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!