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Date: Sat, 17 Mar 2012 06:49:44 -0700
From: Eric Dumazet <>
To: Indan Zupancic <>
Cc: Will Drewry <>,,,,,,,,,,,,,,,,,,,,,,,,,,
Subject: Re: [PATCH v14 01/13] sk_run_filter: add BPF_S_ANC_SECCOMP_LD_W

Le samedi 17 mars 2012 à 21:14 +1100, Indan Zupancic a écrit :
> On Wed, March 14, 2012 19:05, Eric Dumazet wrote:
> > Le mercredi 14 mars 2012 à 08:59 +0100, Indan Zupancic a écrit :
> >
> >> The only remaining question is, is it worth the extra code to release
> >> up to 32kB of unused memory? It seems a waste to not free it, but if
> >> people think it's not worth it then let's just leave it around.
> >
> > Quite frankly its not an issue, given JIT BPF is not yet default
> > enabled.
> And what if assuming JIT BPF would be default enabled?

OK, so here are the reasons why I chose not doing this :

1) When I wrote this code, I _wanted_ keeping the original BPF around
for post morterm analysis. When we are 100% confident code is bug free,
we might remove the "BPF source code", but I am not convinced.

2) Most filters are less than 1 Kbytes, and who run thousands of BPF
network filters on a machine ? Do you have real cases ? Because in these
cases, the vmalloc() PAGE granularity might be a problem anyway.

Some filters are setup for a very short period of time...
(tcpdump for example setup a "ret 0" at the very beginning of a capture
). Doing the extra kmalloc()/copy/kfree() is a loss.

tcpdump -n -s 0 -c 1000 arp

[29211.083449] JIT code: ffffffffa0cbe000: 31 c0 c3
[29211.083481] flen=4 proglen=55 pass=3 image=ffffffffa0cc0000
[29211.083487] JIT code: ffffffffa0cc0000: 55 48 89 e5 48 83 ec 60 48 89 5d f8 44 8b 4f 68
[29211.083494] JIT code: ffffffffa0cc0010: 44 2b 4f 6c 4c 8b 87 e0 00 00 00 be 0c 00 00 00
[29211.083500] JIT code: ffffffffa0cc0020: e8 04 32 38 e0 3d 06 08 00 00 75 07 b8 ff ff 00
[29211.083506] JIT code: ffffffffa0cc0030: 00 eb 02 31 c0 c9 c3

> The current JIT doesn't handle negative offsets: The stuff that's handled
> by __load_pointer(). Easiest solution would be to make it non-static and
> call it instead of doing bpf_error. I guess __load_pointer was added later
> and the JIT code didn't get updated.

I dont think so, check git history if you want :)

> But gcc refuses to inline load_pointer, instead it inlines __load_pointer
> and does the important checks first. Considering the current assembly code
> does a call too, it could as well call load_pointer() directly. That would
> save a lot of assembly code, handle all negative cases too and be pretty
> much the same speed. The only question is if this slow down some other
> archs than x86. What do you think?

You miss the point : 99.999 % of offsets are positive in filters.

Best is to not call load_pointer() and only call skb_copy_bits() if the
data is not in skb head, but in some fragment.

I dont know, I never had to use negative offsets in my own filters.
So in the BPF JIT I said : If we have a negative offset in a filter,
just disable JIT code completely for this filter (lines 478-479).

Same for fancy instructions like BPF_S_ANC_NLATTR /

Show me a real use first.

I am pragmatic : I spend time coding stuff if there is a real need.

> The EMIT_COND_JMP(f_op, f_offset); should be in an else case, otherwise
> it's superfluous. It's a harmless bug though. I haven't spotted anything
> else yet.

Its not superflous, see my comment at the end of this mail.

> You can get rid of all the "if (is_imm8(offsetof(struct sk_buff, len)))"
> code by making sure everything is near: Somewhere at the start, just
> add 127 to %rdi and a BUILD_BUG_ON(sizeof(struct sk_buff) > 255).

This code is optimized away by the compiler, you know that ?

Adding "add 127 to rdi" is one more instruction, adding dependencies and
making out slow path code more complex (calls to skb_copy_bits() in
bpf_jit.S ...). Thats a bad idea.

> diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
> index 7c1b765..7e0f575 100644
> --- a/arch/x86/net/bpf_jit_comp.c
> +++ b/arch/x86/net/bpf_jit_comp.c
> @@ -581,8 +581,9 @@ cond_branch:			f_offset = addrs[i + filter[i].jf] - addrs[i];
>  					if (filter[i].jf)
>  						EMIT_JMP(f_offset);
>  					break;
> +				} else {
> +					EMIT_COND_JMP(f_op, f_offset);
>  				}
> -				EMIT_COND_JMP(f_op, f_offset);
>  				break;
>  			default:
>  				/* hmm, too complex filter, give up with jit compiler */

I see no change in your patch in the code generation.

if (filter[i].jt == 0), we want to EMIT_COND_JMP(f_op, f_offset);
because we know at this point that filter[i].jf != 0) [ line 536 ]

if (filter[i].jt != 0), the break; in line 583 prevents the
EMIT_COND_JMP(f_op, f_offset);

Thanks !

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