linux-kernel - Re: [RFC] perf probe: Fix offline module name missmatch issue

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Date:	Tue, 26 Apr 2016 16:15:20 +0530
From:	Ravi Bangoria <ravi.bangoria@...ux.vnet.ibm.com>
To:	"Wangnan (F)" <wangnan0@...wei.com>,
	Masami Hiramatsu <mhiramat@...nel.org>
Cc:	linux-kernel@...r.kernel.org, acme@...nel.org,
	peterz@...radead.org, mingo@...hat.com,
	alexander.shishkin@...ux.intel.com,
	masami.hiramatsu.pt@...achi.com, namhyung@...nel.org,
	srikar@...ux.vnet.ibm.com, naveen.n.rao@...ux.vnet.ibm.com
Subject: Re: [RFC] perf probe: Fix offline module name missmatch issue



On Tuesday 26 April 2016 02:45 PM, Wangnan (F) wrote:
>
>
> On 2016/4/26 16:56, Ravi Bangoria wrote:
>> Thanks Masami for reviewing.
>>
>> Please find my replies to your comment.
>>
>> On Tuesday 26 April 2016 02:54 AM, Masami Hiramatsu wrote:
>>> Hi Ravi,
>>>
>>> On Mon, 25 Apr 2016 16:08:27 +0530
>>> Ravi Bangoria <ravi.bangoria@...ux.vnet.ibm.com> wrote:
>>>
>>>> Perf can add a probe on kernel module which has not been loaded yet.
>>>> Current implementation finds module name from path. But if filename
>>>> is different from actual module name then perf fails to register
>>>> probe while loading module because of mismatch in names. For example,
>>>> samples/kobject/kobject-example.ko is loaded as kobject_example.
>>> Ah! right, good catch!
>>> Have some comment below;
>>>
>>>> diff --git a/tools/perf/util/probe-event.c 
>>>> b/tools/perf/util/probe-event.c
>>>> index 8319fbb..05d0905 100644
>>>> --- a/tools/perf/util/probe-event.c
>>>> +++ b/tools/perf/util/probe-event.c
>>>> @@ -265,6 +265,65 @@ static bool kprobe_warn_out_range(const char 
>>>> *symbol, unsigned long address)
>>>>       return true;
>>>>   }
>>>>   +/*
>>>> + * NOTE:
>>>> + * '.gnu.linkonce.this_module' section of kernel module elf directly
>>>> + * maps to 'struct module' from linux/module.h. This section contains
>>>> + * actual module name which will be used by kernel after loading it.
>>>> + * But, we cannot use 'struct module' here since linux/module.h is 
>>>> not
>>>> + * exposed to user-space. Offset of 'name' has remained same from 
>>>> long
>>>> + * time, so hardcoding it here.
>>>> + */
>>>> +#ifdef __LP64__
>>>> +#define MOD_NAME_OFFSET 24
>>>> +#else
>>>> +#define MOD_NAME_OFFSET 12
>>>> +#endif
>>>> +
>>>> +/*
>>>> + * @module can be module name of module file path. In case of path,
>>>> + * inspect elf and find out what is actual module name.
>>>> + * Caller has to free mod_name after using it.
>>>> + */
>>>> +char *find_module_name(const char *module)
>>> Could you make this function static, since there is no caller outside
>>> this file?
>>
>> Yes. no caller outside of this file. But,
>>
>> In this patch, function find_module_name is defined outside of
>> #ifdef HAVE_DWARF_SUPPORT while it's being called from inside of
>> #ifdef HAVE_DWARF_SUPPORT.
>>
>> If I make it static and if there is no dwarf support, there will be 
>> compilation
>> error about function defined but not used.
>>
>> And in second patch("perf probe: Fix module probe issue if no dwarf 
>> support"),
>> I'm calling it from outside of #ifdef HAVE_DWARF_SUPPORT.
>>
>> So I have two options:
>> 1. merge both the patches and make definition as static
>> 2. make function static in second patch
>>
>> I've chose second approach and sent v2. But please let me know if 
>> there is
>> better way to do it.
>>
>
> Try __maybe_unused directive?
>

Thanks Wangnan for suggestion,

Actually I tried to use __maybe_unused with definition of 
find_module_name but
it throws following compilation error:

util/probe-event.c:289:1: error: expected ‘,’ or ‘;’ before ‘{’ token
  {
  ^
util/probe-event.c:288:14: error: ‘find_module_name’ declared ‘static’ 
but never defined [-Werror=unused-function]
  static char *find_module_name(const char *module) __maybe_unused
               ^
   CC       util/zlib.o
cc1: all warnings being treated as errors


I've to declare prototype of function with __maybe_unused before it's 
definition to
resolve this error. And, anyway this is temporary and need to be removed 
in patch 2,
I think no need to do this change.

Regards,
Ravi