Re: [PATCH bpf-next 1/4] bpf, x64: Fix tailcall infinite loop caused by freplace

[Leon Hwang <leon.hwang@linux.dev>]

On 29/8/24 00:01, Alexei Starovoitov wrote:
> On Tue, Aug 27, 2024 at 7:36 PM Leon Hwang <leon.hwang@linux.dev> wrote:
>>
>>
>>
>> On 28/8/24 04:50, Alexei Starovoitov wrote:
>>> On Tue, Aug 27, 2024 at 5:48 AM Leon Hwang <leon.hwang@linux.dev> wrote:
>>>>
>>>>> I wonder if disallowing to freplace programs when
>>>>> replacement.tail_call_reachable != replaced.tail_call_reachable
>>>>> would be a better option?
>>>>>
>>>>
>>>> This idea is wonderful.
>>>>
>>>> We can disallow attaching tail_call_reachable freplace prog to
>>>> not-tail_call_reachable bpf prog. So, the following 3 cases are allowed.
>>>>
>>>> 1. attach tail_call_reachable freplace prog to tail_call_reachable bpf prog.
>>>> 2. attach not-tail_call_reachable freplace prog to tail_call_reachable
>>>> bpf prog.
>>>> 3. attach not-tail_call_reachable freplace prog to
>>>> not-tail_call_reachable bpf prog.
>>>
>>> I think it's fine to disable freplace and tail_call combination altogether.
>>
>> I don't think so.
>>
>> My XDP project heavily relies on freplace and tailcall combination.
> 
> Pls share the link to the code.
> 

I'm willing to share it with you. But it's an in-house project of my
company. Sorry.

>>>
>>> And speaking of the patch. The following:
>>> -                       if (tail_call_reachable) {
>>> -
>>> LOAD_TAIL_CALL_CNT_PTR(bpf_prog->aux->stack_depth);
>>> -                               ip += 7;
>>> -                       }
>>> +                       LOAD_TAIL_CALL_CNT_PTR(bpf_prog->aux->stack_depth);
>>> +                       ip += 7;
>>>
>>> Is too high of a penalty for every call for freplace+tail_call combo.
>>>
>>> So disable it in the verifier.
>>>
>>
>> I think, it's enough to disallow attaching tail_call_reachable freplace
>> prog to not-tail_call_reachable prog in verifier.
>>
>> As for this code snippet in x64 JIT:
>>
>>                         func = (u8 *) __bpf_call_base + imm32;
>>                         if (tail_call_reachable) {
>>                                 LOAD_TAIL_CALL_CNT_PTR(bpf_prog->aux->stack_depth);
>>                                 ip += 7;
>>                         }
>>                         if (!imm32)
>>                                 return -EINVAL;
>>                         ip += x86_call_depth_emit_accounting(&prog, func, ip);
>>                         if (emit_call(&prog, func, ip))
>>                                 return -EINVAL;
>>
>> when a subprog is tail_call_reachable, its caller has to propagate
>> tail_call_cnt_ptr by rax. It's fine to attach tail_call_reachable
>> freplace prog to this subprog as for this case.
>>
>> When the subprog is not tail_call_reachable, its caller is unnecessary
>> to propagate tail_call_cnt_ptr by rax. Then it's disallowed to attach
>> tail_call_reachable freplace prog to the subprog. However, it's fine to
>> attach not-tail_call_reachable freplace prog to the subprog.
>>
>> In conclusion, if disallow attaching tail_call_reachable freplace prog
>> to not-tail_call_reachable prog in verifier, the above code snippet
>> won't be changed.
> 
> As long as there are no more JIT changes it's ok to go
> with this partial verifier restriction,
> but if more issues are found we'll have to restrict it further.

OK. I'll do the restriction in verifier.

Thanks,
Leon