netdev - Re: [PATCH v6 net-next 1/6] net: filter: add "load 64-bit immediate" eBPF instruction

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Date:	Mon, 25 Aug 2014 18:38:28 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Alexei Starovoitov <ast@...mgrid.com>
Cc:	David 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>,
	Brendan Gregg <brendan.d.gregg@...il.com>,
	Namhyung Kim <namhyung@...nel.org>,
	"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 v6 net-next 1/6] net: filter: add "load 64-bit immediate"
 eBPF instruction

On Mon, Aug 25, 2014 at 6:35 PM, Alexei Starovoitov <ast@...mgrid.com> wrote:
> On Mon, Aug 25, 2014 at 6:06 PM, David Miller <davem@...emloft.net> wrote:
>> From: Alexei Starovoitov <ast@...mgrid.com>
>> Date: Mon, 25 Aug 2014 18:00:53 -0700
>>
>>> add BPF_LD_IMM64 instruction to load 64-bit immediate value into a register.
>>
>> I think you need to rethink this.
>>
>> I understand that you want to be able to compile arbitrary C code into
>> eBPF, but you have to restrict strongly what data the eBPF code can get
>> to.
>
> I believe verifier already does restrict it. I don't see any holes in
> the architecture. I'm probably not explaining it clearly though :(
>
>> Arbitrary pointer loads is asking for trouble.
>
> Of course.
> There is no arbitrary pointer from user space.
> Verifier checks all pointers.
> I guess this commit log description is confusing.
> It says:
> BPF_LD_IMM64(R1, const_imm_map_ptr)
> that's what appears in the program _after_ it goes through verifier.
> User space cannot pass a pointer into the kernel.

If you don't intend for userspace to load a program that contains this
instruction, then why does it need to be an instruction that the
verifier rewrites?  Why not have an instruction "load immediate
relocated pointer" that contains a reference to a relocation table and
have the JIT do it?  That might be easier to understand than having
the verifier do it, and it'll avoid committing to ABIs before we need
them.

--Andy
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