linux-kernel - Re: [PATCH v7 net-next 1/3] filter: add Extended BPF interpreter and converter

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Date:	Sun, 09 Mar 2014 07:49:34 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Alexei Starovoitov <ast@...mgrid.com>
Cc:	"David S. Miller" <davem@...emloft.net>,
	Daniel Borkmann <dborkman@...hat.com>,
	Ingo Molnar <mingo@...nel.org>, Will Drewry <wad@...omium.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	"H. Peter Anvin" <hpa@...or.com>,
	Hagen Paul Pfeifer <hagen@...u.net>,
	Jesse Gross <jesse@...ira.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Tom Zanussi <tom.zanussi@...ux.intel.com>,
	Jovi Zhangwei <jovi.zhangwei@...il.com>,
	Eric Dumazet <edumazet@...gle.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	Pekka Enberg <penberg@....fi>,
	Arjan van de Ven <arjan@...radead.org>,
	Christoph Hellwig <hch@...radead.org>,
	linux-kernel@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: [PATCH v7 net-next 1/3] filter: add Extended BPF interpreter
 and converter

On Sat, 2014-03-08 at 15:15 -0800, Alexei Starovoitov wrote:

> +			if (BPF_SRC(fp->code) == BPF_K &&
> +			    (int)fp->k < 0) {
> +				/* extended BPF immediates are signed,
> +				 * zero extend immediate into tmp register
> +				 * and use it in compare insn
> +				 */
> +				insn->code = BPF_ALU | BPF_MOV | BPF_K;
> +				insn->a_reg = 2;
> +				insn->imm = fp->k;
> +				insn++;
> +
> +				insn->a_reg = 6;
> +				insn->x_reg = 2;
> +				bpf_src = BPF_X;
> +			} else {
> +				insn->a_reg = 6;
> +				insn->x_reg = 7;
> +				insn->imm = fp->k;
> +				bpf_src = BPF_SRC(fp->code);
> +			}
> +			/* common case where 'jump_false' is next insn */
> +			if (fp->jf == 0) {
> +				insn->code = BPF_JMP | BPF_OP(fp->code) |
> +					bpf_src;
> +				tgt = i + fp->jt + 1;
> +				EMIT_JMP;
> +				break;
> +			}
> +			/* convert JEQ into JNE when 'jump_true' is next insn */
> +			if (fp->jt == 0 && BPF_OP(fp->code) == BPF_JEQ) {
> +				insn->code = BPF_JMP | BPF_JNE | bpf_src;
> +				tgt = i + fp->jf + 1;
> +				EMIT_JMP;
> +				break;
> +			}
> +			/* other jumps are mapped into two insns: Jxx and JA */
> +			tgt = i + fp->jt + 1;
> +			insn->code = BPF_JMP | BPF_OP(fp->code) | bpf_src;
> +			EMIT_JMP;
> +
> +			insn++;
> +			insn->code = BPF_JMP | BPF_JA;
> +			tgt = i + fp->jf + 1;
> +			EMIT_JMP;
> +			break;
> +
> +		/* ldxb 4*([14]&0xf) is remaped into 3 insns */
> +		case BPF_LDX | BPF_MSH | BPF_B:
> +			insn->code = BPF_LD | BPF_ABS | BPF_B;
> +			insn->a_reg = 7;
> +			insn->imm = fp->k;
> +
> +			insn++;
> +			insn->code = BPF_ALU | BPF_AND | BPF_K;
> +			insn->a_reg = 7;
> +			insn->imm = 0xf;
> +
> +			insn++;
> +			insn->code = BPF_ALU | BPF_LSH | BPF_K;
> +			insn->a_reg = 7;
> +			insn->imm = 2;
> +			break;
> +
> +		/* RET_K, RET_A are remaped into 2 insns */
> +		case BPF_RET | BPF_A:
> +		case BPF_RET | BPF_K:
> +			insn->code = BPF_ALU | BPF_MOV |
> +				(BPF_RVAL(fp->code) == BPF_K ? BPF_K : BPF_X);
> +			insn->a_reg = 0;
> +			insn->x_reg = 6;
> +			insn->imm = fp->k;
> +
> +			insn++;
> +			insn->code = BPF_RET | BPF_K;
> +			break;


What the hell is this ?

All this magical values, like 2, 6, 7, 10.

I am afraid nobody will be able to read this but you.



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