_name, current_fd, out_name,
AT_SYMLINK_FOLLOW):
  No such file or directory
error: linkat (proc_fd, proc_name, AT_FDCWD, out_name, AT_SYMLINK_FOLLOW):
  No such file or directory
error: linkat (proc_fd, proc_name, current_fd, out_name,
AT_SYMLINK_FOLLOW):
  No such file or directory
successes: 1, failures: 5

And under a privileged user:

success: linkat (fd, "", AT_FDCWD, out_name, AT_EMPTY_PATH)
error: linkat (fd, "", current_fd, out_name, AT_EMPTY_PATH):
  No such file or directory
error: linkat (AT_FDCWD, proc_name, AT_FDCWD, out_name, AT_SYMLINK_FOLLOW):
  No such file or directory
error: linkat (AT_FDCWD, proc_name, current_fd, out_name,
AT_SYMLINK_FOLLOW):
  No such file or directory
error: linkat (proc_fd, proc_name, AT_FDCWD, out_name, AT_SYMLINK_FOLLOW):
  No such file or directory
error: linkat (proc_fd, proc_name, current_fd, out_name,
AT_SYMLINK_FOLLOW):
  No such file or directory
successes: 1, failures: 5

(Seen on tmpfs and XFS, 4.7.x kernels.)

I double-checked with strace, and the test case does not appear to be
broken.  But the exhibited behavior is truly bizarre, and it means that
it is very difficult to give a name to an O_TMPFILE file.

The test case is broken because it does not account for the fact that an 
O_TMPFILE file can only be linked once in this way.  This is still a bit 
counter-intuitive, but it means that O_TMPFILE works.

Florian