[TCP]: Fix TCP header misalignment

[TCP]: Fix TCP header misalignment

[Herbert Xu]

Hi Dave:

[TCP]: Fix TCP header misalignment

Indeed my previous change to alloc_pskb has made it possible
for the TCP header to be misaligned iff the MTU is not a multiple
of 4 (and less than a page).  So I suspect the optimised IPsec
MTU calculation is giving you just such an MTU :)

This patch fixes it by changing alloc_pskb to make sure that
the size is at least 32-bit aligned.  This does not cause the
problem fixed by the previous patch because max_header is always
32-bit aligned which means that in the SG/NOTSO case this will
be a no-op.

I thought about putting this in the callers but all the current
callers are from TCP.  If and when we get a non-TCP caller we
can always create a TCP wrapper for this function and move the
alignment over there.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

Cheers,
-- 
Visit Openswan at http://www.openswan.org/
Email: Herbert Xu ~{PmV>HI~} <herbert@gondor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
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--
diff --git a/include/net/sock.h b/include/net/sock.h
index 567e468..6f67b91 100644
--- a/include/net/sock.h
+++ b/include/net/sock.h
@@ -1236,6 +1236,9 @@ static inline struct sk_buff *sk_stream_alloc_pskb(struct sock *sk,
 {
 	struct sk_buff *skb;
 
+	/* The TCP header must be at least 32-bit aligned.  */
+	size = ALIGN(size, 4);
+
 	skb = alloc_skb_fclone(size + sk->sk_prot->max_header, gfp);
 	if (skb) {
 		skb->truesize += mem;

Re: [TCP]: Fix TCP header misalignment

[David Miller]

From: Herbert Xu <herbert@gondor.apana.org.au>
Date: Sun, 18 Nov 2007 20:15:30 +0800

> [TCP]: Fix TCP header misalignment
> 
> Indeed my previous change to alloc_pskb has made it possible
> for the TCP header to be misaligned iff the MTU is not a multiple
> of 4 (and less than a page).  So I suspect the optimised IPsec
> MTU calculation is giving you just such an MTU :)
> 
> This patch fixes it by changing alloc_pskb to make sure that
> the size is at least 32-bit aligned.  This does not cause the
> problem fixed by the previous patch because max_header is always
> 32-bit aligned which means that in the SG/NOTSO case this will
> be a no-op.
> 
> I thought about putting this in the callers but all the current
> callers are from TCP.  If and when we get a non-TCP caller we
> can always create a TCP wrapper for this function and move the
> alignment over there.
> 
> Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

I'm getting this without IPSEC FWIW.  I believe it occurs when I talk
to sites with sub-1500 MTUs.

I'll test your patch and let you know how it goes, thanks.

Re: [TCP]: Fix TCP header misalignment

[Herbert Xu]

David Miller <davem@davemloft.net> wrote:
>
> I'm getting this without IPSEC FWIW.  I believe it occurs when I talk
> to sites with sub-1500 MTUs.

It is possible to get this with any MTU that's not a multiple
of 4.  I was basing my comment above on the fact that such MTUs
are usually rare.  However, perhaps that is no longer the case
on today's Internet :)

Cheers,
-- 
Visit Openswan at http://www.openswan.org/
Email: Herbert Xu ~{PmV>HI~} <herbert@gondor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt