netdev - Re: [PATCH v2 net] bpf: x86: fix epilogue generation for eBPF programs

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Date:	Wed, 3 Dec 2014 07:51:25 -0800
From:	Alexei Starovoitov <ast@...mgrid.com>
To:	Z Lim <zlim.lnx@...il.com>
Cc:	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Daniel Borkmann <dborkman@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...nel.org>,
	Network Development <netdev@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 net] bpf: x86: fix epilogue generation for eBPF programs

On Tue, Dec 2, 2014 at 10:38 PM, Z Lim <zlim.lnx@...il.com> wrote:
> Hi Alexei,
>
> On Sat, Nov 29, 2014 at 2:46 PM, Alexei Starovoitov <ast@...mgrid.com> wrote:
>> classic BPF has a restriction that last insn is always BPF_RET.
>> eBPF doesn't have BPF_RET instruction and this restriction.
>> It has BPF_EXIT insn which can appear anywhere in the program
>> one or more times and it doesn't have to be last insn.
>
> Just to confirm, in valid eBPF, BPF_EXIT *must* be present at least
> once, correct?
> Does an eBPF JIT implementation need to check for it?

yes. of course. At least one bpf_exit is always there
and there are no loops. verifier is checking for it.
So no need for jit to check it again.

> I'll cook up a patch for arm64 if you haven't already done so.
> Any related test case I should run through?

Pending socket samples are generating such code by llvm.
I was planning to add an explicit test to test_bpf, but feel free
to beat me to it.
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