Re: [PATCH nf-next] netfilter: nf_log_syslog: no longer acquire sk_callback_lock in nf_log_dump_sk_uid_gid()

[Florian Westphal <fw@strlen.de>]

Eric Dumazet <edumazet@google.com> wrote:
> After commit 983512f3a87f ("net: Drop the lock in skb_may_tx_timestamp()")
> from Sebastian Andrzej Siewior, apply the same logic in nf_log_dump_sk_uid_gid()
> to avoid touching sk_callback_lock.
> 
> Signed-off-by: Eric Dumazet <edumazet@google.com>
> ---
>  net/netfilter/nf_log_syslog.c | 18 ++++++++++++++----
>  1 file changed, 14 insertions(+), 4 deletions(-)
> 
> diff --git a/net/netfilter/nf_log_syslog.c b/net/netfilter/nf_log_syslog.c
> index 41503847d9d7fb21824fabb1b57b45f2622b9310..dee6be17440c6c0f202fc2ba67129437879e45a6 100644
> --- a/net/netfilter/nf_log_syslog.c
> +++ b/net/netfilter/nf_log_syslog.c
> @@ -165,18 +165,28 @@ static struct nf_logger nf_arp_logger __read_mostly = {
>  static void nf_log_dump_sk_uid_gid(struct net *net, struct nf_log_buf *m,
>  				   struct sock *sk)
>  {
> +	const struct socket *sock;
> +	const struct file *file;
> +
>  	if (!sk || !sk_fullsock(sk) || !net_eq(net, sock_net(sk)))
>  		return;
>  
> -	read_lock_bh(&sk->sk_callback_lock);
> -	if (sk->sk_socket && sk->sk_socket->file) {
> -		const struct cred *cred = sk->sk_socket->file->f_cred;
> +	/* The sk pointer remains valid as long as the skb is. The sk_socket and
> +	 * file pointer may become NULL if the socket is closed. Both structures
> +	 * (including file->cred) are RCU freed which means they can be accessed
> +	 * within a RCU read section.
> +	 */
> +	rcu_read_lock();
> +	sock = READ_ONCE(sk->sk_socket);
> +	file = sock ? READ_ONCE(sock->file) : NULL;
> +	if (file) {
> +		const struct cred *cred = file->f_cred;
>  
>  		nf_log_buf_add(m, "UID=%u GID=%u ",
>  			       from_kuid_munged(&init_user_ns, cred->fsuid),
>  			       from_kgid_munged(&init_user_ns, cred->fsgid));
>  	}
> -	read_unlock_bh(&sk->sk_callback_lock);
> +	rcu_read_unlock();

Thanks Eric!  The explicit rcu_read_lock()/unlock are not required,
caller guarantees RCU read-side section.

Aside of that nit:
Reviewed-by: Florian Westphal <fw@strlen.de>