[Bug libc/15305] New: POSIX problem: pathconf/fpathconf modifies errno for XFS filesystems on Linux

[Bug libc/15305] New: POSIX problem: pathconf/fpathconf modifies errno for XFS filesystems on Linux

[licquia at linuxfoundation dot org]

http://sourceware.org/bugzilla/show_bug.cgi?id=15305

             Bug #: 15305
           Summary: POSIX problem: pathconf/fpathconf modifies errno for
                    XFS filesystems on Linux
           Product: glibc
           Version: unspecified
            Status: NEW
          Severity: normal
          Priority: P2
         Component: libc
        AssignedTo: unassigned@sourceware.org
        ReportedBy: licquia@linuxfoundation.org
                CC: drepper.fsp@gmail.com
    Classification: Unclassified


Created attachment 6950
  --> http://sourceware.org/bugzilla/attachment.cgi?id=6950
test program to demonstrate the problem

When pointed at a file on a XFS filesystem under Linux, pathconf(path,
_PC_CHOWN_RESTRICTED) attempts to open /proc/sys/fs/xfs/restrict_chown to
determine what it should do.  This open fails (because the path is not
supported on recent kernels), which modifies errno.

>From http://pubs.opengroup.org/onlinepubs/009695399/functions/pathconf.html:

"Otherwise, pathconf() or fpathconf() shall return the current variable value
for the file or directory without changing errno."

(describing non-error situations)

I've attached a test program, which takes a path to test as its argument. 
Here's the behavior I see on Fedora 18, with ext4 as the regular filesystem and
an XFS loopback filesystem on ./mountpoint:

[licquia@lflap5 lsb-bug-3763]$ ./test-pathconf-errno /etc
pathconf returned 1
errno not changed
[licquia@lflap5 lsb-bug-3763]$ ./test-pathconf-errno ./mountpoint
pathconf returned 1
errno changed!
 pre = 0
 post = 2

The behavior is identical with git HEAD using LD_PRELOAD.

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[Bug libc/15305] POSIX problem: pathconf/fpathconf modifies errno for XFS filesystems on Linux

[carlos at redhat dot com]

http://sourceware.org/bugzilla/show_bug.cgi?id=15305

Carlos O'Donell <carlos at redhat dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |carlos at redhat dot com

--- Comment #1 from Carlos O'Donell <carlos at redhat dot com> 2013-03-28 15:02:15 UTC ---
Jeff,

Thanks for submitting this. 

Take care that you're quoting POSIX 2004, and the most recent standard is 2008.
In this case the 2008 standard is still the same.

I agree that there is a problem here and we'll try to fix this for 2.18.

Siddhesh Poyarekar noted that the XFS proc file we check in glibc is now really
out of date e.g.
...
    case XFS_SUPER_MAGIC:
      /* Read the value from /proc/sys/fs/xfs/restrict_chown.  If we cannot
         read it default to assume the restriction is in place.  */
      fd = open_not_cancel_2 ("/proc/sys/fs/xfs/restrict_chown", O_RDONLY);
      if (fd != -1)
        {
          char buf[2];
          if (TEMP_FAILURE_RETRY (read_not_cancel (fd, buf, 2)) == 2
              && buf[0] >= '0' && buf[0] <= '1')
            retval = buf[0] - '0';

          close_not_cancel_no_status (fd);
        }
      break;
...
The open_not_cancel_2 fails and that sets errno when it shouldn't.

I don't think we've ever had a good test for this.

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[Bug libc/15305] POSIX problem: pathconf/fpathconf modifies errno for XFS filesystems on Linux

[siddhesh at redhat dot com]

http://sourceware.org/bugzilla/show_bug.cgi?id=15305

Siddhesh Poyarekar <siddhesh at redhat dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |ASSIGNED
                 CC|                            |siddhesh at redhat dot com
         AssignedTo|unassigned at sourceware    |siddhesh at redhat dot com
                   |dot org                     |

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[Bug libc/15305] POSIX problem: pathconf/fpathconf modifies errno for XFS filesystems on Linux

[siddhesh at redhat dot com]

http://sourceware.org/bugzilla/show_bug.cgi?id=15305

Siddhesh Poyarekar <siddhesh at redhat dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|ASSIGNED                    |RESOLVED
         Resolution|                            |FIXED

--- Comment #2 from Siddhesh Poyarekar <siddhesh at redhat dot com> 2013-04-03 06:49:32 UTC ---
Fixed in master.

commit d755bba40f880c01ced8740a26fecc85534454b9
Author: Siddhesh Poyarekar <siddhesh@redhat.com>
Date:   Wed Apr 3 10:56:45 2013 +0530

    Preserve errno across _PC_CHOWN_RESTRICTED call on XFS

    Fix BZ #15305.

    On kernel versions earlier than 2.6.29, the Linux kernel exported a
    sysctl called restrict_chown for xfs, which could be used to allow
    chown to users other than the owner.  2.6.29 removed this support,
    causing the open_not_cancel_2 to fail and thus modify errno.  The fix
    is to save and restore errno so that the caller sees it as unmodified.

    Additionally, since the code to check the sysctl is not useful on
    newer kernels, we add an ifdef so that in future the code block gets
    rmeoved completely.

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[Bug libc/15305] POSIX problem: pathconf/fpathconf modifies errno for XFS filesystems on Linux

[fweimer at redhat dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=15305

Florian Weimer <fweimer at redhat dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
              Flags|                            |security-

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