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

[Mat Martineau <mathew.j.martineau@linux.intel.com>]

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

> On Tue, Feb 1, 2022 at 1:46 PM Eric Dumazet <eric.dumazet@gmail.com> wrote:
>>
>> 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.
>>
>> 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 <mathew.j.martineau@linux.intel.com>
>> Cc: Talal Ahmad <talalahmad@google.com>
>> Cc: Arjun Roy <arjunroy@google.com>
>> Cc: Soheil Hassas Yeganeh <soheil@google.com>
>> Cc: Willem de Bruijn <willemb@google.com>
>
> Acked-by: Soheil Hassas Yeganeh <soheil@google.com>
>
> I wish there were some packetdrill tests for MPTCP. Thank you for the fix!
>

Soheil -

I have good news, there are packetdrill tests for MPTCP:

https://github.com/multipath-tcp/packetdrill

This is still in a fork. I think Davide has talked to Neal about 
upstreaming the MPTCP changes before but there may be some code that needs 
refactoring before that could happen.


>> ---
>>  net/ipv4/tcp_input.c | 2 ++
>>  1 file changed, 2 insertions(+)
>>
>> diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
>> index dc49a3d551eb919baf5ad812ef21698c5c7b9679..bfe4112e000c09ba9d7d8b64392f52337b9053e9 100644
>> --- a/net/ipv4/tcp_input.c
>> +++ b/net/ipv4/tcp_input.c
>> @@ -1660,6 +1660,8 @@ static struct sk_buff *tcp_shift_skb_data(struct sock *sk, struct sk_buff *skb,
>>             (mss != tcp_skb_seglen(skb)))
>>                 goto out;
>>
>> +       if (!tcp_skb_can_collapse(prev, skb))
>> +               goto out;
>>         len = skb->len;
>>         pcount = tcp_skb_pcount(skb);
>>         if (tcp_skb_shift(prev, skb, pcount, len))
>> --
>> 2.35.0.rc2.247.g8bbb082509-goog
>>
>

--
Mat Martineau
Intel