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

[Josh Poimboeuf]

On Tue, Aug 02, 2016 at 05:16:10PM -0400, Steven Rostedt wrote:
> On Tue, 2 Aug 2016 16:00:11 -0500
> Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote:
> 
>    [<ffffffff81061d8b>] nmi_raise_cpu_backtrace+0x1b/0x20
> > 
> > The ret_stack is out of sync with the stack dump because the stack dump
> > was started with the regs from the NMI, instead of being started from
> > the current frame.
> > 
> > So I guess there are a couple of ways to fix it:
> > 
> >   a) keep track of the return address pointer like we discussed above;
> > 
> >      or
> > 
> >   b) have the unwinder count the # of skipped frames which refer to
> >      'return_to_handler', and pass that as the initial index value to
> >      ftrace_graph_ret_addr().
> > 
> > Option a) would be much cleaner.  But to fix it for both mcount and
> > fentry, we couldn't override 'fp' so I guess we'd need to add a new
> > field to ftrace_ret_stack.
> 
> Actually, what about calling ftrace_graph_ret_addr() to figure out the
> next stack conversion only if reliable or CONFIG_FRAME_POINTER is not
> enabled?
> 
> 	unsigned long real_addr = addr;
> 
> 	[...]
> 
> 	if (!IS_ENABLED(CONFIG_FRAME_POINTER) || reliable)
> 		real_addr = ftrace_graph_ret_addr(task, graph, addr);
> 	if (addr != real_addr)
> 		ops->address(data, addr, 0);
> 	ops->address(data, real_addr, reliable);
> 
> Then we only need the fp use case when FRAME_POINTER is not set. As
> mcount forces FRAME_POINTER, we only need to worry about the fentry
> case.

Hm, I'm confused.  First, I don't see where mcount forces FRAME_POINTER.

Second, I don't see why that even matters.  If mcount and frame pointers
are enabled, then the 'fp' field of ftrace_ret_stack is needed for the
gcc sanity check, right?  So we couldn't override 'fp', and the old
"stateful index" version of ftrace_graph_ret_addr() would have to be
used in the code above for reliable addresses, and we'd still have the
same out-of-sync bug.

Or am I missing something?

-- 
Josh