Re: Naming O_TMPFILE files

[Florian Weimer <fw@deneb.enyo.de>]

* Michael Kerrisk:

> On 09/23/2016 10:41 AM, Florian Weimer wrote:
>>> 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, could you elaborate on the "can only be linked once in 
> this way"? In my experiments, it's possible to link multiple times 
> to the O_TMPFILE file.

It's in the quoted bits above.  After the first success, subsequent
attempts at linking the same file descriptor (with AT_EMPTY_PATH or
the /proc/self/fd kludge) fail.  This is what confused me so much, and
makes the behavior seem extremely erratic.