linux-kernel - Re: [PATCH V4 8/9] perf buildid-cache: add ability to add kcore to the cache

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Date:	Tue, 08 Oct 2013 11:05:37 +0300
From:	Adrian Hunter <adrian.hunter@...el.com>
To:	Namhyung Kim <namhyung@...nel.org>
CC:	Arnaldo Carvalho de Melo <acme@...stprotocols.net>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	linux-kernel@...r.kernel.org, David Ahern <dsahern@...il.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Jiri Olsa <jolsa@...hat.com>, Mike Galbraith <efault@....de>,
	Paul Mackerras <paulus@...ba.org>,
	Stephane Eranian <eranian@...gle.com>
Subject: Re: [PATCH V4 8/9] perf buildid-cache: add ability to add kcore to
 the cache

On 08/10/13 10:41, Namhyung Kim wrote:
> On Mon,  7 Oct 2013 09:50:17 +0300, Adrian Hunter wrote:
>> kcore can be used to view the running kernel object code.
>> However, kcore changes as modules are loaded and unloaded,
>> and when the kernel decides to modify its own code.
>> Consequently it is useful to create a copy of kcore at a
>> particular time.  Unlike vmlinux, kcore is not unique
>> for a given build-id.  And in addition, the kallsyms
>> and modules files are also needed.  The tool therefore
>> creates a directory:
>>
>> 	~/.debug/[kernel.kcore]/<build-id>/<YYYYmmddHHMMSShh>
>>
>> which contains: kcore, kallsyms and modules.
> 
> Hmm..  I think the problem is that the kallsyms and kcore also have
> module information and the build-id of kernel can identify the core
> kernel part only.  So why not splitting modules from kcore and kallsyms?
> 
> As the modules have their own build-id, we can extract module info from
> kcore and kallsyms and put them under ~/.debug/[module]/<build-id>.
> While at it, we can even synthesize symbol table and inject it into the
> module kcore and get rid of the module kallsyms file.
> 
> This way, we can identify all binaries using build-id only, no?

No.

kcore doesn't just give the module object code - it gives it
linked in to the kernel.  Unless you have the modules at the same
addresses the linking doesn't match what you traced.

> 
>>
>> Note that the copied kcore contains only code sections.
>> See the kcore_copy() function for how that is determined.
>>
>> The tool will not make additional copies of kcore if there
>> is already one with the same modules at the same addresses.
>>
>> Currently, perf tools will not look for kcore in the cache.
>> That is addressed in another patch.
>>
> [SNIP]
> 
>> +static int build_id_cache__kcore_buildid(const char *proc_dir, char *sbuildid)
>> +{
>> +	char root_dir[PATH_MAX];
>> +	char notes[PATH_MAX];
>> +	u8 build_id[BUILD_ID_SIZE];
>> +	char *p;
>> +
>> +	strlcpy(root_dir, proc_dir, sizeof(root_dir));
>> +
>> +	p = strrchr(root_dir, '/');
>> +	if (!p)
>> +		return -1;
>> +	*p = '\0';
>> +
>> +	snprintf(notes, sizeof(notes), "%s/sys/kernel/notes", root_dir);
> 
> Please use scnprintf() rather than snprintf().  I can see them in other
> places (and on other patches) too.

OK

> 
> Thanks,
> Namhyung
> 
>> +
>> +	if (sysfs__read_build_id(notes, build_id, sizeof(build_id)))
>> +		return -1;
>> +
>> +	build_id__sprintf(build_id, sizeof(build_id), sbuildid);
>> +
>> +	return 0;
>> +}
> 
> 

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