[Bug 269954] Re: ssh -X breaks Xauthority on NFS mounted home dir

[Ari Mujunen]

I'm also running 8.04.1 with linux-image-2.6.24-19-generic
(2.6.24-19.41) and I can confirm this problem.

By running 'while true; do date; ls -il .Xauthority ; sleep 1; done' on
both my Ubuntu client and Debian etch NFS server (running
2.6.18-6-amd64) I can see that doing a 'ssh -X third-machine' indeed
replaces the ~/.Xauthority file in my home directory: the inode number
changes on the server but not on my Ubuntu desktop.

With the 'defaults' mount options in my /etc/fstab, my desktop continues
to show old ~/.Xauthority inode number and stat() data for 60 seconds,
then 'ls -l' starts returning 'ls: cannot access .Xauthority: Stale NFS
file handle'.  A command line 'stat .Xauthority' returns the same error.
Doing any of 'ls .', 'cat .Xauthority >/dev/null', 'touch .Xauthority'
will immediately cure this, letting 'ls -l .Xauthority' show the correct
and updated info, the same as in the NFS server.

With the 'noac' mount option in my /etc/fstab the behavior is otherwise
the same as in the above, but the 60 second timeout disappears and 'stat
.Xauthority' starts to return 'Stale NFS file handle' immediately after
'ssh -X third-machine' has replaced the file at 'third-machine' and the
NFS server.

With the 'sync' mount option in /etc/fstab the behavior is just like in
the case of 'defaults' mount option.

I find it quite likely that this has actually nothing to do with the way
OpenSSH does the updating of '.Xauthority' file (i.e. many other
applications revise files by replacing them with a new version) but the
way '~/.Xauthority' is apparently often used, by just 'stat()'ting it,
leads to exposure of this problem.  It looks to me more like a kernel
NFS client problem.

I'm a bit puzzled why 'lstat64(".Xauthority", ...)' would fail with
stale NFS handle whereas a 'open(".Xauthority", ...)' will work just
fine---why the former gets out-of-date directory info and the latter
gets the correct, updated one?

-- 
ssh -X breaks Xauthority on NFS mounted home dir
https://bugs.launchpad.net/bugs/269954
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