linux-kernel - Re: [PATCH Next V2] net: restore the iterator to its original state when an error occurs

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Message-ID: <20251201111557.15cb9415@kernel.org>
Date: Mon, 1 Dec 2025 11:15:57 -0800
From: Jakub Kicinski <kuba@...nel.org>
To: Edward Adam Davis <eadavis@...com>
Cc: davem@...emloft.net, edumazet@...gle.com, eperezma@...hat.com,
 horms@...nel.org, jasowang@...hat.com, kvm@...r.kernel.org,
 linux-kernel@...r.kernel.org, mst@...hat.com, netdev@...r.kernel.org,
 pabeni@...hat.com, sgarzare@...hat.com, stefanha@...hat.com,
 syzbot+ci3edb9412aeb2e703@...kaller.appspotmail.com,
 syzbot@...ts.linux.dev, syzbot@...kaller.appspotmail.com,
 syzkaller-bugs@...glegroups.com, virtualization@...ts.linux.dev,
 xuanzhuo@...ux.alibaba.com
Subject: Re: [PATCH Next V2] net: restore the iterator to its original state
 when an error occurs

On Mon,  1 Dec 2025 11:41:07 +0800 Edward Adam Davis wrote:
> On Fri, 28 Nov 2025 09:39:46 -0800, Jakub Kicinski wrote:
> > > In zerocopy_fill_skb_from_iter(), if two copy operations are performed
> > > and the first one succeeds while the second one fails, it returns a
> > > failure but the count in iterator has already been decremented due to
> > > the first successful copy. This ultimately affects the local variable
> > > rest_len in virtio_transport_send_pkt_info(), causing the remaining
> > > count in rest_len to be greater than the actual iterator count. As a
> > > result, packet sending operations continue even when the iterator count
> > > is zero, which further leads to skb->len being 0 and triggers the warning
> > > reported by syzbot [1].  
> > 
> > Please follow the subsystem guidelines for posting patches:
> > https://www.kernel.org/doc/html/next/process/maintainer-netdev.html
> > Your patch breaks zerocopy tests.  
> I see that they all timed out. I'm not familiar with this test, how can
> I get more details about it?

IIRC its was the packetdrill tests:

tools/testing/selftests/net/packetdrill/tcp_fastopen_server_basic-zero-payload.pkt
tools/testing/selftests/net/packetdrill/tcp_zerocopy_basic.pkt
tools/testing/selftests/net/packetdrill/tcp_zerocopy_batch.pkt
tools/testing/selftests/net/packetdrill/tcp_zerocopy_client.pkt
tools/testing/selftests/net/packetdrill/tcp_zerocopy_closed.pkt
tools/testing/selftests/net/packetdrill/tcp_zerocopy_epoll_edge.pkt
tools/testing/selftests/net/packetdrill/tcp_zerocopy_epoll_exclusive.pkt
tools/testing/selftests/net/packetdrill/tcp_zerocopy_epoll_oneshot.pkt
tools/testing/selftests/net/packetdrill/tcp_zerocopy_fastopen-client.pkt
tools/testing/selftests/net/packetdrill/tcp_zerocopy_fastopen-server.pkt
tools/testing/selftests/net/packetdrill/tcp_zerocopy_maxfrags.pkt
tools/testing/selftests/net/packetdrill/tcp_zerocopy_small.pkt

If you have the packetdrill command installed those _should_ be
relatively easy to run via standard kselftest commands