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

[Thomas-Mich Richter]

On 08/11/2017 07:19 AM, Wangnan (F) wrote:
> 
> 
> 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.
> 

Thanks for this patch, unfortunately it does not work. The result is the same
as before. Here is the trace output from /sys/kernel/debug/tracing/trace:        
   perf-18119 [001] d..2 66832.765154: : f_mode 0 offset:0 orig:0
   perf-18119 [001] d..2 66832.765189: : f_mode 0 offset:0 orig:0
   perf-18119 [001] d..2 66832.765203: : f_mode 0 offset:1 orig:0
   perf-18119 [001] d..2 66832.765216: : f_mode 0 offset:1 orig:0
   perf-18119 [001] d..2 66832.765229: : f_mode 0 offset:2 orig:1
   perf-18119 [001] d..2 66832.765242: : f_mode 0 offset:2 orig:1
            
However when I alter your patch and right shift _f_mode by 32 bits
like this:

> -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 >> 32);
> +
>         if (err)
>                 return 0;
>         if (f_mode & FMODE_WRITE)
> 

the test case succeeds (here is part of the ./perf test -v bpf output):

38.3: BPF prologue generation                  :
--- start ---
test child forked, pid 20477
Kernel build dir is set to /lib/modules/4.12.0perf1+/build
set env: KBUILD_DIR=/lib/modules/4.12.0perf1+/build
unset env: KBUILD_OPTS

[...]

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,
unsigned long offset, unsigned long orig)
{
fmode_t f_mode = (fmode_t)(_f_mode >> 32);
char m_fmt[] = "f_mode %x offset:%lx orig:%lx\n";
bpf_trace_printk(m_fmt, sizeof(m_fmt), f_mode, offset, orig);

[...]

Found 1 probe_trace_events.
Opening /sys/kernel/debug/tracing//kprobe_events write=1
Writing event: p:perf_bpf_probe/func _text+8769704 f_mode=+132(%r2):x32 offset=%r3:s64 orig=%r4:s32
[...]
Parsing probe_events: p:perf_bpf_probe/func _text+8769704 f_mode=+132(%r2):x32 offset=%r3:s64 orig=%r4:s32
Group:perf_bpf_probe Event:func probe:p
Writing event: -:perf_bpf_probe/func
test child finished with 0
---- end ----
BPF filter subtest 2: Ok

The trace output looks correct too:

            perf-20477 [001] d..2 67093.798264: : f_mode 2001d offset:6e orig:1
            perf-20477 [001] d..2 67093.798288: : f_mode 6001f offset:6e orig:1


I did some more digging and added a printk statement into function bpf_probe_read()
to get some output into the kernel log.
     Aug 11 09:57:07 s8360047 kernel: ____bpf_probe_read dst:0000000063df7940
         0x6001f00000000 size:8 unsafe_ptr:0000000067829684 ret:0

I think the error is in file  utils/bpf-prologue.c function gen_read_mem().
Register 1 is the destination address (pointing to the stack of the bpf program?)
where the value of struct-file.f_mode member will be stored.
Register 2 is unconditionally set to 8.
Register 3 is the pointer to struct-file.f_mode member (source address).

Function BPF_FUNC_probe_read is then invoked to get the value of struct-file.f_mode
from r3 to store it in r1. This ends up in function bpf_probe_read() and
probe_kernel_read() which copies 8 byte byte wise from source to destination.

I think there a 2 issues here:
1. We copy 8 bytes instead of 4 (sizeof(struct-file.f_mode member is 4 bytes).
2. Since this is a byte copy the eBPF program treats the r1 value wrong.
   
For little endian platforms this is ok since the eBPF program sees register r1 as 
0x6001f00000000 (which is treated as 00 00 00 00 00 06 00 1f for arithmetic operation)
and the bit test for FMODE_WRITE (0x2) succeeds.

For big endian platforms this is wrong and the eBPF program sees register r1 also
as 0x6001f00000000, but does not swap the byte for arithmetic operations.
So the bit test for FMODE_WRITE (0x2) always fails.


Maybe the solution is to add an endianness convertion into the gen_read_mem()
function.

Thanks a lot for your help and toughts.
-- 
Thomas Richter, Dept 3303, IBM LTC Boeblingen Germany
--
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