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

From: Yonghong Song
Date: Tue Jun 30 2020 - 16:37:44 EST




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@xxxxxxxxxx>
---
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;
}