Re: [HELP] failed to resolve CO-RE relocation

[Hengqi Chen <hengqi.chen@gmail.com>]

Hi, Namhyung

On 2023/4/27 10:21, Namhyung Kim wrote:
> Hello Andrii,
> 
> On Wed, Apr 26, 2023 at 6:19 PM Andrii Nakryiko
> <andrii.nakryiko@gmail.com> wrote:
>>
>> On Wed, Apr 26, 2023 at 5:14 PM Namhyung Kim <namhyung@kernel.org> wrote:
>>>
>>> Hello,
>>>
>>> I'm having a problem of loading perf lock contention BPF program [1]
>>> on old kernels.  It has collect_lock_syms() to get the address of each
>>> CPU's run-queue lock.  The kernel 5.14 changed the name of the field
>>> so there's bpf_core_field_exists to check the name like below.
>>>
>>>         if (bpf_core_field_exists(rq_new->__lock))
>>>                 lock_addr = (__u64)&rq_new->__lock;
>>>         else
>>>                 lock_addr = (__u64)&rq_old->lock;
>>
>> I suspect compiler rewrites it to something like
>>
>>    lock_addr = (__u64)&rq_old->lock;
>>    if (bpf_core_field_exists(rq_new->__lock))
>>         lock_addr = (__u64)&rq_new->__lock;
>>
>> so rq_old relocation always happens and ends up being not guarded
>> properly. You can try adding barrier_var(rq_new) and
>> barrier_var(rq_old) around if and inside branches, that should
>> pessimize compiler
>>
>> alternatively if you do
>>
>> if (bpf_core_field_exists(rq_new->__lock))
>>     lock_addr = (__u64)&rq_new->__lock;
>> else if (bpf_core_field_exists(rq_old->lock))
>>     lock_addr = (__u64)&rq_old->lock;
>> else
>>     lock_addr = 0; /* or signal error somehow */
>>
>> It might work as well.
> 
> Thanks a lot for your comment!
> 
> I've tried the below code but no luck. :(
> 
>         barrier_var(rq_old);
>         barrier_var(rq_new);
> 
>         if (bpf_core_field_exists(rq_old->lock)) {
>             barrier_var(rq_old);
>             lock_addr = (__u64)&rq_old->lock;

Have you tried `BPF_CORE_READ(rq_old, lock)` ?

>         } else if (bpf_core_field_exists(rq_new->__lock)) {
>             barrier_var(rq_new);
>             lock_addr = (__u64)&rq_new->__lock;
>         } else
>             lock_addr = 0;
> 
> 
> ; int BPF_PROG(collect_lock_syms)
> 0: (b7) r8 = 0                        ; R8_w=0
> 1: (b7) r7 = 1                        ; R7_w=1
> 2: <invalid CO-RE relocation>
> failed to resolve CO-RE relocation <byte_off> [381] struct
> rq___old.lock (0:0 @ offset 0)
> processed 3 insns (limit 1000000) max_states_per_insn 0 total_states 0
> peak_states 0 mark_read
> 
> Thanks,
> Namhyung

Cheers,
Hengqi