Re: [TEST FAILURE] bpf: s390: missed/kprobe_recursion

[Jiri Olsa <olsajiri@gmail.com>]

On Fri, Jan 24, 2025 at 12:23:35PM +0100, Jiri Olsa wrote:
> On Thu, Jan 23, 2025 at 02:32:38PM -0800, Martin KaFai Lau wrote:
> > Hi Jiri,
> > 
> > The "missed/kprobe_recursion" fails consistently on s390. It seems to start
> > failing after the recent bpf and bpf-next tree ffwd.
> > 
> > An example:
> > https://github.com/kernel-patches/bpf/actions/runs/12934431612/job/36076956920
> > 
> > Can you help to take a look?
> > 
> > afaict, it only happens on s390 so far, so cc IIya if there is any recent
> > change that may ring the bell.
> 
> hi,
> I need to check more but I wonder it's the:
>   7495e179b478 s390/tracing: Enable HAVE_FTRACE_GRAPH_FUNC
> 
> which seems to add recursion check and bail out before we have
> a chance to trigger it in bpf code

so the test attaches bpf program test1 to bpf_fentry_test1 via kprobe.multi

	SEC("kprobe.multi/bpf_fentry_test1")
	int test1(struct pt_regs *ctx)
	{
		bpf_kfunc_common_test();
		return 0;
	}

and several other programs are attached to bpf_kfunc_common_test function


I can't test this on s390, but looks like following is happening:

kprobe.multi uses fprobe, so the test kernel path goes:

    bpf_fentry_test1
      ftrace_graph_func
        function_graph_enter_regs
	   fprobe_entry
	     kprobe_multi_link_prog_run
	       test1 (bpf program)
	         bpf_kfunc_common_test
		   kprobe_ftrace_handler
		     kprobe_perf_func
		       trace_call_bpf
		         -> bpf_prog_active check fails, missed count is incremented


kprobe_ftrace_handler calls/takes ftrace_test_recursion_trylock (ftrace recursion lock)

but s390 now calls/takes ftrace_test_recursion_trylock already in ftrace_graph_func,
so s390 stops at kprobe_ftrace_handler and does not get to trace_call_bpf to increment
prog->missed counters

adding Sven, Masami, any idea?

if the ftrace_test_recursion_trylock is needed ftrace_graph_func on s390, then
I think we will need to fix our test to skip s390 arch

thanks,
jirka