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

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Date:   Wed, 14 Oct 2020 19:54:10 +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 07:47:50PM +0300, Vladimir Oltean wrote:
> On Wed, Oct 14, 2020 at 06:17:19PM +0200, Christian Eggers wrote:
> > __skb_put_padto() is called in order to ensure a minimal size of the
> > sk_buff. The required minimal size is ETH_ZLEN + the size required for
> > the tail tag.
> >
> > The current argument misses the size for the tail tag. The expression
> > "skb->len + padlen" can be simplified to ETH_ZLEN.
> >
> > Too small sk_buffs typically result from cloning in
> > dsa_skb_tx_timestamp(). The cloned sk_buff may not meet the minimum size
> > requirements.
> >
> > Fixes: e71cb9e00922 ("net: dsa: ksz: fix skb freeing")
> > Signed-off-by: Christian Eggers <ceggers@...i.de>
> > ---
> 
> Reviewed-by: Vladimir Oltean <olteanv@...il.com>

Actually no, I take that back.

This statement:

> The expression "skb->len + padlen" can be simplified to ETH_ZLEN.

is false.
skb->len + padlen == ETH_ZLEN only if skb->len is less than ETH_ZLEN.
Otherwise, skb->len + padlen == skb->len.

Otherwise said, the frame must be padded to
max(skb->len, ETH_ZLEN) + tail tag length.

So please keep the "skb->len + padlen + len".

Thanks,
-Vladimir