netdev - Re: [PATCH net-next] net: Silence false field-spanning write warning in ip_tunnel_info_opts

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Message-ID: <202501090852.AA32AF8BD@keescook>
Date: Thu, 9 Jan 2025 08:52:50 -0800
From: Kees Cook <kees@...nel.org>
To: Gal Pressman <gal@...dia.com>
Cc: "David S. Miller" <davem@...emloft.net>,
	Jakub Kicinski <kuba@...nel.org>, netdev@...r.kernel.org,
	Eric Dumazet <edumazet@...gle.com>, Paolo Abeni <pabeni@...hat.com>,
	David Ahern <dsahern@...nel.org>, Simon Horman <horms@...nel.org>,
	linux-hardening@...r.kernel.org, Cosmin Ratiu <cratiu@...dia.com>
Subject: Re: [PATCH net-next] net: Silence false field-spanning write warning
 in ip_tunnel_info_opts_set() memcpy

On Thu, Jan 09, 2025 at 11:00:24AM +0200, Gal Pressman wrote:
> On 08/01/2025 1:28, Kees Cook wrote:
> >> This resolves the following warning:
> >>  memcpy: detected field-spanning write (size 8) of single field "_Generic(info, const struct ip_tunnel_info * : ((const void *)((info) + 1)), struct ip_tunnel_info * : ((void *)((info) + 1)) )" at include/net/ip_tunnels.h:662 (size 0)
> > 
> > Then you can drop this macro and just use: info->options
> > 
> > Looks like you'd need to do it for all the types in struct metadata_dst, but at least you could stop hiding it from the compiler. :)
> 
> Can you please explain the "do it for all the types in struct
> metadata_dst" part?
> AFAICT, struct ip_tunnel_info is the only one that's extendable, I don't
> think others need to be modified.

Ah, sorry. If that's the case, then just ip_tunnel_info is fine. (Is all
of the metadata_dst trailing byte allocation logic just for
ip_tunnel_info?)

-Kees

-- 
Kees Cook