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

[Namhyung Kim <namhyung@xxxxxxxxxx>]

Hello Andrii,

On Wed, Apr 26, 2023 at 6:19 PM Andrii Nakryiko
<andrii.nakryiko@xxxxxxxxx> wrote:
>
> On Wed, Apr 26, 2023 at 5:14 PM Namhyung Kim <namhyung@xxxxxxxxxx> 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;
        } 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