netdev - Re: [PATCH v2 net-next] bpf: enable verifier to add 0 to packet ptr

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Date:   Fri, 3 Feb 2017 15:28:53 -0800
From:   William Tu <u9012063@...il.com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Daniel Borkmann <daniel@...earbox.net>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Mihai Budiu <mbudiu@...are.com>
Subject: Re: [PATCH v2 net-next] bpf: enable verifier to add 0 to packet ptr

Thanks. I got it. I will resubmit v3 patch!

On Fri, Feb 3, 2017 at 2:53 PM, Alexei Starovoitov
<alexei.starovoitov@...il.com> wrote:
> On Fri, Feb 03, 2017 at 11:29:19PM +0100, Daniel Borkmann wrote:
>> On 02/03/2017 10:10 PM, William Tu wrote:
>> >Hi Alexei,
>> >
>> >why it is bogus? on my system, it fails without the patch applied.
>> >
>> >--William
>> >
>> >On Fri, Feb 3, 2017 at 12:55 PM, Alexei Starovoitov
>> ><alexei.starovoitov@...il.com> wrote:
>> >>On Fri, Feb 03, 2017 at 09:22:45AM -0800, William Tu wrote:
>> >>>The patch fixes the case when adding a zero value to the packet
>> >>>pointer.  The verifer reports the following error:
>> >>>   [...]
>> >>>     R0=imm0,min_value=0,max_value=0
>> >>>     R1=pkt(id=0,off=0,r=4)
>> >>>     R2=pkt_end R3=fp-12
>> >>>     R4=imm4,min_value=4,max_value=4
>> >>>     R5=pkt(id=0,off=4,r=4)
>> >>>   269: (bf) r2 = r0   // r2 becomes imm0
>> >>>   270: (77) r2 >>= 3
>> >>>   271: (bf) r4 = r1   // r4 becomes pkt ptr
>> >>>   272: (0f) r4 += r2  // r4 += 0
>> >>>   addition of negative constant to packet pointer is not allowed
>> >>>
>> >>>Signed-off-by: William Tu <u9012063@...il.com>
>> >>>Signed-off-by: Mihai Budiu <mbudiu@...are.com>
>> [...]
>> >>>       {
>> >>>+             "direct packet access: test14 (pkt_ptr += 0, good access)",
>> >>>+             .insns = {
>> >>>+                     BPF_LDX_MEM(BPF_W, BPF_REG_2, BPF_REG_1,
>> >>>+                                 offsetof(struct __sk_buff, data)),
>> >>>+                     BPF_LDX_MEM(BPF_W, BPF_REG_3, BPF_REG_1,
>> >>>+                                 offsetof(struct __sk_buff, data_end)),
>> >>>+                     BPF_MOV64_REG(BPF_REG_0, BPF_REG_2),
>> >>>+                     BPF_ALU64_IMM(BPF_ADD, BPF_REG_0, 0),
>> >>
>> >>wait. the test is bogus.
>> >>please write the proper test for the feature
>> >>and check that it fails before the patch and passes afterwards.
>>
>> But still same code path that is executed in verifier as BPF_K and
>> CONST_IMM tracked reg both share the same path under add_imm label
>> in check_packet_ptr_add(), no? So it becomes r2=pkt(id=0,off=0,r=0);
>> r0 = r2; r0 += 0 here in this test. Probably okay as well, though
>> there could be risk that in future both don't share the same path
>> for some reason. I guess you were referring to either adding tests
>> for BPF_K /and/ CONST_IMM reg or just the latter, right?
>
> yes. Sorry I wasn't clear.
> imo the 'r0 += 0' is not something that verifier should recognize,
> since such nop insns shouldn't be generated by the compiler.
> It happened that the code path in verifier covers that case
> as well, but I think we really need to test 'rX += rY' case
> where rY is recognized as imm0, since that what the original
> use case was about.
>
>