Re: [PATCH net-next 2/3] net: icmp: add skb drop reasons to ping_queue_rcv_skb()

[Paolo Abeni]

Hello,

On Mon, 2022-03-14 at 19:32 +0800, menglong8.dong@xxxxxxxxx wrote:
> From: Menglong Dong <imagedong@xxxxxxxxxxx>
> 
> In order to get the reasons of skb drops, replace sock_queue_rcv_skb()
> used in ping_queue_rcv_skb() with sock_queue_rcv_skb_reason().
> Meanwhile, use kfree_skb_reason() instead of kfree_skb().
> 
> As we can see in ping_rcv(), 'skb' will be freed if '-1' is returned
> by ping_queue_rcv_skb(). In order to get the drop reason of 'skb',
> make ping_queue_rcv_skb() return the drop reason.
> 
> As ping_queue_rcv_skb() is used as 'ping_prot.backlog_rcv()', we can't
> change its return type. (Seems ping_prot.backlog_rcv() is not used?)
> Therefore, make it return 'drop_reason * -1' to keep the origin logic.
> 
> Signed-off-by: Menglong Dong <imagedong@xxxxxxxxxxx>
> ---
>  net/ipv4/ping.c | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/net/ipv4/ping.c b/net/ipv4/ping.c
> index 3ee947557b88..cd4eb211431a 100644
> --- a/net/ipv4/ping.c
> +++ b/net/ipv4/ping.c
> @@ -936,12 +936,13 @@ EXPORT_SYMBOL_GPL(ping_recvmsg);
>  
>  int ping_queue_rcv_skb(struct sock *sk, struct sk_buff *skb)
>  {
> +	enum skb_drop_reason reason;

Please insert an empty line between variable declaration and code.

>  	pr_debug("ping_queue_rcv_skb(sk=%p,sk->num=%d,skb=%p)\n",
>  		 inet_sk(sk), inet_sk(sk)->inet_num, skb);
> -	if (sock_queue_rcv_skb(sk, skb) < 0) {
> -		kfree_skb(skb);
> +	if (sock_queue_rcv_skb_reason(sk, skb, &reason) < 0) {
> +		kfree_skb_reason(skb, reason);
>  		pr_debug("ping_queue_rcv_skb -> failed\n");
> -		return -1;
> +		return -reason;

This changes the return value for the release callback.  Such callback
has a long and non trivial call chain via sk_backlog_rcv. 

It *should* be safe, but have you considered factoring out an
__ping_queue_rcv_skb() variant returning the full drop reason, use it
in the next patch and build ping_queue_rcv_skb() on top of such helper
to that backlog_rcv() return code will not change?

The above should additionally avoid the IMHO not so nice:

	reason = ping_queue_rcv_skb(sk, skb2) * -1;

in the next patch - it will become:

	reason = __ping_queue_rcv_skb(sk, skb2);

Thanks!

Paolo