netdev - Re: [PATCH 1/2][next] UAPI: ethtool: Use __struct_group() in struct ethtool_link

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Message-ID: <2b7e1535-2d7a-4c7c-9687-9ddd42392802@embeddedor.com>
Date: Wed, 23 Oct 2024 15:30:25 -0600
From: "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To: Andrew Lunn <andrew@...n.ch>, "Gustavo A. R. Silva"
 <gustavoars@...nel.org>
Cc: Michael Chan <michael.chan@...adcom.com>,
 Andrew Lunn <andrew+netdev@...n.ch>, "David S. Miller"
 <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>,
 Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
 Potnuri Bharat Teja <bharat@...lsio.com>,
 Christian Benvenuti <benve@...co.com>, Satish Kharat <satishkh@...co.com>,
 Manish Chopra <manishc@...vell.com>, netdev@...r.kernel.org,
 linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH 1/2][next] UAPI: ethtool: Use __struct_group() in struct
 ethtool_link_settings

On 21/10/24 14:11, Andrew Lunn wrote:
>>   struct ethtool_link_settings {
>> -	__u32	cmd;
>> -	__u32	speed;
>> -	__u8	duplex;
>> -	__u8	port;
>> -	__u8	phy_address;
>> -	__u8	autoneg;
>> -	__u8	mdio_support;
>> -	__u8	eth_tp_mdix;
>> -	__u8	eth_tp_mdix_ctrl;
>> -	__s8	link_mode_masks_nwords;
>> -	__u8	transceiver;
>> -	__u8	master_slave_cfg;
>> -	__u8	master_slave_state;
>> -	__u8	rate_matching;
>> -	__u32	reserved[7];
>> +	/* New members MUST be added within the __struct_group() macro below. */
>> +	__struct_group(ethtool_link_settings_hdr, hdr, /* no attrs */,
>> +		__u32	cmd;
>> +		__u32	speed;
>> +		__u8	duplex;
>> +		__u8	port;
>> +		__u8	phy_address;
>> +		__u8	autoneg;
>> +		__u8	mdio_support;
>> +		__u8	eth_tp_mdix;
>> +		__u8	eth_tp_mdix_ctrl;
>> +		__s8	link_mode_masks_nwords;
>> +		__u8	transceiver;
>> +		__u8	master_slave_cfg;
>> +		__u8	master_slave_state;
>> +		__u8	rate_matching;
>> +		__u32	reserved[7];
>> +	);
>>   	__u32	link_mode_masks[];
> 
> Dumb C question. What are the padding rules for a union, compared to
> base types? Do we know for sure the compiler is not going pad this
> structure differently because of the union?

We've been using the struct_group() family of helpers in Linux for years,
and we haven't seen any issues with padding an alignment. So, it seems
to do its job just fine. :)

Thanks
--
Gustavo

> 
> It is however nicely constructed. The 12 __u8 making 3 32bit words, so
> we have a total of 12 32bit words, or 6 64bit words, before the
> link_mode_masks[], so i don't think padding is technically an issue,
> but it would be nice to know the C standard guarantees this.
> 
> 	Andrew