Re: [dpdk-dev] [RFC 0/2] gcc 10 disable stringop-overflow warning

[Stephen Hemminger <stephen@networkplumber.org>]

On Tue, 31 Mar 2020 15:08:26 +0100
Kevin Traynor <ktraynor@redhat.com> wrote:

> struct virtchnl_rss_key {
>         u16 vsi_id;
>         u16 key_len;
>         u8 key[1];         /* RSS hash key, packed bytes */
>         ^^^^^^^^^
> };
> 
> Then in iavf_configure_rss_key()
> 
> 	len = sizeof(*rss_key) + vf->vf_res->rss_key_size - 1;
>                                              ^^^^^^^^^^^^
> 	rss_key = rte_zmalloc("rss_key", len, 0);
>                   ^^^ extra space beyond key[1] allocated here
> 
> <snip>
> 	rte_memcpy(rss_key->key, vf->rss_key, vf->vf_res->rss_key_size);
> 	                    ^^^                           ^^^^^^^^^^^^
> 
> At runtime we have allocated extra space at the end of the struct for
> key, and the same size used in the malloc is also considered when
> finding the right branches in the memcpy fns. But the compiler does not
> know value of size and it simply sees there can be casts of a 1 byte key
> to 16 or 32 bytes in some branches of the memcpy fns, so gives a warning.

The standard way to do such a thing is to use an empty
array. The Linux kernel has just gone through cleaning up
all these zero length (and it this case one) array at
end of function.