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

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Date:	Thu, 9 Jan 2014 10:11:47 -0800
From:	Kees Cook <keescook@...omium.org>
To:	Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
Cc:	Andrew Morton <akpm@...ux-foundation.org>, pavel@....cz,
	Joe Perches <joe@...ches.com>,
	Geert Uytterhoeven <geert@...ux-m68k.org>,
	Jiri Kosina <jkosina@...e.cz>,
	Al Viro <viro@...iv.linux.org.uk>,
	"David S. Miller" <davem@...emloft.net>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] lib/vsprintf: add %pT format specifier

On Thu, Jan 9, 2014 at 4:52 AM, Tetsuo Handa
<penguin-kernel@...ove.sakura.ne.jp> wrote:
> Hello.
>
> Since addition of %pT itself seems to be agreed, I refreshed this patch using
> linux-next-20140109. Please pick up if this patch is OK for you; I will start
> making patches for killing most of direct ->comm readers.

Looks good; thanks for chasing this. :)

>
> Regards.
> ----------------------------------------
> >From 0d1f03d59a477459f3d3c190593d9e78f5d67de8 Mon Sep 17 00:00:00 2001
> From: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
> Date: Thu, 9 Jan 2014 21:32:22 +0900
> Subject: [PATCH] lib/vsprintf: add %pT format specifier
>
> Since task_struct->comm can be modified by other threads while the current
> thread is reading it, it is recommended to use get_task_comm() for reading it.
>
> However, since get_task_comm() holds task_struct->alloc_lock spinlock,
> some users cannot use get_task_comm(). Also, a lot of users are directly
> reading from task_struct->comm even if they can use get_task_comm().
> Such users might obtain inconsistent result.
>
> This patch introduces %pT format specifier for printing task_struct->comm.
> Currently %pT does not provide consistency. I'm planning to change to use RCU
> in the future. By using RCU, the comm name read from task_struct->comm will be
> guaranteed to be consistent. But before modifying set_task_comm() to use RCU,
> we need to kill direct ->comm users who do not use get_task_comm().
>
> An example for converting direct ->comm users is shown below. Since many debug
> printings use p == current, you can pass NULL instead of p if p == current.
>
>   pr_info("comm=%s\n", p->comm);       => pr_info("comm=%pT\n", p);
>   pr_info("comm=%s\n", current->comm); => pr_info("comm=%pT\n", NULL);
>
> Signed-off-by: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
> Reviewed-by: Pavel Machek <pavel@....cz>

Reviewed-by: Kees Cook <keescook@...omium.org>

-Kees

-- 
Kees Cook
Chrome OS Security
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