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 21:57:40 +0800
From:   Jason Xing <kerneljasonxing@...il.com>
To:     Yunsheng Lin <linyunsheng@...wei.com>
Cc:     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,
        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 8:54 PM Jason Xing <kerneljasonxing@...il.com> wrote:
>
> On Wed, Oct 27, 2021 at 8:40 PM Yunsheng Lin <linyunsheng@...wei.com> wrote:
> >
> > On 2021/10/27 16:56, Jason Xing wrote:
> > > On Wed, Oct 27, 2021 at 4:07 PM Jason Xing <kerneljasonxing@...il.com> wrote:
> > >>
> > >> 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 seems the below is more matched with the gro_flush_oldest() instead
> > of the above code block:
> > https://elixir.bootlin.com/linux/v5.15-rc3/source/net/core/dev.c#L6118
> >
>
> What you said is the @skb->list but not the list between skbs which is
> connected by skb->next when the new incoming skb needs to get merged.
> The @skb->list->next/prev is not the same as @skb->next.
>
> > >
> > > I just submitted another patch to explain how it happens, please help
> > > me review both patches.
> > >
> > > Link: https://lore.kernel.org/lkml/20211027084944.4508-1-kerneljasonxing@gmail.com/
> > >

Emm, I think you're right, Yunsheng. The gro_flush_oldest() fetches
the list of @skb->list.
Do you think the tail of skb's next pointer should be set to NULL?

Thanks,
Jason

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