[PATCH] bpf, powerpc: fix jit for seccomp_data access

[Michael Ellerman]

Mark Lord <mlord at pobox.com> writes:

> I am using SECCOMP to filter syscalls on a ppc32 platform,
> and noticed that the JIT compiler was failing on the BPF
> even though the interpreter was working fine.
>
> The issue was that the compiler was missing one of the instructions
> used by SECCOMP, so here is a patch to enable JIT for that instruction.
>
> Signed-Off-By:  Mark Lord <mlord at pobox.com>

Thanks.

What is the failure mode without this patch? I assume when you have the
JIT enabled the seccomp filter fails to load entirely? Or do we have
logic to fall back to the interpreter?

Either way we should probably back port to stable. I assume this has
been broken since we originally added 32-bit support, so:

Fixes: eb84bab0fb38 ("ppc: Kconfig: Enable BPF JIT on ppc32")
Cc: stable at vger.kernel.org # v4.1+

cheers


> --- old/arch/powerpc/net/bpf_jit_comp.c 2018-02-16 14:07:01.000000000 -0500
> +++ linux/arch/powerpc/net/bpf_jit_comp.c       2018-02-20 14:41:20.805227494 -0500
> @@ -329,6 +329,9 @@ static int bpf_jit_build_body(struct bpf
>                         BUILD_BUG_ON(FIELD_SIZEOF(struct sk_buff, len) != 4);
>                         PPC_LWZ_OFFS(r_A, r_skb, offsetof(struct sk_buff, len));
>                         break;
> +               case BPF_LDX | BPF_W | BPF_ABS: /* A = *((u32 *)(seccomp_data + K)); */
> +                       PPC_LWZ_OFFS(r_A, r_skb, K);
> +                       break;
>                 case BPF_LDX | BPF_W | BPF_LEN: /* X = skb->len; */
>                         PPC_LWZ_OFFS(r_X, r_skb, offsetof(struct sk_buff, len));
>                         break;
> -- 
> Mark Lord
> Real-Time Remedies Inc.
> mlord at pobox.com