Re: [nfsd4] potentially hardware breaking regression in 4.14-rc and 4.13.11

[bfields@fieldses.org (J. Bruce Fields)]

On Fri, Nov 10, 2017 at 03:26:27PM -0800, Patrick McLean wrote:
> 
> 
> On 2017-11-10 10:42 AM, Linus Torvalds wrote:
> > On Thu, Nov 9, 2017 at 5:58 PM, Patrick McLean <chutzpah@gentoo.org> wrote:
> >>
> >> Something must have changed since 4.13.8 to trigger this though.
> > 
> > Arnd pointed to some commits that might be relevant for the cp210x
> > module, but those are all already in 4.13.8, so if 4.13.8 really is
> > rock solid for you, I don't think that's it.
> > 
> > I really don't see anything that looks even half-way suspicious in
> > that 4.13.8..11 range. But as mentioned, compiler interactions can be
> > _really_ subtle.
> > 
> > And hey, it can be a real kernel bug too, that just happens to be
> > exposed by RANDSTRUCT, so a bisect really would be very nice.
> 
> I am working on bisecting the issue now, but I think I have some more
> evidence pointing to a compiler issue related to RANDSTRUCT. There are
> actually 3 issues that we have seen. Sometimes we get the null pointer
> deref in the initial message, sometimes we get the GPF, and sometimes we
> see an issue where the NFS clients see all files as root-owned
> directories.

That suggests that stat.uid is 0 and stat.mode & S_IFMT is 0040000 in
the stat structure that nfsd passed to vfs_getattr().

No idea what sort of information is useful when tracking down this kind
of bug, but you could also run wireshark and take a look at the server's
GETATTR replies to see if there's some other corruption.

--b.

> Any given kernel will always see the same issue, but after
> a "make mrproper" and recompile (with the same .config), the issue will
> often change. I suspect that all 3 of these problems are actually the
> same issue manifesting itself in different ways depending on what seed
> the RANDSTRUCT gcc plugin is using.
> 
> > 
> > Because in the end, compiler bugs are very rare. They are particularly
> > annoying when they do happen, though, so they loom big in the mind of
> > people who have had to chase them down.
> >