linux-kernel - Re: [PATCH net] net: dsa: ksz: don't pad a cloned sk

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Message-ID: <20201016140634.acu6v5igxih2xukf@skbuf>
Date:   Fri, 16 Oct 2020 17:06:34 +0300
From:   Vladimir Oltean <olteanv@...il.com>
To:     Andrew Lunn <andrew@...n.ch>
Cc:     Christian Eggers <ceggers@...i.de>,
        Woojung Huh <woojung.huh@...rochip.com>,
        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
Subject: Re: [PATCH net] net: dsa: ksz: don't pad a cloned sk_buff

On Fri, Oct 16, 2020 at 04:00:36PM +0200, Andrew Lunn wrote:
> On Fri, Oct 16, 2020 at 09:35:27AM +0200, Christian Eggers wrote:
> > If the supplied sk_buff is cloned (e.g. in dsa_skb_tx_timestamp()),
> > __skb_put_padto() will allocate a new sk_buff with size = skb->len +
> > padlen. So the condition just tested for (skb_tailroom(skb) >= padlen +
> > len) is not fulfilled anymore. Although the real size will usually be
> > larger than skb->len + padlen (due to alignment), there is no guarantee
> > that the required memory for the tail tag will be available
> > 
> > Instead of letting __skb_put_padto allocate a new (too small) sk_buff,
> > lets take the already existing path and allocate a new sk_buff ourself
> > (with sufficient size).
> 
> Hi Christian
> 
> What is not clear to me is why not change the __skb_put_padto() call
> to pass the correct length?

There is a second call to skb_put that increases the skb->len further
from the tailroom area. See Christian's other patch.

I would treat this patch as "premature" until we fully understand what's
going on there.