Re: [PATCH net] tcp: add missing tcp_skb_can_collapse() test in tcp_shift_skb_data()

On Tue, 2022-02-01 at 12:01 -0800, Mat Martineau wrote:

On Tue, 1 Feb 2022, Soheil Hassas Yeganeh wrote:

quoted

On Tue, Feb 1, 2022 at 1:46 PM Eric Dumazet [off-list ref] wrote:

quoted

From: Eric Dumazet <edumazet@google.com>

tcp_shift_skb_data() might collapse three packets into a larger one.

P_A, P_B, P_C  -> P_ABC

Historically, it used a single tcp_skb_can_collapse_to(P_A) call,
because it was enough.

In commit 85712484110d ("tcp: coalesce/collapse must respect MPTCP extensions"),
this call was replaced by a call to tcp_skb_can_collapse(P_A, P_B)

But the now needed test over P_C has been missed.

This probably broke MPTCP.

Indeed it looks like it could cause MPTCP data stream corruption, in
case of multiple substreams, if we hit this code-path. Thanks for
catching and fixing it!

quoted

quoted

Then later, commit 9b65b17db723 ("net: avoid double accounting for pure zerocopy skbs")
added an extra condition to tcp_skb_can_collapse(), but the missing call
from tcp_shift_skb_data() is also breaking TCP zerocopy, because P_A and P_C
might have different skb_zcopy_pure() status.

Fixes: 85712484110d ("tcp: coalesce/collapse must respect MPTCP extensions")
Fixes: 9b65b17db723 ("net: avoid double accounting for pure zerocopy skbs")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Paolo Abeni <pabeni@redhat.com>
Cc: Mat Martineau <redacted>
Cc: Talal Ahmad <redacted>
Cc: Arjun Roy <redacted>
Cc: Soheil Hassas Yeganeh <redacted>
Cc: Willem de Bruijn <willemb@google.com>

Acked-by: Soheil Hassas Yeganeh <redacted>

Acked-by: Paolo Abeni <pabeni@redhat.com>

quoted

I wish there were some packetdrill tests for MPTCP. Thank you for the fix!

Do you have by chance a drill for the zero-copy case? it may help
creating the MPTCP one, too.

Thanks!

Paolo