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

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

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.

> 
> 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.

Thanks,
Leon