Re: perf test BPF subtest bpf-prologue test fails on s390x

[Wangnan (F)]

On 2017/8/11 2:13, Arnaldo Carvalho de Melo wrote:
> Thomas, please try to find who wrote that test and CC him, I'm doing it
> this time, Wang, can you please take a look at this?
>
> I also added lkml to the CC list, here we have more users of perf, lkml
> is the more developer centric perf list, as perf touches way too many
> places in the kernel.
>
> Best regards,
>
> - Arnaldo
>
> Em Wed, Aug 09, 2017 at 11:24:19AM +0200, Thomas Richter escreveu:
> > I investigate perf test BPF for s390x and have a question regarding
> > the 38.3 subtest (bpf-prologue test) which fails on s390x.
> >
> > When I turn on trace_printk in tests/bpf-script-test-prologue.c
> > I see this output in /sys/kernel/debug/tracing/trace:
> >
> > [root@s8360047 perf]# cat /sys/kernel/debug/tracing/trace
> > perf-30229 [000] d..2 170161.535791: : f_mode 2001d00000000 offset:0 orig:0
> > perf-30229 [000] d..2 170161.535809: : f_mode 6001f00000000 offset:0 orig:0
> > perf-30229 [000] d..2 170161.535815: : f_mode 6001f00000000 offset:1 orig:0
> > perf-30229 [000] d..2 170161.535819: : f_mode 2001d00000000 offset:1 orig:0
> > perf-30229 [000] d..2 170161.535822: : f_mode 2001d00000000 offset:2 orig:1
> > perf-30229 [000] d..2 170161.535825: : f_mode 6001f00000000 offset:2 orig:1
> > perf-30229 [000] d..2 170161.535828: : f_mode 6001f00000000 offset:3 orig:1
> > perf-30229 [000] d..2 170161.535832: : f_mode 2001d00000000 offset:3 orig:1
> > perf-30229 [000] d..2 170161.535835: : f_mode 2001d00000000 offset:4 orig:0
> > perf-30229 [000] d..2 170161.535841: : f_mode 6001f00000000 offset:4 orig:0
> >
> > [...]
> >
> > There are 3 parameters the eBPF program tests/bpf-script-test-prologue.c
> > accesses: f_mode (member of struct file at offset 140) offset and orig.
> > They are parameters of the lseek() system call triggered in this
> > test case in function llseek_loop().
> >
> > What is really strange is the value of f_mode. It is an 8 byte
> > value, whereas in the probe event it is defined as a 4 byte value.
> > The lower 4 bytes are all zero and do not belong to member f_mode.
> > The correct value should be 2001d for read-only and 6001f for
> > read-write open mode.
> >
> > Here is the output of the 'perf test -vv bpf' trace:
> >     Try to find probe point from debuginfo.
> >     Matched function: null_lseek [2d9310d]
> >     Probe point found: null_lseek+0
> >     Searching 'file' variable in context.
> >     Converting variable file into trace event.
> >     converting f_mode in file
> >     f_mode type is unsigned int.
> >     Opening /sys/kernel/debug/tracing//README write=0
> >     Searching 'offset' variable in context.
> >     Converting variable offset into trace event.
> >     offset type is long long int.
> >     Searching 'orig' variable in context.
> >     Converting variable orig into trace event.
> >     orig type is int.
> >     Found 1 probe_trace_events.
> >     Opening /sys/kernel/debug/tracing//kprobe_events write=1
> >     Writing event: p:perf_bpf_probe/func _text+8794224 f_mode=+140(%r2):x32
> > 	offset=%r3:s64 orig=%r4:s32

Thank you for your information. This is an endianess problem.

Here f_mode is x32, so perf probe and debuginfo in vmlinux is correct.
However, BPF prologue generator doesn't obey type, but unconditionally
fetch 8 bytes (on 64-bit machine) and pass it to parameter of
bpf_func__null_lseek. This is reasonable, because all args should be
unsigned long. However, to recover its original value, we need a casting.

Please help me verify if the following fix works on your platform:

diff --git a/tools/perf/tests/bpf-script-test-prologue.c 
b/tools/perf/tests/bpf-script-test-prologue.c
index b4ebc75..43f1e16 100644
--- a/tools/perf/tests/bpf-script-test-prologue.c
+++ b/tools/perf/tests/bpf-script-test-prologue.c
@@ -26,9 +26,11 @@ static void (*bpf_trace_printk)(const char *fmt, int 
fmt_size, ...) =
        (void *) 6;

 SEC("func=null_lseek file->f_mode offset orig")
-int bpf_func__null_lseek(void *ctx, int err, unsigned long f_mode,
+int bpf_func__null_lseek(void *ctx, int err, unsigned long _f_mode,
                         unsigned long offset, unsigned long orig)
 {
+       fmode_t f_mode = (fmode_t)_f_mode;
+
        if (err)
                return 0;
        if (f_mode & FMODE_WRITE)

Thank you.