Re: [PATCH bpf-next 2/4] bpf, arm64: Fix tailcall infinite loop caused by freplace

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

On 2024/9/5 17:13, Puranjay Mohan wrote:
> Xu Kuohai <xukuohai@huaweicloud.com> writes:
> 
>> On 8/27/2024 10:23 AM, Leon Hwang wrote:
>>>
>>>
>>> On 26/8/24 22:32, Xu Kuohai wrote:
>>>> On 8/25/2024 9:09 PM, Leon Hwang wrote:
>>>>> Like "bpf, x64: Fix tailcall infinite loop caused by freplace", the same
>>>>> issue happens on arm64, too.
>>>>>
>>>
>>> [...]
>>>
>>>>
>>>> This patch makes arm64 jited prologue even more complex. I've posted a
>>>> series [1]
>>>> to simplify the arm64 jited prologue/epilogue. I think we can fix this
>>>> issue based
>>>> on [1]. I'll give it a try.
>>>>
>>>> [1]
>>>> https://lore.kernel.org/bpf/20240826071624.350108-1-xukuohai@huaweicloud.com/
>>>>
>>>
>>> Your patch series seems great. We can fix it based on it.
>>>
>>> Please notify me if you have a successful try.
>>>
>>
>> I think the complexity arises from having to decide whether
>> to initialize or keep the tail counter value in the prologue.
>>
>> To get rid of this complexity, a straightforward idea is to
>> move the tail call counter initialization to the entry of
>> bpf world, and in the bpf world, we only increase and check
>> the tail call counter, never save/restore or set it. The
>> "entry of the bpf world" here refers to mechanisms like
>> bpf_prog_run, bpf dispatcher, or bpf trampoline that
>> allows bpf prog to be invoked from C function.
>>
>> Below is a rough POC diff for arm64 that could pass all
>> of your tests. The tail call counter is held in callee-saved
>> register x26, and is set to 0 by arch_run_bpf.
> 
> I like this approach as it removes all the complexity of handling tcc in

I like this approach, too.

> different cases. Can we go ahead with this for arm64 and make
> arch_run_bpf a weak function and let other architectures override this
> if they want to use a similar approach to this and if other archs want to
> do something else they can skip implementing arch_run_bpf.
> 

Hi Alexei,

What do you think about this idea?

Thanks,
Leon