netdev - Re: bpf: undefined shift in __bpf_prog

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Date:	Fri, 4 Dec 2015 11:10:15 -0800
From:	Alexei Starovoitov <alexei.starovoitov@...il.com>
To:	Dmitry Vyukov <dvyukov@...gle.com>
Cc:	Alexei Starovoitov <ast@...nel.org>,
	netdev <netdev@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	syzkaller <syzkaller@...glegroups.com>,
	Kostya Serebryany <kcc@...gle.com>,
	Alexander Potapenko <glider@...gle.com>,
	Sasha Levin <sasha.levin@...cle.com>,
	Eric Dumazet <edumazet@...gle.com>,
	Andrey Ryabinin <ryabinin.a.a@...il.com>
Subject: Re: bpf: undefined shift in __bpf_prog_run

On Fri, Dec 04, 2015 at 08:03:47PM +0100, Dmitry Vyukov wrote:
> > is it with some random seccomp program?
> > If normal libseccomp generates such programs than it needs to be fixed.
> 
> Yes, it is with completely random seccomp program.
> 
> >> Such shifts have undefined behavior according to C standard and behave
> >> differently on different archs. I guess we don't want to rely on any
> >> kind of undefined behavior in bpf/seccomp. And generally want to
> >> completely define results of all operations in bpf.
> >
> > bpf is an engine and we're not going to slow down each shift operation
> > by extra run-time checks or masks.
> > In other words bpf shift instruction == shift in C. Both undefined
> > with for large operands.
> > If seccomp is relying on undefined behavior, it should be fixed.
> 
> But note that it is not that result of such operation is undefined, it
> is overall kernel behavior that becomes undefined.

not true.
just don't generate random bpf programs with such shifts.
kernel is fine.

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