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

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

[Eric Dumazet]

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>
---
 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

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

[Soheil Hassas Yeganeh]

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!

> ---
>  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
>

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

[Mat Martineau]

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

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

[Paolo Abeni]

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

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!

> > > 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>

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

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

[patchwork-bot+netdevbpf]

Hello:

This patch was applied to netdev/net.git (master)
by Jakub Kicinski <kuba@kernel.org>:

On Tue,  1 Feb 2022 10:46:40 -0800 you 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.
> 
> [...]

Here is the summary with links:
  - [net] tcp: add missing tcp_skb_can_collapse() test in tcp_shift_skb_data()
    https://git.kernel.org/netdev/net/c/b67985be4009

You are awesome, thank you!
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