netdev - Re: Endless loop in udp with MSG_SPLICE_READ - Re: [syzbot] [fs?] INFO: task hung in pipe

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Date: Mon, 31 Jul 2023 14:34:01 +0100
From: David Howells <dhowells@...hat.com>
To: Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc: dhowells@...hat.com, Jakub Kicinski <kuba@...nel.org>,
    syzbot <syzbot+f527b971b4bdc8e79f9e@...kaller.appspotmail.com>,
    bpf@...r.kernel.org, brauner@...nel.org, davem@...emloft.net,
    dsahern@...nel.org, edumazet@...gle.com,
    linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
    netdev@...r.kernel.org, pabeni@...hat.com,
    syzkaller-bugs@...glegroups.com, viro@...iv.linux.org.uk
Subject: Re: Endless loop in udp with MSG_SPLICE_READ - Re: [syzbot] [fs?] INFO: task hung in pipe_release (4)

Willem de Bruijn <willemdebruijn.kernel@...il.com> wrote:

> Is the MSG_CONFIRM needed to trigger this?

It's not actually necessary.  The syz test was doing it.

> Is this indeed trivially sidestepped if downgrading from splicing to
> regular copying with fragmentation?

That works.  The logging looks like:

==>splice_to_socket() 5535
udp_sendmsg(8,8)
__ip_append_data(copy=-1,len=8, mtu=8192 skblen=8189 maxfl=8188)
copy 8 = 9 - 0 - 1 - 0
length 8 -= 8 + 0
<==splice_to_socket() = 8

It looks like pagedlen being non-zero might be the issue.

David