linux-kernel - Re: vsprintf: Add support for userspace strings

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Message-ID: <55506F86.9020106@nod.at>
Date:	Mon, 11 May 2015 10:59:50 +0200
From:	Richard Weinberger <richard@....at>
To:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	akpm@...ux-foundation.org
CC:	linux-kernel@...r.kernel.org
Subject: Re: vsprintf: Add support for userspace strings

Am 11.05.2015 um 02:24 schrieb Masami Hiramatsu:
> On 2015/05/11 4:42, Richard Weinberger wrote:
>> While debugging issues I often add (trace_)printks to strategic positions.
>> Dealing with user provided string is complicated as an extra buffer a
>> copy_from_user() is needed.
>> This adds a new format string to allow direct printing of such strings.
>>
>> My initial plan was to use %pU but 'U' is already taken, therefore
>> I used the next letter which comes in mind when one thinks of userpace,
>> 'L'.
>> The %pL format string works exactly like %s.
> 
> BTW, if you need to do this for debug, you can also use ftrace's kprobe-tracer
> (and perf probe) which allows you to dump userspace strings :)

Sounds promising!

But I fail to use it:
$ perf probe -vv -L do_sys_open:0-3 -k /boot/vmlinux-4.1.0-rc2-3.g3541e77-vanilla
Line range is 0 to 3
Use vmlinux: /boot/vmlinux-4.1.0-rc2-3.g3541e77-vanilla
Failed to find path of kernel module.
  Error: Failed to show lines. Reason: No such file or directory (Code: -2)

Any idea what's the issue?

Thanks,
//richard
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