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

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Message-ID: <554FF6B3.4030205@hitachi.com>
Date:	Mon, 11 May 2015 09:24:19 +0900
From:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
To:	Richard Weinberger <richard@....at>, akpm@...ux-foundation.org
CC:	linux-kernel@...r.kernel.org
Subject: Re: vsprintf: Add support for userspace strings

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 :)

Here is an example.
-----
[mhiramat@...alhost perf]$ ./perf probe -L do_sys_open:0-3
<do_sys_open@...r/src/debug/kernel-3.10.0-229.1.2.el7/linux-3.10.0-229.1.2.el7.x
      0  long do_sys_open(int dfd, const char __user *filename, int flags, umode
      1  {
                struct open_flags op;
      3         int lookup = build_open_flags(flags, mode, &op);
[mhiramat@...alhost perf]$ ./perf probe -V do_sys_open
Available variables at do_sys_open
        @<do_sys_open+0>
                char*   filename
                int     dfd
                int     flags
                int     lookup
                struct open_flags       op
                umode_t mode
[mhiramat@...alhost perf]$ sudo ./perf probe do_sys_open filename:string
Added new event:
  probe:do_sys_open    (on do_sys_open with filename:string)

You can now use it in all perf tools, such as:

        perf record -e probe:do_sys_open -aR sleep 1


[mhiramat@...alhost perf]$ sudo ./perf record -e probe:do_sys_open -a ls &> /dev/null
[mhiramat@...alhost perf]$ sudo ./perf script | more
              ls  7238 [003] 1629305.250347: probe:do_sys_open: (ffffffff811c5e40) filename_string="/etc/ld.so.cache"
              ls  7238 [003] 1629305.250384: probe:do_sys_open: (ffffffff811c5e40) filename_string="/lib64/libselinux.so.1"
              ls  7238 [003] 1629305.250501: probe:do_sys_open: (ffffffff811c5e40) filename_string="/lib64/libcap.so.2"
              ls  7238 [003] 1629305.250562: probe:do_sys_open: (ffffffff811c5e40) filename_string="/lib64/libacl.so.1"
              ls  7238 [003] 1629305.250631: probe:do_sys_open: (ffffffff811c5e40) filename_string="/lib64/libc.so.6"
              ls  7238 [003] 1629305.250706: probe:do_sys_open: (ffffffff811c5e40) filename_string="/lib64/libpcre.so.1"
              ls  7238 [003] 1629305.250769: probe:do_sys_open: (ffffffff811c5e40) filename_string="/lib64/liblzma.so.5"
              ls  7238 [003] 1629305.250838: probe:do_sys_open: (ffffffff811c5e40) filename_string="/lib64/libdl.so.2"
              ls  7238 [003] 1629305.250898: probe:do_sys_open: (ffffffff811c5e40) filename_string="/lib64/libattr.so.1"
              ls  7238 [003] 1629305.250959: probe:do_sys_open: (ffffffff811c5e40) filename_string="/lib64/libpthread.so.0"
              ls  7238 [003] 1629305.251591: probe:do_sys_open: (ffffffff811c5e40) filename_string=""
              ls  7238 [003] 1629305.251695: probe:do_sys_open: (ffffffff811c5e40) filename_string="."
[mhiramat@...alhost perf]$ sudo ./perf probe -d \*
Removed event: probe:do_sys_open
-----

Thank you,

-- 
Masami HIRAMATSU
Linux Technology Research Center, System Productivity Research Dept.
Center for Technology Innovation - Systems Engineering
Hitachi, Ltd., Research & Development Group
E-mail: masami.hiramatsu.pt@...achi.com
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