linux-kernel - Re: [PATCH] lib/vsprintf: add %pT[C012] format specifier

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Date:	Sun, 29 Dec 2013 09:32:44 +0900
From:	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:	joe@...ches.com, akpm@...ux-foundation.org
Cc:	geert@...ux-m68k.org, jkosina@...e.cz, viro@...iv.linux.org.uk,
	davem@...emloft.net, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] lib/vsprintf: add %pT[C012] format specifier

Joe Perches wrote:
> On Sat, 2013-12-28 at 12:08 -0800, Andrew Morton wrote:
> > On Sat, 28 Dec 2013 11:53:25 -0800 Joe Perches <joe@...ches.com> wrote:
> > 
> > > > > #define PRINTK_PID      "\002"
> > > > > #define PRINTK_TASK_ID  "\003"  /* "comm:pid" */
> 
> > > > >
> > > > >         printk(PRINTK_TASK_ID ": hair on fire\n");
> > > > >
> > > > > It's certainly compact.  I doubt if there's any existing code which
> > > > > deliberately prints control chars?

What about using bytes from \x7F to \xFF ?

We are not passing multibyte characters like UTF-8 in the format string (are
we?) because the non-first byte of multibyte characters by error matching %
will cause security problem (format string bug) because the format string is
parsed as char array.

Then, we could do something like below.

  pr_info("%s", current->comm);   => pr_info("\x7F\x80");
  pr_info("%d", current->pid);    => pr_info("\x7F\x81");
  pr_info("%10d", current->pid);  => pr_info("\x7F10\x81");

If precision field support is unnecessary, we could use only \x80 to \xFF .

  pr_info("%s", current->comm);   => pr_info("\x80");
  pr_info("%d", current->pid);    => pr_info("\x81");
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