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

[Al Viro <viro@zeniv.linux.org.uk>]

On Sat, May 18, 2019 at 05:21:42PM +0100, Al Viro wrote:
> On Sat, May 18, 2019 at 05:00:39PM +0200, Dmitry Vyukov wrote:
> > 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'?
> 
> In this case?  Take a look at
> 
> commit fd0002870b453c58d0d8c195954f5049bc6675fb
> Author: David Howells <dhowells@redhat.com>
> Date:   Tue Aug 28 14:45:06 2018 +0100
> 
>     vfs: Implement a filesystem superblock creation/configuration context
> 
> and compare with
> 
> commit f18edd10d3c7d6127b1fa97c8f3299629cf58ed5
> Author: David Howells <dhowells@redhat.com>
> Date:   Thu Nov 1 23:07:25 2018 +0000
> 
>     vfs: Implement a filesystem superblock creation/configuration context
> 
> There might have been intermediate forms, but that should illustrate what
> happened.

While we are at it, even the latter form has *not* made it into the
mainline.  It got split, reordered and massaged quite a bit; the counterpart
of the code in question that went into mainline is

+       fc = fs_context_for_reconfigure(path->dentry, sb_flags, MS_RMT_MASK);
+       if (IS_ERR(fc))
+               return PTR_ERR(fc);

in commit 8d0347f6c3a9d4953ddd636a31c6584da082e084
Author: David Howells <dhowells@redhat.com>
Date:   Sun Nov 4 09:28:36 2018 -0500

    convert do_remount_sb() to fs_context