netdev - Re: [PATCH net 1/2] net/mlx4_en: Change min MTU size to ETH_MIN

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Message-ID: <34fbdbfd-5684-af7d-0d50-b0b18312127e@gmail.com>
Date:   Tue, 4 Dec 2018 09:04:15 -0800
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     David Laight <David.Laight@...LAB.COM>,
        'Tariq Toukan' <tariqt@...lanox.com>,
        "David S. Miller" <davem@...emloft.net>
Cc:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Eran Ben Elisha <eranbe@...lanox.com>
Subject: Re: [PATCH net 1/2] net/mlx4_en: Change min MTU size to ETH_MIN_MTU



On 12/04/2018 08:59 AM, David Laight wrote:
> From: Tariq Toukan
>> Sent: 02 December 2018 12:35
>> From: Eran Ben Elisha <eranbe@...lanox.com>
>>
>> NIC driver minimal MTU size shall be set to ETH_MIN_MTU, as defined in
>> the RFC791 and in the network stack. Remove old mlx4_en only define for
>> it, which was set to wrong value.
> ...
>>
>> -	/* MTU range: 46 - hw-specific max */
>> -	dev->min_mtu = MLX4_EN_MIN_MTU;
>> +	/* MTU range: 68 - hw-specific max */
>> +	dev->min_mtu = ETH_MIN_MTU;
>>  	dev->max_mtu = priv->max_mtu;
> 
> Where does 68 come from?

Min IPv4 MTU per RFC791

> The minimum size of an ethernet packet including the mac addresses
> and CRC is 64 bytes - but that would never be an 'mtu'.
> 
> Since 64 - 46 = 18, the 46 probably excludes both MAC addresses,
> the ethertype/length and the CRC.
> This is 'sort of' the minimum mtu for an ethernet frame.
> 
> I'm not sure which values are supposed to be in dev->min/max_mtu.

I am not sure we really care, only syzkaller can possibly.