tcp: fix tcp_inet6_sk() for 32bit kernels

It turns out that struct ipv6_pinfo is not located as we think.

inet6_sk_generic() and tcp_inet6_sk() disagree on 32bit kernels by 4-bytes,
because struct tcp_sock has 8-bytes alignment,
but ipv6_pinfo size is not a multiple of 8.

sizeof(struct ipv6_pinfo): 116 (not padded to 8)

I actually first coded tcp_inet6_sk() as this patch does, but thought
that "container_of(tcp_sk(sk), struct tcp6_sock, tcp)" was cleaner.

As Julian told me : Nobody should use tcp6_sock.inet6
directly, it should be accessed via tcp_inet6_sk() or inet6_sk().

This happened when we added the first u64 field in struct tcp_sock.

Fixes: 93a77c11 ("tcp: add tcp_inet6_sk() helper")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Bisected-by: Julian Anastasov <ja@ssi.bg>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/tcp_ipv6.c

@@ -93,12 +93,13 @@ static struct tcp_md5sig_key *tcp_v6_md5_do_lookup(const struct sock *sk,
 /* Helper returning the inet6 address from a given tcp socket.
  * It can be used in TCP stack instead of inet6_sk(sk).
  * This avoids a dereference and allow compiler optimizations.
+ * It is a specialized version of inet6_sk_generic().
  */
 static struct ipv6_pinfo *tcp_inet6_sk(const struct sock *sk)
 {
-	struct tcp6_sock *tcp6 = container_of(tcp_sk(sk), struct tcp6_sock, tcp);
+	unsigned int offset = sizeof(struct tcp6_sock) - sizeof(struct ipv6_pinfo);
 
-	return &tcp6->inet6;
+	return (struct ipv6_pinfo *)(((u8 *)sk) + offset);
 }
 
 static void inet6_sk_rx_dst_set(struct sock *sk, const struct sk_buff *skb)