Re: [Syzkaller & bisect] There is BUG: unable to handle kernel NULL pointer dereference in xfs_extent_free_diff_items in v6.4-rc3

[Pengfei Xu <pengfei.xu@intel.com>]

On 2023-05-24 at 07:52:59 +1000, Dave Chinner wrote:
> On Tue, May 23, 2023 at 05:14:24PM +0800, Pengfei Xu wrote:
> >   I did not do well in two points, which led to the problem of this useless
> >   bisect info:
> >   1. Should double check "V4 Filesystem" related issue carefully, and should
> >      give reason of problem.
> >   2. Double check the bisect bad and good dmesg info, this time actually
> >      "good(actually not good)" dmesg also contains "BUG" related
> >      dmesg, but it doesn't contain the keyword "xfs_extent_free_diff_items"
> >      dmesg info, and give the wrong bisect info.
> >      Sorry for inconvenience...
> 
> I think you misunderstand.
> 
> The bisect you did was correct - the commit it
> identified was certainly does expose the underlying issue.
> 
> The reason the bisect, while correct, is actually useless is that it
> the underlying issue that the commit tripped over is not caused by
> the change in the commit. The underlying issue has been there for a
> long while - probably a decade - and it's that old, underlying issue
> that has caused the new code to fail.
> 
> IOWs, the problem is not the new code (i.e. it is not a regression
> in the new code identified by the bisect), the problem is in other
> code that has been silently propagating undetected corruption for
> years. Hence the bisect is not actually useful in diagnosing the
> root cause of the problem.
> 
  Thanks a lot Dave's description! It's clear.
  Anyway I will remove "CONFIG_XFS_SUPPORT_V4" in syzkaller fuzzing test
  next time to avoid the noise.

  Thanks also to Eric Biggers, Bagas Sanjaya and all community's help!

  Thanks!
  BR.

> Cheers,
> 
> Dave.
> -- 
> Dave Chinner
> david@fromorbit.com