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

[Robert Richter]

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;
}

Maybe this is even better.

-Robert

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

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