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

[Dmitry Vyukov]

.On Sat, May 18, 2019 at 6:21 PM Al Viro <viro@xxxxxxxxxxxxxxxxxx> 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@xxxxxxxxxx> wrote:
> > >
> > > On Fri, May 17, 2019 at 3:48 PM Al Viro <viro@xxxxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxxxxxxxxx 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@xxxxxxxxxx>
> 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@xxxxxxxxxx>
> 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.  Diff of those two contains (among other things) this:
> @@ -985,6 +989,9 @@
>  +      fc = vfs_new_fs_context(path->dentry->d_sb->s_type,
>  +                              path->dentry, sb_flags, MS_RMT_MASK,
>  +                              FS_CONTEXT_FOR_RECONFIGURE);
> ++      err = PTR_ERR(fc);
> ++      if (IS_ERR(fc))
> ++              goto err;
>  +
>  +      err = parse_monolithic_mount_data(fc, data, data_size);
>  +      if (err < 0)
>
> IOW, Dan's fix folded into the offending commit.  And that kind of
> pattern is not rare; I would argue that appending Dan's patch at
> the end of queue and leaving the crap in between would be a fucking
> bad idea - it would've left a massive bisection hazard *and* made
> life much more unpleasant when the things got to merging into the
> mainline (or reviewing, for that matter).
>
> What would you prefer to happen in such situations?  Commit summaries
> modified enough to confuse CI tools into *NOT* noticing that those
> are versions of the same patch?  Some kind of metadata telling the
> same tools that such-and-such commits got folded in (and they might
> have been split in process, with parts folded into different spots
> in the series, at that)?
>
> Because "never fold in, never reorder, just accumulate patches in
> the end of the series" is not going to fly.  For a lot of reasons.

I don't advocate for stopping folding/amending/rebasing patches in any
way. I understand this is required to get sane commits.

But what I said in the previous email still applies:
 - either include the tag into the first commit version that fixes the
reported bug
 - or link report and the fixing commit manually using the final commit title

As far as I understand in this case it would be adding Tested-by (or
some other tag) to f18edd10d3c7d6127b1fa97c8f3299629cf58ed5.
We can't do this now, so this should work:

#syz fix:
vfs: Implement a filesystem superblock creation/configuration context