Re: [PATCH net-next v3 2/3] net: support SO_PRIORITY cmsg

[Willem de Bruijn <willemdebruijn.kernel@gmail.com>]

Anna Emese Nyiri wrote:
> The Linux socket API currently allows setting SO_PRIORITY at the
> socket level, applying a uniform priority to all packets sent through
> that socket. The exception to this is IP_TOS, when the priority value
> is calculated during the handling of
> ancillary data, as implemented in commit <f02db315b8d88>
> ("ipv4: IP_TOS and IP_TTL can be specified as ancillary data").
> However, this is a computed
> value, and there is currently no mechanism to set a custom priority
> via control messages prior to this patch.
> 
> According to this pacth, if SO_PRIORITY is specified as ancillary data,

typo: patch

> the packet is sent with the priority value set through
> sockc->priority, overriding the socket-level values
> set via the traditional setsockopt() method. This is analogous to
> the existing support for SO_MARK, as implemented in commit
> <c6af0c227a22> ("ip: support SO_MARK cmsg").

If both cmsg SO_PRIORITY and IP_TOS are passed, then the one that
takes precedence is the last one in the cmsg list.
 
> Suggested-by: Ferenc Fejes <fejes@inf.elte.hu>
> Signed-off-by: Anna Emese Nyiri <annaemesenyiri@gmail.com>

Reviewed-by: Willem de Bruijn <willemb@google.com>

> diff --git a/net/ipv4/ip_sockglue.c b/net/ipv4/ip_sockglue.c
> index cf377377b52d..f6a03b418dde 100644
> diff --git a/net/ipv4/raw.c b/net/ipv4/raw.c
> index 0e9e01967ec9..4304a68d1db0 100644
> --- a/net/ipv4/raw.c
> +++ b/net/ipv4/raw.c
> @@ -358,7 +358,7 @@ static int raw_send_hdrinc(struct sock *sk, struct flowi4 *fl4,
>  	skb_reserve(skb, hlen);
>  
>  	skb->protocol = htons(ETH_P_IP);
> -	skb->priority = READ_ONCE(sk->sk_priority);
> +	skb->priority = sockc->priority;

This has the side effect that raw_send_hdrinc will now interpret cmsg
IP_TOS, where it previously did not (as only sockcm_cookie was passed,
not all of ipcm_cookie). This is an improvement, but good to make
explicit.