netdev - Re: [PATCH] arm64: net: bpf: don't BUG() on large shifts

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Message-ID: <20160106221247.GA95332@ast-mbp.thefacebook.com>
Date:	Wed, 6 Jan 2016 14:12:49 -0800
From:	Alexei Starovoitov <alexei.starovoitov@...il.com>
To:	Rabin Vincent <rabin@....in>
Cc:	davem@...emloft.net, netdev@...r.kernel.org, zlim.lnx@...il.com,
	yang.shi@...aro.org, will.deacon@....com, catalin.marinas@....com,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH] arm64: net: bpf: don't BUG() on large shifts

On Wed, Jan 06, 2016 at 09:31:27PM +0100, Rabin Vincent wrote:
> On Tue, Jan 05, 2016 at 09:55:58AM -0800, Alexei Starovoitov wrote:
> > this one is better to be addressed in verifier instead of eBPF JITs.
> > Please reject it in check_alu_op() instead.
> 
> AFAICS the eBPF verifier is not called on the eBPF filters generated by
> the BPF->eBPF conversion in net/core/filter.c, so performing this check
> only in check_alu_op() will be insufficient.  So I think we'd need to
> add this check to bpf_check_classic() too.  Or am I missing something?

correct. the check is needed in bpf_check_classic() too and I believe
it's ok to tighten it up in this case, since >32 shift is
invalid/undefined anyway. We can either accept it as nop or K&=31
or error. I think returning error is more user friendly long term, though
there is a small risk of rejecting previously loadable broken programs.

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