linux-kernel - Re: [PATCH 1/2] remove all uses of printf's %n

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Date:	Tue, 17 Sep 2013 07:34:50 -0700
From:	Kees Cook <keescook@...omium.org>
To:	Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
Cc:	Geert Uytterhoeven <geert@...ux-m68k.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Joe Perches <joe@...ches.com>,
	George Spelvin <linux@...izon.com>,
	Dan Carpenter <dan.carpenter@...cle.com>,
	Al Viro <viro@...iv.linux.org.uk>,
	Jan Beulich <JBeulich@...e.com>,
	Motohiro KOSAKI <kosaki.motohiro@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 1/2] remove all uses of printf's %n

On Tue, Sep 17, 2013 at 6:06 AM, Tetsuo Handa
<penguin-kernel@...ove.sakura.ne.jp> wrote:
> Kees Cook wrote:
>> On Mon, Sep 16, 2013 at 1:09 AM, Geert Uytterhoeven
>> <geert@...ux-m68k.org> wrote:
>> > On Mon, Sep 16, 2013 at 9:43 AM, Kees Cook <keescook@...omium.org> wrote:
>> >> All users of %n are calculating padding size when using seq_file, so
>> >> instead use the new last_len member for discovering the length of the
>> >> written strings.
>> >
>> > Would it make sense to provide a seq_pad(...) function instead, to avoid
>> > exposing more seq_file internals to its callers?
>>
>> We'd still need to track how much to pad.
>
> If we add "size_t pad_until;" to "struct seq_file", we can do
>
> void seq_setwidth(struct seq_file *m, size_t size)
> {
>         m->pad_until = m->count + size;
> }
>
> void seq_pad(struct seq_file *m, char c)
> {
>         int size = m->pad_until - m->count;
>         if (size > 0)
>                 seq_printf(m, "%*s", size, "");
>         if (c)
>                 seq_putc(m, c);
> }
>
> and use like
>
>   seq_setwidth(m, 21);
>   seq_printf(m, "%s%d", con->name, con->index);
>   seq_pad(m, '\n');

Ooh, I like this a lot! Much cleaner.

-Kees

-- 
Kees Cook
Chrome OS Security
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