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

Dave Chinner
2023-05-23T21:53:11Z

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.

Cheers,

Dave.
--
Dave Chinner
david@fromorbit.com