[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@google.com>
Bisected-by: Julian Anastasov <ja@ssi.bg>
---
 net/ipv6/tcp_ipv6.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

From: Eric Dumazet <edumazet@google.com>
Date: Mon,  1 Apr 2019 03:09:20 -0700

> 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@google.com>
> Bisected-by: Julian Anastasov <ja@ssi.bg>

This looks like a net-next change, because the Fixes tag commit is only
there.  So that's where I have applied it.

Maybe we should change the name of the tcp6_sock.inet6 member if it
should never be accessed directly.

On Mon, Apr 1, 2019 at 10:22 AM David Miller <davem@davemloft.net> wrote:
>
>
> This looks like a net-next change, because the Fixes tag commit is only
> there.  So that's where I have applied it.

Oops, wrong script, yes this is a net-next change.

>
> Maybe we should change the name of the tcp6_sock.inet6 member if it
> should never be accessed directly.

Or add an __alignof__(u64) if this does not increase memory needs.

Hello,

On Mon, 1 Apr 2019, Eric Dumazet wrote:

> 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@google.com>
> Bisected-by: Julian Anastasov <ja@ssi.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)

	Thanks Eric! It is working here.

Regards

--
Julian Anastasov <ja@ssi.bg>