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SubjectRe: [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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