linux-kernel - Re: BUG: unable to handle kernel paging request in do

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Message-ID: <CACT4Y+Z8760uYQP0jKgJmVC5sstqTv9pE6K6YjK_feeK6-Obfg@mail.gmail.com>
Date:   Fri, 17 May 2019 16:08:35 +0200
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     syzbot <syzbot+73c7fe4f77776505299b@...kaller.appspotmail.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>, sabin.rapan@...il.com,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>
Subject: Re: BUG: unable to handle kernel paging request in do_mount

On Fri, May 17, 2019 at 3:48 PM Al Viro <viro@...iv.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@...kaller.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.