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Subject: Re: INFO: task hung in pipe_write (2)
To:     syzbot <syzbot+3c01db6025f26530cf8d@syzkaller.appspotmail.com>,
        agruenba@redhat.com, darrick.wong@oracle.com,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
        syzkaller-bugs@googlegroups.com, viro@zeniv.linux.org.uk
References: <000000000000ac6a360592eb26c1@google.com>
From:   Rasmus Villemoes <linux@rasmusvillemoes.dk>
Message-ID: <d9a957b3-9f0a-20b5-588a-64ca4722d433@rasmusvillemoes.dk>
Date:   Thu, 19 Sep 2019 22:55:44 +0200
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On 19/09/2019 19.19, syzbot wrote:
> Hello,
> 
> syzbot found the following crash on:
> 
> HEAD commit:    288b9117 Add linux-next specific files for 20190918
> git tree:       linux-next
> console output: https://syzkaller.appspot.com/x/log.txt?x=17e86645600000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=f6126e51304ef1c3
> dashboard link:
> https://syzkaller.appspot.com/bug?extid=3c01db6025f26530cf8d
> compiler:       gcc (GCC) 9.0.0 20181231 (experimental)
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=11855769600000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=143580a1600000
> 
> The bug was bisected to:
> 
> commit cfb864757d8690631aadf1c4b80022c18ae865b3
> Author: Darrick J. Wong <darrick.wong@oracle.com>
> Date:   Tue Sep 17 16:05:22 2019 +0000
> 
>     splice: only read in as much information as there is pipe buffer space

The middle hunk (the one before splice_pipe_to_pipe()) accesses
opipe->{buffers, nrbufs}, but opipe is not locked at that point. So
maybe we end up passing len==0, which seems (once there's room in opipe)
it would put a zero-length pipe_buffer in opipe - and that probably
violates an invariant somewhere.

But does the splice_pipe_to_pipe() case even need that extra logic?
Doesn't it handle short writes correctly already?

Rasmus