Re: [RFC PATCH v1.9 07/14] x86/stacktrace: add function for detecting reliable stack traces

[Josh Poimboeuf <jpoimboe@redhat.com>]

On Thu, Apr 07, 2016 at 01:55:52PM +0200, Petr Mladek wrote:
> On Fri 2016-03-25 14:34:54, Josh Poimboeuf wrote:
> > For live patching and possibly other use cases, a stack trace is only
> > useful if you can be assured that it's completely reliable.  Add a new
> > save_stack_trace_tsk_reliable() function to achieve that.
> > 
> > Scenarios which indicate that a stack strace may be unreliable:
> > 
> > - interrupt stacks
> > - preemption
> > - corrupted stack data
> > - newly forked tasks
> > - running tasks
> > - the user didn't provide a large enough entries array
> > 
> > Also add a config option so arch-independent code can determine at build
> > time whether the function is implemented.
> > 
> > diff --git a/arch/x86/kernel/dumpstack.c b/arch/x86/kernel/dumpstack.c
> > index 3b10518..9c68bfc 100644
> > --- a/arch/x86/kernel/dumpstack.c
> > +++ b/arch/x86/kernel/dumpstack.c
> > @@ -145,6 +145,42 @@ int print_context_stack_bp(struct thread_info *tinfo,
> >  }
> >  EXPORT_SYMBOL_GPL(print_context_stack_bp);
> >  
> > +int print_context_stack_reliable(struct thread_info *tinfo,
> > +				 unsigned long *stack, unsigned long *bp,
> > +				 const struct stacktrace_ops *ops,
> > +				 void *data, unsigned long *end, int *graph)
> > +{
> > +	struct stack_frame *frame = (struct stack_frame *)*bp;
> > +	struct stack_frame *last_frame = frame;
> > +	unsigned long *ret_addr = &frame->return_address;
> > +
> > +	if (test_ti_thread_flag(tinfo, TIF_FORK))
> > +		return -EINVAL;
> 
> Why exactly is a stack of a forked task unreliable, please?
> 
> There was some discussion about the possible stack state and the patch
> state after forking, see
> http://thread.gmane.org/gmane.linux.kernel/2184163/focus=2191057
> 
> Anyway, I think that the stack should be ready for use when the process
> is visible in the task list. It means that it should be reliable.

To be honest I don't remember exactly why I added this check.  I think
it's related to the fact that newly forked tasks don't yet have a stack.
I should have definitely added a comment.  I'll need to revisit it.

> > +	while (valid_stack_ptr(tinfo, ret_addr, sizeof(*ret_addr), end)) {
> > +		unsigned long addr = *ret_addr;
> > +
> > +		if (frame <= last_frame || !__kernel_text_address(addr) ||
> > +		    in_preempt_schedule_irq(addr))
> 
> I wonder how exactly this works :-)
> 
> First, __kernel_text_address() returns true also for dynamically generated
> ftrace handlers, see is_ftrace_trampoline(). Do we have a guarantee
> that these functions generate a valid stack frame? We might want to
> ignore these here.

This is a good point.  I think the ftrace code does the right thing.
But we don't necessarily have a way to guarantee that.  Maybe we should
consider it unreliable.

> Second, if I get it correctly, in_preempt_schedule_irq() works because
> this functions is called only for tasks that are _not_ running.
> It means that we must be exactly at
> 
>      preempt_schedule_irq()
>        __schedule()
> 	  context_switch()
> 	    switch_to()
> 
> It means that preempt_schedule_irq() must be on the stack if at
> least one of the other functions is not inlined.
> 
> As Jiri Kosina explained to me. We check it because it is
> called on exit from an interrupt handler. The interrupt might
> came at any time, for example, right before a function saves
> the stack frame. This is why it makes the stack unreliable.
> 
> If I get it correctly, this is the only location when the
> running process might get rescheduled from irq context. Other
> possibilities keeps the process running and the stack is
> marked unreliable elsewhere.

Right.  I guess that needs a better comment too.

> Well, I wonder if we should be more suspicious and make
> sure that only the regular process stack is used.

Notice the save_stack_stack_reliable() function, which is called by
dump_trace() when the task is running on an interrupt or exception
stack.  It returns -EINVAL, so the stack gets marked unreliable.  Does
that address your concern, or did you mean something else?

-- 
Josh