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SubjectRe: OOPS in nfsd, affects all 2.2 and 2.4 kernels
On Friday October 27, ajlill@ajlc.waterloo.on.ca wrote:
> This was first reported in 2.2.12, according to Deja. Solaris clients,
> on rare occaisons, will send some command to a linux server which
> causes a null resp->fh.fh_dentry to be passed to routines in
> /usr/src/linux/fs/nfsd/nfsxdr.c. This causes an oops, and then the nfs
> server subsystem stop functioning. A fix is to check that this is not
> null before de-referencing it in the following three routines. I
> looked and this bug is present in the latest 2.2 and 2.4 kernels.
>
> Whatever condition causes this is very rare. We had a linux server
> supporting 100 Solaris and HP-UX boxes running flawlessly for 8
> months, then one day something triggered this bug, and it wouldn't go
> away until I implemented this fix. There were no apparent side effects
> to doing this, although you may want to print some informative message
> to try and track down the real culprit.
>
> THis patch is against 2.2.16, but the code looks unchanged in
> 2.4.0.

Thanks for sending me this.

This problem that you are addressing is caused when solaris sends a
zero length write (I assume to implement the "access" system call, but
I haven't checked).
If you look at the code at the top of nfsd_write in nfsd/vfs.c, you
will see that if cnt (the number of bytes to write) is zero, then it
jumps straight out without setting an error or initialising fhp (by
calling nfsd_open).
As there is no error, nfsd tried to return status information (hence
the call the nfssvc_encode_attrstat) but doesn't have a valid file
handle. So it Oopses.

The correct fix, which is in 2.4 and would have recently gone into the
2.2.18 pre-patches (but I haven't actually looked at them) is to move
the test for (!cnt) to after the call to nfsd_open.

NeilBrown
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