linux-kernel - Re: [PATCH v4] net: ip, diag -- Add diag interface for raw sockets

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Date:   Tue, 27 Sep 2016 08:39:59 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Cyrill Gorcunov <gorcunov@...il.com>
Cc:     David Ahern <dsa@...ulusnetworks.com>, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, David Miller <davem@...emloft.net>,
        kuznet@....inr.ac.ru, jmorris@...ei.org, yoshfuji@...ux-ipv6.org,
        kaber@...sh.net, avagin@...nvz.org, stephen@...workplumber.org
Subject: Re: [PATCH v4] net: ip, diag -- Add diag interface for raw sockets

On Tue, 2016-09-27 at 18:08 +0300, Cyrill Gorcunov wrote:

> +static struct sock *raw_sock_get(struct net *net, const struct inet_diag_req_v2 *r)
> +{
> +	struct raw_hashinfo *hashinfo = raw_get_hashinfo(r);
> +	struct sock *sk = NULL, *s;
> +	int slot;
> +
> +	if (IS_ERR(hashinfo))
> +		return ERR_CAST(hashinfo);
> +
> +	read_lock(&hashinfo->lock);
> +	for (slot = 0; slot < RAW_HTABLE_SIZE; slot++) {
> +		sk_for_each(s, &hashinfo->ht[slot]) {
> +			sk = raw_lookup(net, s, r);
> +			if (sk)
> +				break;
> +		}
> +	}
> +	if (sk && !atomic_inc_not_zero(&sk->sk_refcnt))
> +		sk = NULL;

Minor detail, but note that raw sockets do not use rcu (yet)

Since you have read_lock(&hashinfo->lock), no writer can suddenly change
sk->sk_refcnt to a zero value.

Therefore, a mere "sock_hold(sk)" should be enough to get a reference on
the socket.

Using atomic_inc_not_zero() also works, but might distract/confuse the
next guy trying to understand this code ;)



> +	read_unlock(&hashinfo->lock);
> +
> +	return sk ? sk : ERR_PTR(-ENOENT);
> +}
> +