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Date:   Sun, 11 Dec 2022 09:38:54 +0000
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Eric Dumazet <edumazet@google.com>
Cc:     syzbot <syzbot+d43608d061e8847ec9f3@syzkaller.appspotmail.com>,
        davem@davemloft.net, jmaloy@redhat.com, kuba@kernel.org,
        linux-kernel@vger.kernel.org, netdev@vger.kernel.org,
        pabeni@redhat.com, syzkaller-bugs@googlegroups.com,
        tipc-discussion@lists.sourceforge.net, ying.xue@windriver.com
Subject: Re: [syzbot] WARNING in _copy_from_iter
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Sender: Al Viro <viro@ftp.linux.org.uk>
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On Thu, Dec 08, 2022 at 08:38:14PM +0100, Eric Dumazet wrote:

> Exposes an old bug in tipc ?
> 
> Seems a new check added by Al in :
> 
> Author: Al Viro <viro@zeniv.linux.org.uk>
> Date:   Thu Sep 15 20:11:15 2022 -0400
> 
>     iov_iter: saner checks for attempt to copy to/from iterator
> 
>     instead of "don't do it to ITER_PIPE" check for ->data_source being
>     false on copying from iterator.  Check for !->data_source for
>     copying to iterator, while we are at it.
> 
>     Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

Lovely...  zero-length sendmsg with uninitialized ->msg_data...

	I would probably argue that it's a bug in tipc_connect(),
fixed by iov_iter_kvec(&m.msg_iter, ITER_SOURCE, NULL, 0, 0);
in there.  Depends - if that kind of uninitialized msg_iter used
as zero length source or zero length destination is a frequent pattern,
might as well make zero-byte copy_...iter() succeed quietly;
I hope it isn't, but that's definitely something I'd missed
when doing that series.

	I'll take a look tomorrow^Win the morning, after I get
some sleep...