Re: [PATCH 05/19] x86/dumpstack: fix function graph tracing stack dump reliability issues

[Steven Rostedt]

On Sat, 30 Jul 2016 08:51:25 -0500
Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote:

> On Fri, Jul 29, 2016 at 10:20:36PM -0400, Steven Rostedt wrote:
> > On Fri, 29 Jul 2016 19:50:59 -0500
> > Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote:
> >   
> > > BTW, it would be really nice if ftrace_graph_ret_addr() were idempotent
> > > so we could get the "real" return address without having to pass in a
> > > state variable.
> > > 
> > > For example we could add an "unsigned long *retp" pointer to
> > > ftrace_ret_stack, which points to the return address on the stack.  Then
> > > we could get rid of the index state variable in ftrace_graph_ret_addr,
> > > and also then there would never be a chance of the stack dump getting
> > > out of sync with the ret_stack.
> > > 
> > > What do you think?
> > >   
> > 
> > I don't want to extend ret_stack as that is allocated 50 of these
> > structures for every task. That said, we have the "fp" field that's
> > used to check for frame pointer corruption when mcount is used. With
> > CC_USING_FENTRY, that field is ignored. Perhaps we could overload that
> > field for this.  
> 
> In that case, I guess we would need two versions of
> ftrace_graph_ret_addr(), with the current implementation still needed
> for mcount+HAVE_FUNCTION_GRAPH_FP_TEST.

How hard would it be in that case?

> 
> Or would you want to get rid of HAVE_FUNCTION_GRAPH_FP_TEST for x86?

No, because there's gcc versions that we still support that mess up
mcount, and could still cause issues with function graph.


> 
> BTW, on a different note, should I put ftrace_graph_ret_addr() to
> kernel/trace/trace_functions_graph.c so other arches can use it?
> 

I guess you could. There doesn't seem to be any x86 specific code in
that right?

-- Steve