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

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Message-ID: <53721db6-f4b1-4394-ab2a-045f214bd2fa@lunn.ch>
Date: Mon, 21 Oct 2024 22:11:50 +0200
From: Andrew Lunn <andrew@...n.ch>
To: "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

>  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?

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