Re: [PATCH net-next] net: Silence false field-spanning write warning in ip_tunnel_info_opts_set() memcpy

[Gal Pressman <gal@nvidia.com>]

On 08/01/2025 1:28, Kees Cook wrote:
> 
> 
> On January 7, 2025 8:55:09 AM PST, Gal Pressman <gal@nvidia.com> wrote:
>> When metadata_dst struct is allocated (using metadata_dst_alloc()), it
>> reserves room for options at the end of the struct.
>>
>> Similar to [1], change the memcpy() to unsafe_memcpy() as it is
>> guaranteed that enough room (md_size bytes) was allocated and the
>> field-spanning write is intentional.
> 
> Why not just add an "options" flex array to struct ip_tunnel_info?
> 
> E.g.:
> 
> struct ip_tunnel_info {
> 	struct ip_tunnel_key	key;
> 	struct ip_tunnel_encap	encap;
> #ifdef CONFIG_DST_CACHE
> 	struct dst_cache	dst_cache;
> #endif
> 	u8			options_len;
> 	u8			mode;
>   u8   options[] __counted_by(options_len);
> };
> 
>>
>> 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. :)
> 
> -Kees
> 
> 

Thanks for the review Kees, I'll take a look into that.