Subject: bug#20597: ‘unlinkat’ bug in Linux 4.0.2 leads to tar test failure

From: [email-masked] (Ludovic Courtès)
Date: Sun, 24 May 2015 15:53:07 +0200

Pádraig Brady <[email-masked]> skribis:

> On 24/05/15 12:33, Ludovic Courtès wrote:

[...]

>> unlinkat(4, "foo_file", 0)              = 0
>> unlinkat(AT_FDCWD, "foo", AT_REMOVEDIR) = 0
>> unlinkat(5, "bar_file", 0)              = 0
>> unlinkat(4, "../bar", AT_REMOVEDIR)     = -1 ENOENT (No such file or directory)
>> --8<---------------cut here---------------end--------------->8---
>> 
>> Contrast this with the same thing on 4.0.4-gnu:
>> 
>> --8<---------------cut here---------------start------------->8---
>> unlinkat(4, "foo_file", 0)              = 0
>> unlinkat(AT_FDCWD, "foo", AT_REMOVEDIR) = 0
>> unlinkat(5, "bar_file", 0)              = 0
>> unlinkat(4, "../bar", AT_REMOVEDIR)     = 0
>> --8<---------------cut here---------------end--------------->8---
>> 
>> So this looks like a 4.0.2 kernel bug that Gnulib’s unlinkat should
>> perhaps work around.
>> 
>> Thoughts?
>
> Maybe. How widely deployed was 4.0.2 (It's not used in Red Hat land for example).
> How many versions was the bug present for?

I don’t know, and I haven’t been able to find traces of a fix in that
area in the kernel.

OTOH, after rereading the analysis at
<http://lists.gnu.org/archive/html/bug-tar/2014-08/msg00010.html>, it
may be that the 4.0.2 behavior is POSIX-conforming, in which case we’d
rather fix tar (or its tests) instead:

  The BSD behavior appears to be in line with POSIX.  unlinkat() with
  AT_REMOVEDIR is equivalent to rmdir(), whose specification says:

    If one or more processes have the directory open when the last
    link is removed, the dot and dot-dot entries, if present, shall
    be removed before rmdir() returns and no new entries may be created
    in the directory, but the directory shall not be removed until
    all references to the directory are closed.

  Without "..", the path resolution of the subsequent unlinkat() call
  should--or at least can--fail.

WDYT?

Thanks,
Ludo’.