netdev - [PATCH net] tcp: fix tcp_inet6

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Date:   Mon,  1 Apr 2019 03:09:20 -0700
From:   Eric Dumazet <edumazet@...gle.com>
To:     "David S . Miller" <davem@...emloft.net>
Cc:     netdev <netdev@...r.kernel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Eric Dumazet <eric.dumazet@...il.com>,
        Julian Anastasov <ja@....bg>
Subject: [PATCH net] 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: 93a77c11ae79 ("tcp: add tcp_inet6_sk() helper")
Signed-off-by: Eric Dumazet <edumazet@...gle.com>
Bisected-by: Julian Anastasov <ja@....bg>
---
 net/ipv6/tcp_ipv6.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/net/ipv6/tcp_ipv6.c b/net/ipv6/tcp_ipv6.c
index eec814fe53b817106bc1d1eaa89dadcb96c974fa..82018bdce863165eba72e1ccf0c12ee558042ae8 100644
--- a/net/ipv6/tcp_ipv6.c
+++ b/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)
-- 
2.21.0.392.gf8f6787159e-goog