linux-kernel - Re: [PATCH] net: macb: Truncate TX1519CNT for trailing NUL

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Message-ID: <202503111540.8EB9B4702@keescook>
Date: Tue, 11 Mar 2025 15:41:29 -0700
From: Kees Cook <kees@...nel.org>
To: Andrew Lunn <andrew@...n.ch>
Cc: Nicolas Ferre <nicolas.ferre@...rochip.com>,
	Claudiu Beznea <claudiu.beznea@...on.dev>,
	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>,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-hardening@...r.kernel.org
Subject: Re: [PATCH] net: macb: Truncate TX1519CNT for trailing NUL

On Tue, Mar 11, 2025 at 03:12:00PM +0100, Andrew Lunn wrote:
> On Mon, Mar 10, 2025 at 03:24:16PM -0700, Kees Cook wrote:
> > GCC 15's -Wunterminated-string-initialization saw that this string was
> > being truncated. Adjust the initializer so that the needed final NUL
> > character will be present.
> 
> This is where we get into the ugliness of the ethtool API for strings.
> It is not actually NUL terminated. The code uses memcpy(), see:
> 
> https://elixir.bootlin.com/linux/v6.13.6/source/drivers/net/ethernet/cadence/macb_main.c#L3193
> 
> The kAPI is that userspace provides a big buffer, and the kernel then
> copies these strings into the buffer at 32 byte offsets. There is no
> requirement for a NUL between them since they are all 32 bytes long.

Oh right! Yes, I always forget these are not NUL terminated. Okay, I
will resend this with a proper __nonstring annotation. I see the title
use in drivers/net/ethernet/cadence/macb_main.c and can confirm it's not
using C String APIs.

-- 
Kees Cook