linux-kernel - Re: [PATCH net] net: gro: set the last skb->next to NULL when it get merged

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Date:   Wed, 27 Oct 2021 16:07:10 +0800
From:   Jason Xing <kerneljasonxing@...il.com>
To:     David Miller <davem@...emloft.net>, kuba@...nel.org,
        alobakin@...me, jonathan.lemon@...il.com,
        Willem de Bruijn <willemb@...gle.com>, pabeni@...hat.com,
        vvs@...tuozzo.com, cong.wang@...edance.com
Cc:     netdev <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Jason Xing <xingwanli@...ishou.com>
Subject: Re: [PATCH net] net: gro: set the last skb->next to NULL when it get merged

On Wed, Oct 27, 2021 at 3:23 PM Jason Xing <kerneljasonxing@...il.com> wrote:
>
> On Tue, Oct 26, 2021 at 9:19 PM <kerneljasonxing@...il.com> wrote:
> >
> > From: Jason Xing <xingwanli@...ishou.com>
> >
> > Setting the @next of the last skb to NULL to prevent the panic in future
> > when someone does something to the last of the gro list but its @next is
> > invalid.
> >
> > For example, without the fix (commit: ece23711dd95), a panic could happen
> > with the clsact loaded when skb is redirected and then validated in
> > validate_xmit_skb_list() which could access the error addr of the @next
> > of the last skb. Thus, "general protection fault" would appear after that.
> >
> > Signed-off-by: Jason Xing <xingwanli@...ishou.com>
> > ---
> >  net/core/skbuff.c | 1 +
> >  1 file changed, 1 insertion(+)
> >
> > diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> > index 2170bea..7b248f1 100644
> > --- a/net/core/skbuff.c
> > +++ b/net/core/skbuff.c
> > @@ -4396,6 +4396,7 @@ int skb_gro_receive(struct sk_buff *p, struct sk_buff *skb)
> >                 skb_shinfo(p)->frag_list = skb;
> >         else
> >                 NAPI_GRO_CB(p)->last->next = skb;
> > +       skb->next = NULL;
> >         NAPI_GRO_CB(p)->last = skb;
>
> Besides, I'm a little bit confused that this operation inserts the
> newest skb into the tail of the flow, so the tail of flow is the
> newest, head oldest. The patch (commit: 600adc18) introduces the flush
> of the oldest when the flow is full to lower the latency, but actually
> it fetches the tail of the flow. Do I get something wrong here? I feel

I have to update this part. The commit 600adc18 evicts and flushes the
oldest flow. But for the current kernel, when
"napi->gro_hash[hash].count >= MAX_GRO_SKBS" happens, the
gro_flush_oldest() flushes the oldest skb of one certain flow,
actually it is the newest skb because it is at the end of the list.

> it is really odd.
>
> Thanks,
> Jason
>
> >         __skb_header_release(skb);
> >         lp = p;
> > --
> > 1.8.3.1
> >