netdev - Re: [PATCH] net: dsa: tag_rtl4_a: Support also egress tags

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Date:   Wed, 17 Feb 2021 15:19:59 +0100
From:   Heiner Kallweit <hkallweit1@...il.com>
To:     Vladimir Oltean <olteanv@...il.com>,
        DENG Qingfang <dqfext@...il.com>
Cc:     Linus Walleij <linus.walleij@...aro.org>,
        Andrew Lunn <andrew@...n.ch>,
        Vivien Didelot <vivien.didelot@...il.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        netdev <netdev@...r.kernel.org>,
        Mauri Sandberg <sandberg@...lfence.com>
Subject: Re: [PATCH] net: dsa: tag_rtl4_a: Support also egress tags

On 17.02.2021 13:42, Vladimir Oltean wrote:
> On Wed, Feb 17, 2021 at 08:38:30PM +0800, DENG Qingfang wrote:
>> On Wed, Feb 17, 2021 at 7:55 AM Linus Walleij <linus.walleij@...aro.org> wrote:
>>> +
>>> +       /* Pad out to at least 60 bytes */
>>> +       if (unlikely(eth_skb_pad(skb)))
>>> +               return NULL;
>>
>> I just found that this will cause double free (eth_skb_pad will free
>> the skb if allocation fails, and dsa_slave_xmit will still try to free
>> it because it returns NULL.
>> Replace eth_skb_pad(skb) with __skb_put_padto(skb, ETH_ZLEN, false) to
>> avoid that.
> 

That's really a pitfall. I had to do the same in r8169:
cc6528bc9a0c ("r8169: fix potential skb double free in an error path")

> Good catch, Qingfang.
>