Re: [PATCH bpf-next] selftests/bpf: Switch test_vmlinux to use hrtimer_range_start_ns.

[Yonghong Song <yhs@fb.com>]

On 6/30/20 11:49 AM, Hao Luo wrote:
> The test_vmlinux test uses hrtimer_nanosleep as hook to test tracing
> programs. But it seems Clang may have done an aggressive optimization,
> causing fentry and kprobe to not hook on this function properly on a
> Clang build kernel.

Could you explain why it does not on clang built kernel? How did you
build the kernel? Did you use [thin]lto?

hrtimer_nanosleep is a global function who is called in several
different files. I am curious how clang optimization can make
function disappear, or make its function signature change, or
rename the function?

> 
> A possible fix is switching to use a more reliable function, e.g. the
> ones exported to kernel modules such as hrtimer_range_start_ns. After
> we switch to using hrtimer_range_start_ns, the test passes again even
> on a clang build kernel.
> 
> Tested:
>   In a clang build kernel, the test fail even when the flags
>   {fentry, kprobe}_called are set unconditionally in handle__kprobe()
>   and handle__fentry(), which implies the programs do not hook on
>   hrtimer_nanosleep() properly. This could be because clang's code
>   transformation is too aggressive.
> 
>   test_vmlinux:PASS:skel_open 0 nsec
>   test_vmlinux:PASS:skel_attach 0 nsec
>   test_vmlinux:PASS:tp 0 nsec
>   test_vmlinux:PASS:raw_tp 0 nsec
>   test_vmlinux:PASS:tp_btf 0 nsec
>   test_vmlinux:FAIL:kprobe not called
>   test_vmlinux:FAIL:fentry not called
> 
>   After we switch to hrtimer_range_start_ns, the test passes.
> 
>   test_vmlinux:PASS:skel_open 0 nsec
>   test_vmlinux:PASS:skel_attach 0 nsec
>   test_vmlinux:PASS:tp 0 nsec
>   test_vmlinux:PASS:raw_tp 0 nsec
>   test_vmlinux:PASS:tp_btf 0 nsec
>   test_vmlinux:PASS:kprobe 0 nsec
>   test_vmlinux:PASS:fentry 0 nsec
> 
> Signed-off-by: Hao Luo <haoluo@google.com>
> ---
>   tools/testing/selftests/bpf/progs/test_vmlinux.c | 16 ++++++++--------
>   1 file changed, 8 insertions(+), 8 deletions(-)
> 
> diff --git a/tools/testing/selftests/bpf/progs/test_vmlinux.c b/tools/testing/selftests/bpf/progs/test_vmlinux.c
> index 5611b564d3b1..29fa09d6a6c6 100644
> --- a/tools/testing/selftests/bpf/progs/test_vmlinux.c
> +++ b/tools/testing/selftests/bpf/progs/test_vmlinux.c
> @@ -63,20 +63,20 @@ int BPF_PROG(handle__tp_btf, struct pt_regs *regs, long id)
>   	return 0;
>   }
>   
> -SEC("kprobe/hrtimer_nanosleep")
> -int BPF_KPROBE(handle__kprobe,
> -	       ktime_t rqtp, enum hrtimer_mode mode, clockid_t clockid)
> +SEC("kprobe/hrtimer_start_range_ns")
> +int BPF_KPROBE(handle__kprobe, struct hrtimer *timer, ktime_t tim, u64 delta_ns,
> +	       const enum hrtimer_mode mode)
>   {
> -	if (rqtp == MY_TV_NSEC)
> +	if (tim == MY_TV_NSEC)
>   		kprobe_called = true;
>   	return 0;
>   }
>   
> -SEC("fentry/hrtimer_nanosleep")
> -int BPF_PROG(handle__fentry,
> -	     ktime_t rqtp, enum hrtimer_mode mode, clockid_t clockid)
> +SEC("fentry/hrtimer_start_range_ns")
> +int BPF_PROG(handle__fentry, struct hrtimer *timer, ktime_t tim, u64 delta_ns,
> +	     const enum hrtimer_mode mode)
>   {
> -	if (rqtp == MY_TV_NSEC)
> +	if (tim == MY_TV_NSEC)
>   		fentry_called = true;
>   	return 0;
>   }
>