Re: [PATCH v2 net-next 2/3] x86: bpf_jit_comp: support BPF_S_ANC_SECCOMP_LD_W instruction

[Eric Dumazet <eric.dumazet@gmail.com>]

On Fri, 2013-04-26 at 22:17 -0400, Xi Wang wrote:
> This patch implements the seccomp BPF_S_ANC_SECCOMP_LD_W instruction
> in x86 JIT.
> 
> Signed-off-by: Xi Wang <xi.wang@gmail.com>
> Cc: Daniel Borkmann <dborkman@redhat.com>
> Cc: Heiko Carstens <heiko.carstens@de.ibm.com>
> Cc: Will Drewry <wad@chromium.org>
> Cc: Eric Dumazet <edumazet@google.com>
> Cc: Russell King <linux@arm.linux.org.uk>
> Cc: David Laight <david.laight@aculab.com>
> Cc: "David S. Miller" <davem@davemloft.net>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Nicolas Schichan <nschichan@freebox.fr>
> ---
>  arch/x86/net/bpf_jit_comp.c | 11 +++++++++++
>  1 file changed, 11 insertions(+)
> 
> diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
> index 8898680..5f1dafb 100644
> --- a/arch/x86/net/bpf_jit_comp.c
> +++ b/arch/x86/net/bpf_jit_comp.c
> @@ -683,6 +683,17 @@ cond_branch:			f_offset = addrs[i + filter[i].jf] - addrs[i];
>  				}
>  				EMIT_COND_JMP(f_op, f_offset);
>  				break;
> +#ifdef CONFIG_SECCOMP_FILTER
> +			case BPF_S_ANC_SECCOMP_LD_W:

I would feel more comfortable if you added :

	if (seen & SEEN_DATAREF) {
		pr_err_once("SECCOMP_LD_W assertion failed\n"):
		goto out;
	}

This way, if BPF is changed in the future, but not the x86 JIT, we
can have a working kernel.

Ideally, we should add a SEEN_SKBREF to make sure rdi value can be
scratched, or you just push %rdi/pop %rdi, its only one byte
instructions.

Or completely optimize the thing and not call seccomp_bpf_load() at all.

(current would be loaded once in r9, task_pt_regs() would be loaded once
in r8)


> +				func = (u8 *)seccomp_bpf_load;
> +				t_offset = func - (image + addrs[i]);
> +				/* seccomp filters don't use %rdi, %r8, %r9
> +				 * it is safe to not save these registers
> +				 */
> +				EMIT1_off32(0xbf, K); /* mov imm32,%edi */
> +				EMIT1_off32(0xe8, t_offset); /* call seccomp_bpf_load */
> +				break;
> +#endif
>  			default:
>  				/* hmm, too complex filter, give up with jit compiler */
>  				goto out;