netdev - Re: [PATCH net 2/3] net: compound page support in skb_seq

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Message-ID: <CAF=yD-Ko7_rkN-pgc5-3qXeBmwNMObD4igWjQfcFVVYY5ES2JQ@mail.gmail.com>
Date:   Sat, 9 Jan 2021 17:09:03 -0500
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Network Development <netdev@...r.kernel.org>
Cc:     David Miller <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Willem de Bruijn <willemb@...gle.com>
Subject: Re: [PATCH net 2/3] net: compound page support in skb_seq_read

On Fri, Jan 8, 2021 at 12:11 PM Willem de Bruijn
<willemdebruijn.kernel@...il.com> wrote:
>
> From: Willem de Bruijn <willemb@...gle.com>
>
> skb_seq_read iterates over an skb, returning pointer and length of
> the next data range with each call.
>
> It relies on kmap_atomic to access highmem pages when needed.
>
> An skb frag may be backed by a compound page, but kmap_atomic maps
> only a single page. There are not enough kmap slots to always map all
> pages concurrently.
>
> Instead, if kmap_atomic is needed, iterate over each page.
>
> As this increases the number of calls, avoid this unless needed.
> The necessary condition is captured in skb_frag_must_loop.
>
> I tried to make the change as obvious as possible. It should be easy
> to verify that nothing changes if skb_frag_must_loop returns false.
>
> Tested:
>   On an x86 platform with
>     CONFIG_HIGHMEM=y
>     CONFIG_DEBUG_KMAP_LOCAL_FORCE_MAP=y
>     CONFIG_NETFILTER_XT_MATCH_STRING=y
>
>   Run
>     ip link set dev lo mtu 1500
>     iptables -A OUTPUT -m string --string 'badstring' -algo bm -j ACCEPT
>     dd if=/dev/urandom of=in bs=1M count=20
>     nc -l -p 8000 > /dev/null &
>     nc -w 1 -q 0 localhost 8000 < in
>
> Signed-off-by: Willem de Bruijn <willemb@...gle.com>
> ---
>  include/linux/skbuff.h |  1 +
>  net/core/skbuff.c      | 28 +++++++++++++++++++++++-----
>  2 files changed, 24 insertions(+), 5 deletions(-)
>
> diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
> index c858adfb5a82..68ffd3f115c1 100644
> --- a/include/linux/skbuff.h
> +++ b/include/linux/skbuff.h
> @@ -1203,6 +1203,7 @@ struct skb_seq_state {
>         struct sk_buff  *root_skb;
>         struct sk_buff  *cur_skb;
>         __u8            *frag_data;
> +       __u16           frag_off;

frags can exceed 64k, so this needs to be __u32, like the other offsets.

I'll have to send a v2.

There's also something to be said for having a

  BUILD_BUG_ON(sizeof(struct skb_seq_state) > sizeof(skb->cb));

as it's getting close. But I won't do that in this stable fix.