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

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Message-ID: <20160730135125.qqw5qz24szlc2crz@treble>
Date:	Sat, 30 Jul 2016 08:51:25 -0500
From:	Josh Poimboeuf <jpoimboe@...hat.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...nel.org>,
	"H . Peter Anvin" <hpa@...or.com>, x86@...nel.org,
	linux-kernel@...r.kernel.org,
	Andy Lutomirski <luto@...capital.net>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Brian Gerst <brgerst@...il.com>,
	Kees Cook <keescook@...omium.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Byungchul Park <byungchul.park@....com>
Subject: Re: [PATCH 05/19] x86/dumpstack: fix function graph tracing stack
 dump reliability issues

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@...hat.com> 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.

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

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?

-- 
Josh