netdev - Re: [PATCH RFC v4 net-next 01/26] net: filter: add "load 64-bit immediate" eBPF instruction

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Date:	Wed, 13 Aug 2014 09:08:24 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Alexei Starovoitov <ast@...mgrid.com>
Cc:	"David S. Miller" <davem@...emloft.net>,
	Ingo Molnar <mingo@...nel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Daniel Borkmann <dborkman@...hat.com>,
	Chema Gonzalez <chema@...gle.com>,
	Eric Dumazet <edumazet@...gle.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	"H. Peter Anvin" <hpa@...or.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Kees Cook <keescook@...omium.org>,
	Linux API <linux-api@...r.kernel.org>,
	Network Development <netdev@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH RFC v4 net-next 01/26] net: filter: add "load 64-bit
 immediate" eBPF instruction

On Wed, Aug 13, 2014 at 12:57 AM, Alexei Starovoitov <ast@...mgrid.com> wrote:
> add BPF_LD_IMM64 instruction to load 64-bit immediate value into register.
> All previous instructions were 8-byte. This is first 16-byte instruction.
> Two consecutive 'struct bpf_insn' blocks are interpreted as single instruction:
> insn[0/1].code = BPF_LD | BPF_DW | BPF_IMM
> insn[0/1].dst_reg = destination register
> insn[0].imm = lower 32-bit
> insn[1].imm = upper 32-bit

This might be unnecessarily difficult for fancy static analysis tools
to reason about.  Would it make sense to assign two different codes
for this?  For example, insn[0].code = code_for_load_low,
insns[1].code = code_for_load_high, along with a verifier check that
they come in matched pairs and that code_for_load_high isn't a jump
target?

(Something else that I find confusing about eBPF: the instruction
mnemonics are very strange.  Have you considered giving them real
names?  For example, load.imm.low instead of BPF_LD | BPF_DW | BPF_IMM
is easier to read and pronounce.)

--Andy
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