linux-kernel - Re: [PATCH net RFC] net: Clear IFF_TX_SKB_SHARING for all Ethernet devices using skb

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Date:   Thu, 22 Oct 2020 12:59:45 -0700
From:   Xie He <xie.he.0141@...il.com>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     "David S. Miller" <davem@...emloft.net>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Neil Horman <nhorman@...driver.com>,
        John Fastabend <john.fastabend@...il.com>
Subject: Re: [PATCH net RFC] net: Clear IFF_TX_SKB_SHARING for all Ethernet
 devices using skb_padto

On Thu, Oct 22, 2020 at 8:22 AM Jakub Kicinski <kuba@...nel.org> wrote:
>
> Are most of these drivers using skb_padto()? Is that the reason they
> can't be sharing the SKB?

Yes, I think if a driver calls skb_pad / skb_padto / skb_put_padto /
eth_skb_pad, the driver can't accept shared skbs because it may modify
the skbs.

> I think the IFF_TX_SKB_SHARING flag is only used by pktgen, so perhaps
> we can make sure pktgen doesn't generate skbs < dev->min_mtu, and then
> the drivers won't pad?

Yes, I see a lot of drivers just want to pad the skb to ETH_ZLEN, or
just call eth_skb_pad. In this case, requiring the shared skb to be at
least dev->min_mtu long can solve the problem for these drivers.

But I also see some drivers that want to pad the skb to a strange
length, and don't set their special min_mtu to match this length. For
example:

drivers/net/ethernet/packetengines/yellowfin.c wants to pad the skb to
a dynamically calculated value.

drivers/net/ethernet/ti/cpsw.c, cpsw_new.c and tlan.c want to pad the
skb to macro defined values.

drivers/net/ethernet/intel/iavf/iavf_txrx.c wants to pad the skb to
IAVF_MIN_TX_LEN (17).

drivers/net/ethernet/intel/ixgbevf/ixgbevf_main.c wants to pad the skb to 17.

Another solution I can think of is to add a "skb_shared" check to
"__skb_pad", so that if __skb_pad encounters a shared skb, it just
returns an error. The driver would think this is a memory allocation
failure. This way we can ensure shared skbs are not modified.