Re: [PATCH 05/15] bpf: avoid gcc-10 stringop-overflow warning

[Daniel Borkmann <daniel@iogearbox.net>]

On 4/30/20 11:30 PM, Arnd Bergmann wrote:
> gcc-10 warns about accesses to zero-length arrays:
> 
> kernel/bpf/core.c: In function 'bpf_patch_insn_single':
> cc1: warning: writing 8 bytes into a region of size 0 [-Wstringop-overflow=]
> In file included from kernel/bpf/core.c:21:
> include/linux/filter.h:550:20: note: at offset 0 to object 'insnsi' with size 0 declared here
>    550 |   struct bpf_insn  insnsi[0];
>        |                    ^~~~~~
> 
> In this case, we really want to have two flexible-array members,
> but that is not possible. Removing the union to make insnsi a
> flexible-array member while leaving insns as a zero-length array
> fixes the warning, as nothing writes to the other one in that way.
> 
> This trick only works on linux-3.18 or higher, as older versions
> had additional members in the union.
> 
> Fixes: 60a3b2253c41 ("net: bpf: make eBPF interpreter images read-only")
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>

Not pretty but looks okay to me, both have the same offset afterwards
in the pahole dump as well.

struct bpf_prog {
[...]
         unsigned int               (*bpf_func)(const void  *, const struct bpf_insn  *); /*    48     8 */
         struct sock_filter         insns[0];             /*    56     0 */
         struct bpf_insn            insnsi[];             /*    56     0 */

         /* size: 56, cachelines: 1, members: 21 */
         /* sum members: 50, holes: 1, sum holes: 4 */
         /* sum bitfield members: 10 bits, bit holes: 1, sum bit holes: 6 bits */
         /* last cacheline: 56 bytes */
};

Applied to bpf-next, thanks!