Re: [PATCH v2 bpf-next 1/3] bpf: Support for pointers beyond pkt_end.

From: John Fastabend <john.fastabend@gmail.com>
To: Alexei Starovoitov <alexei.starovoitov@gmail.com>,
	John Fastabend <john.fastabend@gmail.com>
Cc: davem@davemloft.net, daniel@iogearbox.net,
	netdev@vger.kernel.org, bpf@vger.kernel.org, kernel-team@fb.com
Subject: Re: [PATCH v2 bpf-next 1/3] bpf: Support for pointers beyond pkt_end.
Date: Thu, 12 Nov 2020 16:35:50 -0800	[thread overview]
Message-ID: <5fadd4e6281_2784420869@john-XPS-13-9370.notmuch> (raw)
In-Reply-To: <20201112235934.gkydiegea4nhin3x@ast-mbp>

Alexei Starovoitov wrote:
> On Thu, Nov 12, 2020 at 11:16:11AM -0800, John Fastabend wrote:
> > Alexei Starovoitov wrote:
> > > From: Alexei Starovoitov <ast@kernel.org>
> > > 
> > > This patch adds the verifier support to recognize inlined branch conditions.
> > > The LLVM knows that the branch evaluates to the same value, but the verifier
> > > couldn't track it. Hence causing valid programs to be rejected.
> > > The potential LLVM workaround: https://reviews.llvm.org/D87428
> > > can have undesired side effects, since LLVM doesn't know that
> > > skb->data/data_end are being compared. LLVM has to introduce extra boolean
> > > variable and use inline_asm trick to force easier for the verifier assembly.
> > > 
> > > Instead teach the verifier to recognize that
> > > r1 = skb->data;
> > > r1 += 10;
> > > r2 = skb->data_end;
> > > if (r1 > r2) {
> > >   here r1 points beyond packet_end and
> > >   subsequent
> > >   if (r1 > r2) // always evaluates to "true".
> > > }
> > > 
> > > Tested-by: Jiri Olsa <jolsa@redhat.com>
> > > Signed-off-by: Alexei Starovoitov <ast@kernel.org>
> > > ---
> > >  include/linux/bpf_verifier.h |   2 +-
> > >  kernel/bpf/verifier.c        | 129 +++++++++++++++++++++++++++++------
> > >  2 files changed, 108 insertions(+), 23 deletions(-)
> > > 
> > 
> > Thanks, we can remove another set of inline asm logic.
> 
> Awesome! Please contribute your C examples to selftests when possible.

Sure will do, its just some mundane header parsing iirc.

> 
> > Acked-by: John Fastabend <john.fastabend@gmail.com>
> >  
> > >  	if (pred >= 0) {
> > > @@ -7517,7 +7601,8 @@ static int check_cond_jmp_op(struct bpf_verifier_env *env,
> > >  		 */
> > >  		if (!__is_pointer_value(false, dst_reg))
> > >  			err = mark_chain_precision(env, insn->dst_reg);
> > > -		if (BPF_SRC(insn->code) == BPF_X && !err)
> > > +		if (BPF_SRC(insn->code) == BPF_X && !err &&
> > > +		    !__is_pointer_value(false, src_reg))
> > 
> > This could have been more specific with !type_is_pkt_pointer() correct? I
> > think its fine as is though.
> 
> I actually meant to use __is_pointer_value() here for two reasons:
> 1. to match dst_reg check just few lines above.

Agree.

> 2. mark_chain_precision() is for scalars only. If in the future
>   is_*_branch_taken() will support other kinds of pointers the more
>   precise !type_is_pkt_pointer() check would need to be modified.
>   That would be unnecessary code churn.

Agree.