linux-kernel - Re: [PATCH 6/6] ebpf: allow BPF_REG_X in src

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Date:	Fri, 4 Sep 2015 16:43:50 -0600
From:	Tycho Andersen <tycho.andersen@...onical.com>
To:	Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:	Kees Cook <keescook@...omium.org>,
	Alexei Starovoitov <ast@...nel.org>,
	Will Drewry <wad@...omium.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Andy Lutomirski <luto@...capital.net>,
	Pavel Emelyanov <xemul@...allels.com>,
	"Serge E. Hallyn" <serge.hallyn@...ntu.com>,
	Daniel Borkmann <daniel@...earbox.net>,
	linux-kernel@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: [PATCH 6/6] ebpf: allow BPF_REG_X in src_reg conditional jumps

Hi Alexei,

On Fri, Sep 04, 2015 at 02:06:19PM -0700, Alexei Starovoitov wrote:
> On Fri, Sep 04, 2015 at 10:04:24AM -0600, Tycho Andersen wrote:
> > The classic converter generates conditional jumps with:
> > 
> > if (BPF_SRC(fp->code) == BPF_K && (int) fp->k < 0) {
> >         ...
> > } else {
> >         insn->dst_reg = BPF_REG_A;
> >         insn->src_reg = BPF_REG_X;
> >         insn->imm = fp->k;
> >         bpf_src = BPF_SRC(fp->code);
> > }
> > 
> > but here, we enforce that the src_reg == BPF_REG_0. We should also allow
> > BPF_REG_X since that's what the converter generates; this enables us to
> > load eBPF programs that were generated by the converter.
> 
> good catch. classic->extended converter is just being untidy.
> It shouldn't be populating unused 'src_reg' field when BPF_SRC == BPF_K
> verifier is doing the right thing. It's rejecting instructions that
> have junk in unused fields to make sure that someday we can extend it
> with something useful.
> The fix should be something like this:
> diff --git a/net/core/filter.c b/net/core/filter.c
> index 13079f03902e..05a04ea87172 100644
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -478,9 +478,9 @@ do_pass:
>                                 bpf_src = BPF_X;
>                         } else {
>                                 insn->dst_reg = BPF_REG_A;
> -                               insn->src_reg = BPF_REG_X;
>                                 insn->imm = fp->k;
>                                 bpf_src = BPF_SRC(fp->code);
> +                               insn->src_reg = bpf_src == BPF_X ? BPF_REG_X : 0;
>                         }

Yep, I just tested this and it works for me. Do you want to manage it
or should I carry it as part of this set?

Tycho
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