Date:   Tue, 18 Dec 2018 22:40:36 +0800
From:   Ian Kent <raven@...maw.net>
To:     Dmitry Vyukov <dvyukov@...gle.com>
Cc:     Al Viro <viro@...iv.linux.org.uk>,
        syzbot <syzbot+5399ed0832693e29f392@...kaller.appspotmail.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>
Subject: Re: kernel BUG at fs/inode.c:LINE!

On Tue, 2018-12-18 at 14:19 +0100, Dmitry Vyukov wrote:
> On Tue, Dec 18, 2018 at 1:42 PM Ian Kent <raven@...maw.net> wrote:
> > 
> > On Tue, 2018-12-18 at 13:27 +0100, Dmitry Vyukov wrote:
> > > On Tue, Dec 18, 2018 at 12:35 PM Ian Kent <raven@...maw.net> wrote:
> > > > 
> > > > On Tue, 2018-12-18 at 18:42 +0800, Ian Kent wrote:
> > > > > On Mon, 2018-12-17 at 07:21 +0000, Al Viro wrote:
> > > > > > On Sun, Dec 16, 2018 at 10:11:04PM -0800, syzbot wrote:
> > > > > > > Hello,
> > > > > > > 
> > > > > > > syzbot found the following crash on:
> > > > > > > 
> > > > > > > HEAD commit:    d14b746c6c1c Add linux-next specific files for
> > > > > > > 20181214
> > > > > > > git tree:       linux-next
> > > > > > > console output:
> > > > > > > https://syzkaller.appspot.com/x/log.txt?x=13706347400000
> > > > > > > kernel config:
> > > > > > > https://syzkaller.appspot.com/x/.config?x=1da6d2d18f803140
> > > > > > > dashboard link:
> > > > > > > https://syzkaller.appspot.com/bug?extid=5399ed0832693e29f392
> > > > > > > compiler:       gcc (GCC) 8.0.1 20180413 (experimental)
> > > > > > > syz repro:
> > > > > > > https://syzkaller.appspot.com/x/repro.syz?x=101032b3400000
> > > > > > > C reproducer:
> > > > > > > https://syzkaller.appspot.com/x/repro.c?x=16534063400000
> > > > > > > 
> > > > > > > IMPORTANT: if you fix the bug, please add the following tag to the
> > > > > > > commit:
> > > > > > > Reported-by: syzbot+5399ed0832693e29f392@...kaller.appspotmail.com
> > > > > > > 
> > > > > > >  slab_pre_alloc_hook mm/slab.h:423 [inline]
> > > > > > >  slab_alloc mm/slab.c:3365 [inline]
> > > > > > >  kmem_cache_alloc+0x2c4/0x730 mm/slab.c:3539
> > > > > > >  __d_alloc+0xc8/0xb90 fs/dcache.c:1599
> > > > > > > ------------[ cut here ]------------
> > > > > > > kernel BUG at fs/inode.c:1566!
> > > > > > >  d_alloc_anon fs/dcache.c:1698 [inline]
> > > > > > >  d_make_root+0x43/0xc0 fs/dcache.c:1885
> > > > > > >  autofs_fill_super+0x6f1/0x1c30 fs/autofs/inode.c:273
> > > > > > 
> > > > > > Huh?  BUG is in iput(), AFAICS, so the stack trace is rather
> > > > > > misreported.
> > > > > > iput() can be called by d_make_root(), provided that dentry
> > > > > > allocation
> > > > > > fails.  So the most straightforward interpretation would be that we
> > > > > > had an allocation failure (injected?), followed by iput() of the
> > > > > > inode
> > > > > > passed to d_make_root().  Which happened to find I_CLEAR in
> > > > > > ->i_state
> > > > > > of that inode somehow, which should be impossible short of seriously
> > > > > > buggered inode refcounting somewhere - the inode has just been
> > > > > > returned
> > > > > > by new_inode(), which clears i_state, and it would have to have
> > > > > > passed
> > > > > > clear_inode() (i.e. has been through inode eviction) since then...
> > > > > 
> > > > > Sorry Al, that's my bad.
> > > > > 
> > > > > See
> > > > > 
> > 
> > 
https://www.ozlabs.org/~akpm/mmotm/broken-out/autofs-fix-possible-inode-leak-in-autofs_fill_super.patch
> > > > > 
> > > > > I think this will fix it, I'll forward it to Andrew if you agree:
> > > > 
> > > > Actually, looking at it again the above patch is plain not needed,
> > > > dropping it and updating the patch which follows it in the series
> > > > is what needs to be done.
> > > > 
> > > > Andrew, what should I do to make this easiest for you to handle,
> > > > a respost with v2 in the subject of the patch affected by dropping
> > > > the above patch?
> > > > 
> > > > Or I could repost the series with above patch dropped and the affected
> > > > patch corrected?
> > > 
> > > Hi Ian,
> > > 
> > > If you going to amend any commits, please add:
> > >     Tested-by: syzbot+5399ed0832693e29f392@...kaller.appspotmail.com
> > > otherwise:
> > >     Reported-by: syzbot+5399ed0832693e29f392@...kaller.appspotmail.com
> > 
> > I was thinking about how to handle loosing that information.
> > 
> > I don't think this amounts amending commits since Andrews mmotm is
> > based on patch series so dropping a patch and updating patches before
> > being merged won't capture this.
> > 
> > Adding the "Tested-by" attribution to the updated patch prior to syzbot
> > actually performing the test might be ok since it will get tested along
> > the way. Although the problem patch itself won't exist any more so ... ?
> 
> I am a bit lost.
> There should be some patch that will fix this (new or amended). Either
> that patch needs to include Reporter/Tested-by tag, or we will need to
> tell syzbot about the fix manually with the "#syz fix:" command.

One of the patches that I forwarded to Andrew was plain wrong, it isn't
actually needed so it needs to be dropped altogether (this one caused
the breakage). Doing so then requires a minor (unrelated) change to the
next of my patches in the series I sent.

Andrew forwards certain patches in his patch queue to to the linux-next
tree. I'm not sure if that is done via a cumulative pull request or via
actual patches.

Whether dropping a patch from Andrews mmotm tree when forwarding his
patch queue to linux-next requires a revert patch rather than just
omitting the dropped patch (which would carry the annotation) or not
I don't know, we'll have to wait and see what Andrew would like me to
do.

> 
> > OTOH, if I repost the series I think I can send them to syzbot for testing
> > before forwarding to Andrew (I've done something like that before but can't
> > remember how now) and add the attribution to the series.
> > 
> > But this all depends on what is best for Andrew and what Al would like to
> > see done.

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