netdev - Re: [PATCH net] net: dsa: ksz: fix padding size of skb

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Date:   Wed, 14 Oct 2020 20:31:03 +0300
From:   Vladimir Oltean <olteanv@...il.com>
To:     Christian Eggers <ceggers@...i.de>
Cc:     Woojung Huh <woojung.huh@...rochip.com>,
        Andrew Lunn <andrew@...n.ch>,
        Vivien Didelot <vivien.didelot@...il.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        Microchip Linux Driver Support <UNGLinuxDriver@...rochip.com>,
        "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, Kurt Kanzenbach <kurt@...utronix.de>
Subject: Re: [PATCH net] net: dsa: ksz: fix padding size of skb

On Wed, Oct 14, 2020 at 07:02:13PM +0200, Christian Eggers wrote:
> > Otherwise said, the frame must be padded to
> > max(skb->len, ETH_ZLEN) + tail tag length.
> At first I thought the same when working on this. But IMHO the padding must
> only ensure the minimum required size, there is no need to pad to the "real"
> size of the skb. The check for the tailroom above ensures that enough memory
> for the "real" size is available.

Yes, that's right, that's the current logic, but what's the point of
your patch, then, if the call to __skb_put_padto is only supposed to
ensure ETH_ZLEN length?
In fact, __skb_put_padto fundamentally does:
- an extension of skb->len to the requested argument, via __skb_put
- a zero-filling of the extra area
So if you include the length of the tag in the call to __skb_put_padto,
then what's the other skb_put() from ksz8795_xmit, ksz9477_xmit,
ksz9893_xmit going to do? Aren't you increasing the frame length twice
by the length of one tag when you are doing this? What problem are you
actually trying to solve?
Can you show a skb_dump(KERN_ERR, skb, true) before and after your change?