linux-kernel - Re: [RFC][PATCH 0/3] ftrace: Add dynamically allocated trampolines

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Message-ID: <alpine.LNX.2.00.1407111523270.8779@pobox.suse.cz>
Date:	Fri, 11 Jul 2014 15:24:28 +0200 (CEST)
From:	Jiri Kosina <jkosina@...e.cz>
To:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
cc:	Josh Poimboeuf <jpoimboe@...hat.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Namhyung Kim <namhyung@...nel.org>,
	"H. Peter Anvin" <hpa@...or.com>, Oleg Nesterov <oleg@...hat.com>,
	Seth Jennings <sjenning@...hat.com>,
	Jiri Slaby <jslaby@...e.cz>
Subject: Re: [RFC][PATCH 0/3] ftrace: Add dynamically allocated trampolines

On Fri, 11 Jul 2014, Masami Hiramatsu wrote:

> >> I did some testing with kpatch and I found one minor issue.  The dynamically
> >> allocated trampoline seems to confuse dump_stack() somewhat.
> >>
> >> I added a dump_stack() call in my ftrace_ops callback function
> >> (kpatch_ftrace_handler) which had a filter on meminfo_proc_show().
> > 
> > Interesting. Are you using dwarf2 unwinder for stack dumping by any 
> > chance? It seems to get things right here. Will look into it more 
> > tomorrow.
> 
> Hmm, can dwarf2 unwinder work on the trampoline method? Since the 
> trampoline just a copy of instructions which will not have CFI(which is 
> stored in dwarf section), I guess it may not work... Frame pointer (push 
> bp and save sp to bp on the entry) can work anyway.

That was exactly my idea and that's why I asked, thanks for confirming.

I am afraid we'll have to declare dynamic trampolines incompatible with 
drawf2 stack dumping.

-- 
Jiri Kosina
SUSE Labs
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