[Patch net] ipv4: clear all TCP_SKB_CB before passing to network layer

[Cong Wang]

From: Cong Wang <***@twopensource.com>

Probably not a big deal, but IP is not the only network protocol,
don't clear skb->cb just for IP.

Also, IPv6 header is not always defined in struct tcp_skb_cb.

Cc: Krzysztof Kolasa <***@winsoft.pl>
Cc: Eric Dumazet <***@google.com>
Signed-off-by: Cong Wang <***@twopensource.com>
Signed-off-by: Cong Wang <***@gmail.com>
---
net/ipv4/tcp_output.c | 4 +---
1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index e13d778..ee356e5 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -1005,9 +1005,7 @@ static int tcp_transmit_skb(struct sock *sk, struct sk_buff *skb, int clone_it,
/* Our usage of tstamp should remain private */
skb->tstamp.tv64 = 0;

- /* Cleanup our debris for IP stacks */
- memset(skb->cb, 0, max(sizeof(struct inet_skb_parm),
- sizeof(struct inet6_skb_parm)));
+ memset(TCP_SKB_CB(skb), 0, sizeof(*TCP_SKB_CB(skb)));

err = icsk->icsk_af_ops->queue_xmit(sk, skb, &inet->cork.fl);

[Eric Dumazet]

Post by Cong Wang
Probably not a big deal, but IP is not the only network protocol,
don't clear skb->cb just for IP.
Also, IPv6 header is not always defined in struct tcp_skb_cb.
---
net/ipv4/tcp_output.c | 4 +---
1 file changed, 1 insertion(+), 3 deletions(-)
diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index e13d778..ee356e5 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -1005,9 +1005,7 @@ static int tcp_transmit_skb(struct sock *sk, struct sk_buff *skb, int clone_it,
/* Our usage of tstamp should remain private */
skb->tstamp.tv64 = 0;
- /* Cleanup our debris for IP stacks */
- memset(skb->cb, 0, max(sizeof(struct inet_skb_parm),
- sizeof(struct inet6_skb_parm)));
+ memset(TCP_SKB_CB(skb), 0, sizeof(*TCP_SKB_CB(skb)));
err = icsk->icsk_af_ops->queue_xmit(sk, skb, &inet->cork.fl);

Usually, each layer is responsible for clearing skb->cb[] at its entry
point. Or more exactly it does not care of previous garbage.

There is no evidence your patch is needed.

I was maybe too defensive when I added this, because I wanted to make
only TCP changes.

We should instead remove the memset() in TCP and fix IP/IPv6 if
necessary.

But this should wait net-next being open.

[Cong Wang]

Post by Eric Dumazet

Post by Cong Wang
Probably not a big deal, but IP is not the only network protocol,
don't clear skb->cb just for IP.
Also, IPv6 header is not always defined in struct tcp_skb_cb.
---
net/ipv4/tcp_output.c | 4 +---
1 file changed, 1 insertion(+), 3 deletions(-)
diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index e13d778..ee356e5 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -1005,9 +1005,7 @@ static int tcp_transmit_skb(struct sock *sk, struct sk_buff *skb, int clone_it,
/* Our usage of tstamp should remain private */
skb->tstamp.tv64 = 0;
- /* Cleanup our debris for IP stacks */
- memset(skb->cb, 0, max(sizeof(struct inet_skb_parm),
- sizeof(struct inet6_skb_parm)));
+ memset(TCP_SKB_CB(skb), 0, sizeof(*TCP_SKB_CB(skb)));
err = icsk->icsk_af_ops->queue_xmit(sk, skb, &inet->cork.fl);

Usually, each layer is responsible for clearing skb->cb[] at its entry
point. Or more exactly it does not care of previous garbage.
There is no evidence your patch is needed.

This is why I said "probably not a big deal" in changelog, I never
say it fixes anything, just a cleanup.

Post by Eric Dumazet
I was maybe too defensive when I added this, because I wanted to make
only TCP changes.
We should instead remove the memset() in TCP and fix IP/IPv6 if
necessary.

That works too and sounds better.

Post by Eric Dumazet
But this should wait net-next being open.

Yeah, agreed, I will update and resend when net-next is reopen.

Thanks.