Re: BUG: unable to handle kernel paging request in do_mount

From: Dmitry Vyukov <dvyukov@google.com>
To: Al Viro <viro@zeniv.linux.org.uk>
Cc: syzbot <syzbot+73c7fe4f77776505299b@syzkaller.appspotmail.com>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	LKML <linux-kernel@vger.kernel.org>,
	sabin.rapan@gmail.com,
	syzkaller-bugs <syzkaller-bugs@googlegroups.com>
Subject: Re: BUG: unable to handle kernel paging request in do_mount
Date: Sat, 18 May 2019 17:00:39 +0200	[thread overview]
Message-ID: <CACT4Y+bQ+zW_9a3F4jY0xcAn_Hdk5yAwX2K3E38z9fttbF0SJA@mail.gmail.com> (raw)
In-Reply-To: <CACT4Y+Z8760uYQP0jKgJmVC5sstqTv9pE6K6YjK_feeK6-Obfg@mail.gmail.com>

On Fri, May 17, 2019 at 4:08 PM Dmitry Vyukov <dvyukov@google.com> wrote:
>
> On Fri, May 17, 2019 at 3:48 PM Al Viro <viro@zeniv.linux.org.uk> wrote:
> >
> > On Fri, May 17, 2019 at 03:17:02AM -0700, syzbot wrote:
> > > This bug is marked as fixed by commit:
> > > vfs: namespace: error pointer dereference in do_remount()
> > > But I can't find it in any tested tree for more than 90 days.
> > > Is it a correct commit? Please update it by replying:
> > > #syz fix: exact-commit-title
> > > Until then the bug is still considered open and
> > > new crashes with the same signature are ignored.
> >
> > Could somebody explain how the following situation is supposed to
> > be handled:
> >
> > 1) branch B1 with commits  C1, C2, C3, C4 is pushed out
> > 2) C2 turns out to have a bug, which gets caught and fixed
> > 3) fix is folded in and branch B2 with C1, C2', C3', C4' is
> > pushed out.  The bug is not in it anymore.
> > 4) B1 is left mouldering (or is entirely removed); B2 is
> > eventually merged into other trees.
> >
> > This is normal and it appears to be problematic for syzbot.
> > How to deal with that?  One thing I will *NOT* do in such
> > situations is giving up on folding the fixes in.  Bisection
> > hazards alone make that a bad idea.
>
> linux-next creates a bit of a havoc.
>
> The ideal way of handling this is including Tested-by: tag into C2'.
> Reported-by: would work too, but people suggested that Reported-by: is
> confusing in this situation because it suggests that the commit fixes
> a bug in some previous commit. Technically, syzbot now accepts any
> tag, so With-inputs-from:
> syzbot+73c7fe4f77776505299b@syzkaller.appspotmail.com would work too.
>
> At this point we obvious can't fix up C2'. For such cases syzbot
> accepts #syz fix command to associate bugs with fixes. So replying
> with "#syz fix: C2'-commit-title" should do.

What is that C2'?