netdev - Re: [PATCH RFC net-next 08/14] bpf: add eBPF verifier

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Message-ID: <CAMEtUuw0y+r4v8vFBy=-idhe_tDDga34siF7+SCgRD+H-qamtg@mail.gmail.com>
Date:	Wed, 2 Jul 2014 17:01:24 -0700
From:	Alexei Starovoitov <ast@...mgrid.com>
To:	Chema Gonzalez <chema@...gle.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>,
	Eric Dumazet <edumazet@...gle.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	Jiri Olsa <jolsa@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"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>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH RFC net-next 08/14] bpf: add eBPF verifier

On Wed, Jul 2, 2014 at 4:35 PM, Chema Gonzalez <chema@...gle.com> wrote:
> On Wed, Jul 2, 2014 at 4:04 PM, Alexei Starovoitov <ast@...mgrid.com> wrote:
>>>> +       reg = regs + BPF_REG_1; /* 1st arg to a function */
>>>> +       reg->ptr = PTR_TO_CTX;
>>> Wait, doesn't this depend on doing "BPF_MOV64_REG(BPF_REG_CTX,
>>> BPF_REG_ARG1)" (the bpf-to-ebpf prologue), which is only enforced on
>>> filters converted from bpf? In fact, shouldn't this set
>>> regs[BPF_REG_CTX] instead of regs[BPF_REG_1] ?
>>
>> nope. it's REG_1.
>> as you said r6=r1 is only emitted by converted classic filters.
>> Verifier will see this 'r6=r1' assignment and will copy the r1 type into r6.
> You're right. I read BPF_MOV64_REG() AT&T-syntax-style.
>
> BTW, check_stack_write() in kernel/bpf/verifier.c has a couple of
> assignments of a slot->ptr to 0 (instead of INVALID_PTR). I assume
> this is unintended.

yes. good catch. Will fix it.
Too bad C compiler silently casts integers to enums
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