linux-kernel - Re: [PATCH] x86, 32-bit: Fix invalid stack address while in softirq

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Date:	Thu, 6 Sep 2012 17:36:47 +0200
From:	Robert Richter <robert.richter@....com>
To:	Steven Rostedt <rostedt@...dmis.org>
CC:	wyang1 <Wei.Yang@...driver.com>, Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	<linux-kernel@...r.kernel.org>,
	<oprofile-list@...ts.sourceforge.net>
Subject: Re: [PATCH] x86, 32-bit: Fix invalid stack address while in softirq

On 06.09.12 17:34:07, Robert Richter wrote:
> On 06.09.12 11:14:42, Steven Rostedt wrote:
> > On Thu, 2012-09-06 at 17:02 +0200, Robert Richter wrote:
> > 
> > > > > --- a/arch/x86/oprofile/backtrace.c
> > > > > +++ b/arch/x86/oprofile/backtrace.c
> > > > > @@ -113,7 +113,7 @@ x86_backtrace(struct pt_regs * const regs, unsigned int depth)
> > > > >  
> > > > >  	if (!user_mode_vm(regs)) {
> > > > >  		unsigned long stack = kernel_stack_pointer(regs);
> > > > > -		if (depth)
> > > > > +		if (depth & stack)
> > > > 
> > > > Can other users of kernel_stack_pointer() be nailed by a return of NULL?
> > > 
> > > It would be save here too, but dump_trace() falls back to the current
> > > stack in case there is no stack address given which we don't want with
> > > oprofile.
> > > 
> > > I was looking at all users of kernel_stack_pointer() and could not
> > > find any direct pointer dereference of the sp. The only potential
> > > problems I found could arise here:
> > > 
> > >  arch/x86/kernel/kprobes.c:resume_execution()
> > >  arch/x86/kernel/time.c:profile_pc()
> > > 
> > > It is not quite clear if we really need code here that checks the
> > > pointer. Since a NULL pointer access has the same effect as if the
> > > stack address would be wrong which would be the case without the
> > > patch, I rather tend not to change the code here.
> > 
> > Then a comment should be in the oprofile code too. Something to the
> > effect that oprofile is special and can cause kernel_stack_pointer() to
> > return NULL in some cases, thus we need to check for it.
> 
> We could return always a non-null stack pointer with:
> 
> unsigned long kernel_stack_pointer(struct pt_regs *regs)
> {
> 	unsigned long context = (unsigned long)regs & ~(THREAD_SIZE - 1);
> 	unsigned long sp = (unsigned long)&regs->sp;
> 	struct thread_info *tinfo;
> 
> 	if (context == (sp & ~(THREAD_SIZE - 1)))
> 		return sp;
> 	
> 	tinfo = (struct thread_info *)context;
> 	if (tinfo->previous_esp)
> 		tinfo->previous_esp;
> 
> 	return (unsigned long)regs;
> }

I meant:

unsigned long kernel_stack_pointer(struct pt_regs *regs)
{
	unsigned long context = (unsigned long)regs & ~(THREAD_SIZE - 1);
	unsigned long sp = (unsigned long)&regs->sp;
	struct thread_info *tinfo;

	if (context == (sp & ~(THREAD_SIZE - 1)))
		return sp;
	
	tinfo = (struct thread_info *)context;
	if (tinfo->previous_esp)
		return tinfo->previous_esp;

	return (unsigned long)regs;
}

-Robert

> 
> Maybe this is even better.
> 
> -Robert
> 
> -- 
> Advanced Micro Devices, Inc.
> Operating System Research Center

-- 
Advanced Micro Devices, Inc.
Operating System Research Center

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