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.
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; }
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; }
On Tue, Jun 30, 2020 at 1:37 PM Yonghong Song yhs@fb.com wrote:
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?
Yonghong,
We didn't enable LTO. It also puzzled me. But I can confirm those fentry/kprobe test failures via many different experiments I've done. After talking to my colleague on kernel compiling tools (Bill, cc'ed), we suspected this could be because of clang's aggressive inlining. We also noticed that all the callsites of hrtimer_nanosleep() are tail calls.
For a better explanation, I can reach out to the people who are more familiar to clang in the compiler team to see if they have any insights. This may not be of high priority for them though.
Hao
On Tue, Jun 30, 2020 at 3:48 PM Hao Luo haoluo@google.com wrote:
On Tue, Jun 30, 2020 at 1:37 PM Yonghong Song yhs@fb.com wrote:
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?
Yonghong,
We didn't enable LTO. It also puzzled me. But I can confirm those fentry/kprobe test failures via many different experiments I've done. After talking to my colleague on kernel compiling tools (Bill, cc'ed), we suspected this could be because of clang's aggressive inlining. We also noticed that all the callsites of hrtimer_nanosleep() are tail calls.
For a better explanation, I can reach out to the people who are more familiar to clang in the compiler team to see if they have any insights. This may not be of high priority for them though.
Hi Yonghong,
Clang is generally more aggressive at inlining than gcc. So even though hrtimer_nanosleep is a global function, clang goes ahead and inlines it into the "nanosleep" syscall, which is in the same file. (We're not currently using {Thin}LTO, so this won't happen in functions outside of kernel/time/hrtimer.c.) Note that if gcc were to change it's inlining heuristics so that it inlined more aggressively, you would be faced with a similar issue.
If you would like to test that it calls hrtimer_nanosleep() and not another function, it might be best to call a syscall not defined in hrtimer.c, e.g. clock_nanosleep().
-bw
Ok, with the help of my colleague Ian Rogers, I think we solved the mystery. Clang actually inlined hrtimer_nanosleep() inside SyS_nanosleep(), so there is no call to that function throughout the path of the nanosleep syscall. I've been looking at the function body of hrtimer_nanosleep for quite some time, but clearly overlooked the caller of hrtimer_nanosleep. hrtimer_nanosleep is pretty short and there are many constants, inlining would not be too surprising. Sigh...
Hao
On Tue, Jun 30, 2020 at 3:48 PM Hao Luo haoluo@google.com wrote:
On Tue, Jun 30, 2020 at 1:37 PM Yonghong Song yhs@fb.com wrote:
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?
Yonghong,
We didn't enable LTO. It also puzzled me. But I can confirm those fentry/kprobe test failures via many different experiments I've done. After talking to my colleague on kernel compiling tools (Bill, cc'ed), we suspected this could be because of clang's aggressive inlining. We also noticed that all the callsites of hrtimer_nanosleep() are tail calls.
For a better explanation, I can reach out to the people who are more familiar to clang in the compiler team to see if they have any insights. This may not be of high priority for them though.
Hao
On 6/30/20 5:10 PM, Hao Luo wrote:
Ok, with the help of my colleague Ian Rogers, I think we solved the mystery. Clang actually inlined hrtimer_nanosleep() inside SyS_nanosleep(), so there is no call to that function throughout the path of the nanosleep syscall. I've been looking at the function body of hrtimer_nanosleep for quite some time, but clearly overlooked the caller of hrtimer_nanosleep. hrtimer_nanosleep is pretty short and there are many constants, inlining would not be too surprising.
Oh thanks for explanation. inlining makes sense. We have many other instances like this in the past where kprobe won't work properly.
Could you reword your commit message then?
causing fentry and kprobe to not hook on this function properly on a Clang build kernel.
The above is a little vague on what happens. What really happens is fentry/kprobe does hook on this function but has no effect since its caller has inlined the function.
Sigh...
Hao
On Tue, Jun 30, 2020 at 3:48 PM Hao Luo haoluo@google.com wrote:
On Tue, Jun 30, 2020 at 1:37 PM Yonghong Song yhs@fb.com wrote:
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?
Yonghong,
We didn't enable LTO. It also puzzled me. But I can confirm those fentry/kprobe test failures via many different experiments I've done. After talking to my colleague on kernel compiling tools (Bill, cc'ed), we suspected this could be because of clang's aggressive inlining. We also noticed that all the callsites of hrtimer_nanosleep() are tail calls.
For a better explanation, I can reach out to the people who are more familiar to clang in the compiler team to see if they have any insights. This may not be of high priority for them though.
Hao
On Tue, Jun 30, 2020 at 7:26 PM Yonghong Song yhs@fb.com wrote:
On 6/30/20 5:10 PM, Hao Luo wrote:
Ok, with the help of my colleague Ian Rogers, I think we solved the mystery. Clang actually inlined hrtimer_nanosleep() inside SyS_nanosleep(), so there is no call to that function throughout the path of the nanosleep syscall. I've been looking at the function body of hrtimer_nanosleep for quite some time, but clearly overlooked the caller of hrtimer_nanosleep. hrtimer_nanosleep is pretty short and there are many constants, inlining would not be too surprising.
Oh thanks for explanation. inlining makes sense. We have many other instances like this in the past where kprobe won't work properly.
Could you reword your commit message then?
causing fentry and kprobe to not hook on this function properly on a Clang build kernel.
The above is a little vague on what happens. What really happens is fentry/kprobe does hook on this function but has no effect since its caller has inlined the function.
Sure, sending a v2 with a more accurate description of the issue.
Hao
On Tue, Jun 30, 2020 at 1:47 PM Hao Luo haoluo@google.com 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.
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
Took me a bit of jumping around to find how it is related to nanosleep call :) But seems like it's unconditionally called, so should be fine.
Acked-by: Andrii Nakryiko andriin@fb.com
tools/testing/selftests/bpf/progs/test_vmlinux.c | 16 ++++++++-------- 1 file changed, 8 insertions(+), 8 deletions(-)
[...]
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