Re: [PATCH v2] arm64: fix unwind_frame() for filtered out fn for function graph tracing

[Pratyush Anand]

On Wednesday 13 September 2017 08:12 AM, Will Deacon wrote:
> On Tue, Sep 12, 2017 at 10:54:28AM +0100, James Morse wrote:
> > Hi Pratyush,
> >
> > On 01/09/17 06:48, Pratyush Anand wrote:
> > > do_task_stat() calls get_wchan(), which further does unbind_frame().
> > > unbind_frame() restores frame->pc to original value in case function
> > > graph tracer has modified a return address (LR) in a stack frame to hook
> > > a function return. However, if function graph tracer has hit a filtered
> > > function, then we can't unwind it as ftrace_push_return_trace() has
> > > biased the index(frame->graph) with a 'huge negative'
> > > offset(-FTRACE_NOTRACE_DEPTH).
> > >
> > > Moreover, arm64 stack walker defines index(frame->graph) as unsigned
> > > int, which can not compare a -ve number.
> > >
> > > Similar problem we can have with calling of walk_stackframe() from
> > > save_stack_trace_tsk() or dump_backtrace().
> > >
> > > This patch fixes unwind_frame() to test the index for -ve value and
> > > restore index accordingly before we can restore frame->pc.
> >
> > I've just spotted arm64's profile_pc, which does this:
> >  From arch/arm64/kernel/time.c:profile_pc():
> > > #ifdef CONFIG_FUNCTION_GRAPH_TRACER
> > > 	frame.graph = -1; /* no task info */
> > > #endif
> >
> > Is this another elaborate way of hitting this problem?
> >
> > I guess the options are skip any return-address restore in the unwinder if
> > frame.graph is -1. (and profile_pc may have a bug here). Or, put
> > current->curr_ret_stack in there.

I think we should go with latter, ie assign frame.graph = 
current->curr_ret_stack in profile_pc().

> > profile_pc() always passes tsk=NULL, so the unwinder assumes its current...
> > kernel/profile.c pulls the pt_regs from a per-cpu irq_regs variable, that is
> > updated by handle_IPI ... so it looks like this should always be current...
>
> Hmmm... is profile_pc the *only* case where frame->graph isn't equal to
> tsk->curr_ret_stack in unwind_frame? If so, maybe unwind_frame should just

Yes, it is the only place.

> use that, and we could kill the graph member of struct stackframe completely?

Humm, not sure, we initialize frame->graph out of the while loop in 
unwind_frame()'s caller and then keep in decrementing it in looped function.

--
Regards
Pratyush