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 <[email protected]>
---
include/linux/filter.h | 6 ++----
1 file changed, 2 insertions(+), 4 deletions(-)
diff --git a/include/linux/filter.h b/include/linux/filter.h
index af37318bb1c5..73d06a39e2d6 100644
--- a/include/linux/filter.h
+++ b/include/linux/filter.h
@@ -545,10 +545,8 @@ struct bpf_prog {
unsigned int (*bpf_func)(const void *ctx,
const struct bpf_insn *insn);
/* Instructions for interpreter */
- union {
- struct sock_filter insns[0];
- struct bpf_insn insnsi[0];
- };
+ struct sock_filter insns[0];
+ struct bpf_insn insnsi[];
};
struct sk_filter {
--
2.26.0
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 <[email protected]>
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!