linux-kernel - Re: A better interface, perhaps: a timed signal flag

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Date:	Fri, 28 Jul 2006 11:05:16 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Theodore Tso <tytso@....edu>
Cc:	Neil Horman <nhorman@...driver.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Segher Boessenkool <segher@...nel.crashing.org>,
	Dave Airlie <airlied@...il.com>, linux-kernel@...r.kernel.org,
	a.zummo@...ertech.it, jg@...edesktop.org
Subject: Re: A better interface, perhaps: a timed signal flag

On Fri, 2006-07-28 at 10:52 -0400, Theodore Tso wrote:

> Good point, and limiting this facility to one such timeout per
> task_struct seems like a reasonable restriction.  The downsides I can
> see about about mapping the tasks' own task struct would be (a) a
> potential security leak either now or in the future if some field in
> the task_struct shouldn't be visible to a non-privileged userspace
> program, and (b) exposing the task_struct might cause some (stupid)
> programs to depend on the task_struct layout.  Allocating an otherwise
> empty 4k page just for this purpose wouldn't be all that horrible,
> though, and would avoid these potential problems.

Actually, if you are going to map a page, then allow the user to do
PAGE_SIZE / sizeof(*flag) timers.  That way the user gets a single page
mapped for this purpose, and can have multiple flags.

I would only limit it to one page though. Since this page can not be
swapped out, if you allow for more than one page, a non privileged user
can map in a bunch of non swappable pages and might be able to perform a
DoS attack.

-- Steve


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