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

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

On 2024/8/30 18:00, Xu Kuohai wrote:
> On 8/30/2024 5:08 PM, Leon Hwang wrote:
>>
>>
>> On 30/8/24 15:37, Xu Kuohai wrote:
>>> On 8/27/2024 10:23 AM, Leon Hwang wrote:
>>>>
>>

[...]

>>
>> This approach is really cool!
>>
>> I want an alike approach on x86. But I failed. Because, on x86, it's an
>> indirect call to "call *rdx", aka "bpf_func(ctx, insnsi)".
>>
>> Let us imagine the arch_run_bpf() on x86:
>>
>> unsigned int __naked arch_run_bpf(const void *ctx, const struct bpf_insn
>> *insnsi, bpf_func_t bpf_func)
>> {
>>     asm (
>>         "pushq %rbp\n\t"
>>         "movq %rsp, %rbp\n\t"
>>         "xor %rax, %rax\n\t"
>>         "pushq %rax\n\t"
>>         "movq %rsp, %rax\n\t"
>>         "callq *%rdx\n\t"
>>         "leave\n\t"
>>         "ret\n\t"
>>     );
>> }
>>
>> If we can change "callq *%rdx" to a direct call, it'll be really
>> wonderful to resolve this tailcall issue on x86.
>>
> 
> Right, so we need static call here, perhaps we can create a custom
> static call trampoline to setup tail call counter.
> 
>> How to introduce arch_bpf_run() for all JIT backends?
>>
> 
> Seems we can not avoid arch specific code. One approach could be
> to define a default __weak function to call bpf_func directly,
> and let each arch to provide its own overridden implementation.
> 

Hi Xu Kuohai,

Can you send a separate patch to fix this issue on arm64?

After you fixing it, I'll send the patch to fix it on x64.

Thanks,
Leon