Subject: Re: [PATCH v2] bpf: fix a bug in verification logic when SUB operation taken on FRAME_PTR

From: "Wangnan (F)" <[email-masked]>
Date: Fri, 19 Jun 2015 08:44:52 +0800

On 2015/6/19 0:00, Alexei Starovoitov wrote:
> On Thu, Jun 18, 2015 at 08:31:45AM +0000, Wang Nan wrote:
>> Original code has a problem, cause following code failed to pass verifier:
>>
>>   r1 <- r10
>>   r1 -= 8
>>   r2 = 8
>>   r3 = unsafe pointer
>>   call BPF_FUNC_probe_read  <-- R1 type=inv expected=fp
>>
>> However, by replacing 'r1 -= 8' to 'r1 += -8' the above program can be
>> loaded successfully.
>>
>> This is because the verifier allows only BPF_ADD instruction on a
>> FRAME_PTR reigster to forge PTR_TO_STACK register, but makes BPF_SUB
>> on FRAME_PTR reigster to get a UNKNOWN_VALUE register.
>>
>> This patch fix it by adding BPF_SUB in stack_relative checking.
> It's not a bug. It's catching ADD only by design.
> If we let it recognize SUB then one might argue we should let it
> recognize multiply, shifts and all other arithmetic on pointers.
> verifier will be getting bigger and bigger. Where do we stop?
> llvm only emits canonical ADD. If you've seen llvm doing SUB,
> let's fix it there.
> So what piece generated this 'r1 -= 8' ?
>

I hit this problem when writing code of automatical parameter generator. The
instruction is generated by myself. Now I have corrected my code.

Thank you.