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Date:   Sat, 24 Nov 2018 21:44:36 +0000
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Slavomir Kaslev <kaslevs@vmware.com>
Cc:     syzbot <syzbot+ce18da013d76d837144d@syzkaller.appspotmail.com>,
        davem@davemloft.net, gregkh@linuxfoundation.org,
        kgraul@linux.ibm.com, linux-kernel@vger.kernel.org,
        netdev@vger.kernel.org, stranche@codeaurora.org,
        syzkaller-bugs@googlegroups.com
Subject: Re: WARNING in csum_and_copy_to_iter
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 <20181124200357.GC2217@ZenIV.linux.org.uk>
 <20181124211918.GA20435@box>
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On Sat, Nov 24, 2018 at 11:20:14PM +0200, Slavomir Kaslev wrote:
> On Sat, Nov 24, 2018 at 08:03:57PM +0000, Al Viro wrote:
> > On Sat, Nov 24, 2018 at 11:40:03AM -0800, syzbot wrote:
> > > Hello,
> > > 
> > > syzbot found the following crash on:
> > > 
> > > HEAD commit:    edeca3a769ad Merge tag 'sound-4.20-rc4' of git://git.kerne..
> > > git tree:       upstream
> > > console output: https://syzkaller.appspot.com/x/log.txt?x=12bee26d400000
> > > kernel config:  https://syzkaller.appspot.com/x/.config?x=73e2bc0cb6463446
> > > dashboard link: https://syzkaller.appspot.com/bug?extid=ce18da013d76d837144d
> > > compiler:       gcc (GCC) 8.0.1 20180413 (experimental)
> > > syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=15ccd1f5400000
> > > 
> > > IMPORTANT: if you fix the bug, please add the following tag to the commit:
> > > Reported-by: syzbot+ce18da013d76d837144d@syzkaller.appspotmail.com
> > 
> > Caused by commit 95506588d2c1d72ca29adef8ae9bf771bcfb4ced
> > Author: Slavomir Kaslev <kaslevs@vmware.com>
> > Date:   Fri Nov 16 11:27:53 2018 +0200
> > 
> >     socket: do a generic_file_splice_read when proto_ops has no splice_read
> > 
> > exposing all ->recvmsg() instances to pipe-backed iov_iter as possible destination.
> > It's not all that hard to fix (I'll probably have a candidate patch by tonight,
> > it's just a matter of adding the only missing primitive), but... shouldn't that
> > patch have sat in -next for at least some testing first?  Because it's very
> > easy to reproduce - splice from e.g. UDP socket will step into it.  Sure, the
> > sky is not falling (unless you set panic-on-WARN, that is); the damn thing
> > would've failed anyway, but...
> 
> My bad for not sending the patch tagged as net-next, feel free to revert it.

No point, IMO - the fix isn't hard and bisect hazard created by the whole thing
is both mild (spurious WARN() in case that used to fail anyway) _and_ won't
disappear from reverting, obviously.  I'll post a fix later tonight...