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

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 <redacted>
Cc: Talal Ahmad <redacted>
Cc: Arjun Roy <redacted>
Cc: Soheil Hassas Yeganeh <redacted>
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

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

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

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

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.

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>

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.

quoted

---
 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

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

Hello:

This patch was applied to netdev/net.git (master)
by Jakub Kicinski [off-list ref]:

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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