linux-kernel - Re: [PATCH net-next 2/3] net: icmp: add skb drop reasons to ping_queue_rcv

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Date:   Wed, 16 Mar 2022 09:34:09 +0800
From:   Menglong Dong <menglong8.dong@...il.com>
To:     Paolo Abeni <pabeni@...hat.com>
Cc:     David Ahern <dsahern@...nel.org>, Jakub Kicinski <kuba@...nel.org>,
        David Miller <davem@...emloft.net>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        Menglong Dong <imagedong@...cent.com>,
        Eric Dumazet <edumazet@...gle.com>, Martin Lau <kafai@...com>,
        Talal Ahmad <talalahmad@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        Alexander Lobakin <alobakin@...me>, dongli.zhang@...cle.com,
        maze@...gle.com, aahringo@...hat.com, Wei Wang <weiwan@...gle.com>,
        yangbo.lu@....com, Florian Westphal <fw@...len.de>,
        tglx@...utronix.de, rpalethorpe@...e.com,
        LKML <linux-kernel@...r.kernel.org>,
        netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next 2/3] net: icmp: add skb drop reasons to ping_queue_rcv_skb()

On Tue, Mar 15, 2022 at 8:49 PM Paolo Abeni <pabeni@...hat.com> wrote:
>
> Hello,
>
> On Mon, 2022-03-14 at 19:32 +0800, menglong8.dong@...il.com wrote:
> > From: Menglong Dong <imagedong@...cent.com>
> >
> > 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@...cent.com>
> > ---
> >  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.

Ok

>
> >       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);
>

Good idea, thanks!

Menglong Dong