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Subject: Re: [PATCH] RFC x86_64 more accurate KSTK_ESP implementation
From: Stefani Seibold <stefani@seibold.net>
To: Ingo Molnar <mingo@elte.hu>
Cc: linux-kernel <linux-kernel@vger.kernel.org>,
       Andrew Morton <akpm@linux-foundation.org>,
       "H. Peter Anvin" <hpa@zytor.com>,
       Americo Wang <xiyou.wangcong@gmail.com>,
       Thomas Gleixner <tglx@linutronix.de>, Andi Kleen <andi@firstfloor.org>
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Date: Sun, 08 Nov 2009 13:51:06 +0100
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Am Sonntag, den 08.11.2009, 12:35 +0100 schrieb Ingo Molnar:
> * Stefani Seibold <stefani@seibold.net> wrote:
>  
> > +
> > +	if (((stkp < stk) || (stkp >= stk + THREAD_SIZE))
> > +	    && regs->ip < PAGE_OFFSET)
> > +		percpu_write(old_rsp, stkp);
> > +}
> 
> that check for regs->ip looks imprecise - why dont you use the 
> user_mode_vm()?
> 
> It's true that the value itself is statistical, but still we dont want 
> to leak a kernel-space regs->sp reason - it's an information leak.
> 

Good idea. Much better ;-)


> > --- linux-2.6.32-rc5.old/arch/x86/kernel/irq_64.c	2009-10-16 02:41:50.000000000 +0200
> > +++ linux-2.6.32-rc5.new/arch/x86/kernel/irq_64.c	2009-11-04 22:29:55.762951577 +0100
> > @@ -53,6 +53,7 @@
> >  	struct irq_desc *desc;
> >  
> >  	stack_overflow_check(regs);
> > +	update_usersp(regs);
> >
> >  
> >  	desc = irq_to_desc(irq);
> >  	if (unlikely(!desc))
> > --- linux-2.6.32-rc5.old/arch/x86/kernel/apic/apic.c	2009-10-16 02:41:50.000000000 +0200
> > +++ linux-2.6.32-rc5.new/arch/x86/kernel/apic/apic.c	2009-11-04 23:12:32.805086991 +0100
> > @@ -831,6 +831,9 @@
> >  {
> >  	struct pt_regs *old_regs = set_irq_regs(regs);
> >  
> > +#ifndef CONFIG_X86_32
> > +	update_usersp(regs);
> > +#endif
> 
> Cleanliness: please eliminate this #ifdef by defining update_usersp() on 
> 32-bit as well, as an empty inline function.
> 
> But, i dont like this patch because it adds overhead to the IRQ 
> fastpath.
> 

Agree, but i saw no other way.
 
> I'd suggest a competely different method: why dont you use an IPI to 
> sample the SP whenever someone wants to read it from /proc and we see 
> that the task is running on a CPU right now?
> 

Sounds like a challenge, i like the idea. I will have a look on it... 

Stefani