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

[Linus Torvalds]

On Wed, Nov 8, 2017 at 4:43 PM, Patrick McLean <chutzpah@xxxxxxxxxx> wrote:
> As of 4.13.11 (and also with 4.14-rc) we have an issue where when
> serving nfs4 sometimes we get the following BUG. When this bug happens,
> it usually also causes the motherboard to no longer POST until we
> externally re-flash the BIOS (using the BMC web interface). If a
> motherboard does not have an external way to flash the BIOS, this would
> brick the hardware.

That sounds like your BIOS is just broken.

The kernel oops is probably just a trigger for that - possibly because
you reboot with a particular state that breaks the BIOS.

Also, are you sure you really need to reflash the BIOS? It's actually
fairly hard to overwrite the BIOS itself, but crashing with bad
hardware state (where "bad" can just mean "unexpected by the BIOS")
can cause the BIOS to not properly re-initialize things, and hang at
boot.

So not booting cleanly from a warm reset is a reasonably common BIOS failure.

And yes, reflashing tends to force a full initialization and thus
"fixes" things, but it may be a big hammer when a cold boot or just a
"reset BIOS to safe defaults" might be sufficient.

In pretty much all cases this is a sign of a nasty BIOS problem,
though, and you may want to look into a firmware update from the
vendor for that.

But on to the kernel side:

> Here is the BUG we are getting:
>> [   58.962528] BUG: unable to handle kernel NULL pointer dereference at 0000000000000230
>> [   58.963918] IP: vfs_statfs+0x73/0xb0

The code disassembles to

   0: 83 c9 08              or     $0x8,%ecx
   3: 40 f6 c6 04          test   $0x4,%sil
   7: 0f 45 d1              cmovne %ecx,%edx
   a: 89 d1                mov    %edx,%ecx
   c: 80 cd 04              or     $0x4,%ch
   f: 40 f6 c6 08          test   $0x8,%sil
  13: 0f 45 d1              cmovne %ecx,%edx
  16: 89 d1                mov    %edx,%ecx
  18: 80 cd 08              or     $0x8,%ch
  1b: 40 f6 c6 10          test   $0x10,%sil
  1f: 0f 45 d1              cmovne %ecx,%edx
  22: 89 d1                mov    %edx,%ecx
  24: 80 cd 10              or     $0x10,%ch
  27: 83 e6 20              and    $0x20,%esi
  2a:* 48 8b b7 30 02 00 00 mov    0x230(%rdi),%rsi <-- trapping instruction
  31: 0f 45 d1              cmovne %ecx,%edx
  34: 83 ca 20              or     $0x20,%edx
  37: 89 f1                mov    %esi,%ecx
  39: 83 e1 10              and    $0x10,%ecx
  3c: 89 cf                mov    %ecx,%edi

and all those odd cmovne and bit-ops are just the bit selection code
in flags_by_mnt(), which is inlined through calculate_f_flags (which
is _also_ inlined) into vfs_statfs().

Sadly, gcc makes a mess of it and actually generates code that looks
like the original C. I would have hoped that gcc could have turned

   if (x & BIT)
        y |= OTHER_BIT;

into

    y |= (x & BIT) shifted-by-the-bit-difference-between BIT/OTHER_BIT;

but that doesn't happen. We actually do it by hand in some other more
critical places, but it's painful to do by hand (because the shift
direction/amount is not trivial to do in C).

Anyway, that cmovne noise makes it a bit hard to see the actual part
that matters (and that traps) but I'm almost certain that it's the
"mnt->mnt_sb->s_flags" loading that is part of calculate_f_flags()
when it then does

     flags_by_sb(mnt->mnt_sb->s_flags);

and I think mnt->mnt_sb is NULL. We know it's not 'mnt' itself that is
NULL, because we wouldn't have gotten this far if it was.

Now, afaik, mnt->mnt_sb should never be NULL in the first place for a
proper path. And the vfs_statfs() code itself hasn't changed in a
while.

Which does seem to implicate nfsd as having passed in a bad path to
vfs_statfs(). But I'm not seeing any changes in nfsd either.

In particular, there are *no* nfsd changes in that 4.13.8..4.13.11
range. There is a bunch of xfs changes, though. What's the underlying
filesystem that you are exporting?

But bringing in Al Viro and Bruce Fields explicitly in case they see
something. And Darrick, just in case it might be xfs.

         Linus