linux-kernel - Re: [PATCH] TIF_NOTSC and SECCOMP prctl

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Date:	Tue, 18 Jul 2006 15:29:41 +0200
From:	andrea@...share.com
To:	Chuck Ebbert <76306.1226@...puserve.com>
Cc:	"bruce@...rew.cmu.edu" <bruce@...rew.cmu.edu>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Arjan van de Ven <arjan@...radead.org>,
	Adrian Bunk <bunk@...sta.de>,
	Lee Revell <rlrevell@...-job.com>,
	Linus Torvalds <torvalds@...l.org>, Ingo Molnar <mingo@...e.hu>
Subject: Re: [PATCH] TIF_NOTSC and SECCOMP prctl

On Tue, Jul 18, 2006 at 06:20:20AM -0400, Chuck Ebbert wrote:
> AFAIC the /proc method of controlling seccomp is so ugly it should
> just go, but what about backwards compatibility?

Given that so far CPUShare seems the only user there should be no
problem, I already uploaded a new CPUShare package that handles both
the old and new interfaces transparently, no matter what kernel runs
under it.

> I have a couple of questions:
> 
> 
> +void disable_TSC(void)
> +{
> +       if (!test_and_set_thread_flag(TIF_NOTSC))
> +               /*
> +                * Must flip the CPU state synchronously with
> +                * TIF_NOTSC in the current running context.
> +                */
> +               hard_disable_TSC();
> +}
> 
> This gets called from sys_prctl().  Do you need to worry about preemption
> between the test_and_set and TSC disable?

I tend to completely forget about preempt.

> Maybe these should be inline?  They're really small and that way you
> don't need #ifdef around the code for them.

I wanted to reduce the bytecode overhead to the minimum when seccomp
is set to y, for that I tried to avoided inlines.

> For x86_64 you need this:
> 
> ftp://ftp.firstfloor.org/pub/ak/x86_64/quilt-current/patches/tif-flags-for-debug-regs-and-io-bitmap-in-ctxsw
> 
> But I don't think Andi plans on pushing it for 2.6.18.

Thanks for the pointer.

For now the patch I posted already works on x86-64 and on all other
archs (x86_64 misses the notsc feature for now, but that's not a
problem, the patch is self contained and we can take care of the notsc
for x86-64 later on).

This is the incremental patch to address the preempt=y kernel builds.

diff -r 373f0be00c40 arch/i386/kernel/process.c
--- a/arch/i386/kernel/process.c	Sun Jul 16 15:51:54 2006 +0200
+++ b/arch/i386/kernel/process.c	Tue Jul 18 14:59:23 2006 +0200
@@ -542,12 +542,14 @@ void hard_disable_TSC(void)
 }
 void disable_TSC(void)
 {
+	preempt_disable();
 	if (!test_and_set_thread_flag(TIF_NOTSC))
 		/*
 		 * Must flip the CPU state synchronously with
 		 * TIF_NOTSC in the current running context.
 		 */
 		hard_disable_TSC();
+	preempt_enable();
 }
 void hard_enable_TSC(void)
 {
-
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