netdev - Re: [PATCH bpf-next] bpf: relax verifier restriction on BPF_MOV

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Date:   Wed, 5 Dec 2018 19:13:40 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Jiong Wang <jiong.wang@...ronome.com>
Cc:     Edward Cree <ecree@...arflare.com>, ast@...nel.org,
        daniel@...earbox.net, netdev@...r.kernel.org,
        oss-drivers@...ronome.com
Subject: Re: [PATCH bpf-next] bpf: relax verifier restriction on BPF_MOV |
 BPF_ALU

On Wed, Dec 05, 2018 at 03:32:50PM +0000, Jiong Wang wrote:
> On 05/12/2018 14:52, Edward Cree wrote:
> > On 05/12/18 09:46, Jiong Wang wrote:
> > > There is NO processed instruction number regression, either with or without
> > > -mattr=+alu32.
> > <snip>
> > > Cilium bpf
> > > ===
> > > bpf_lb-DLB_L3.o         2110/2110    1730/1733
> > That looks like a regression of 3 insns in the 32-bit case.
> > May be worth investigating why.
> 
> Will look into this.
> 
> > 
> > > +			dst_reg = insn->dst_reg;
> > > +			regs[dst_reg] = regs[src_reg];
> > > +			if (BPF_CLASS(insn->code) == BPF_ALU) {
> > > +				/* Update type and range info. */
> > > +				regs[dst_reg].type = SCALAR_VALUE;
> > > +				coerce_reg_to_size(&regs[dst_reg], 4);
> > Won't this break when handed a pointer (as root, so allowed to leak
> >   it)?  E.g. (pointer + x) gets turned into scalar x, rather than
> >   unknown scalar in range [0, 0xffffffff].
> 
> Initially I was gating this to scalar_value only, later was thinking it
> could be extended to ptr case if ptr leak is allowed.
> 
> But, your comment remind me min/max value doesn't mean real min/max value
> for ptr types value, it means the offset only if I am understanding the
> issue correctly. So, it will break pointer case.

correct. In case of is_pointer_value() && root -> mark_reg_unknown() has to be called

The explanation of additional 3 steps from another email makes sense to me.

Can you take a look why it helps default (llvm alu64) case too ?
bpf_overlay.o           3096/2898

Thanks