On 28/8/24 04:50, Alexei Starovoitov wrote:
> On Tue, Aug 27, 2024 at 5:48 AM Leon Hwang <leon.hwang@xxxxxxxxx> 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
