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

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Message-ID: <571F2D50.4010305@linux.vnet.ibm.com>
Date:	Tue, 26 Apr 2016 14:26:48 +0530
From:	Ravi Bangoria <ravi.bangoria@...ux.vnet.ibm.com>
To:	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, wangnan0@...wei.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

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.

Regards,
Ravi