linux-kernel - Re: [PATCH v15 21/25] x86/asm: Create stack frames in rwsem functions

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Date:	Tue, 12 Jan 2016 08:36:48 -0600
From:	Josh Poimboeuf <jpoimboe@...hat.com>
To:	Borislav Petkov <bp@...en8.de>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
	linux-kernel@...r.kernel.org, live-patching@...r.kernel.org,
	Michal Marek <mmarek@...e.cz>,
	Peter Zijlstra <peterz@...radead.org>,
	Andy Lutomirski <luto@...nel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andi Kleen <andi@...stfloor.org>,
	Pedro Alves <palves@...hat.com>,
	Namhyung Kim <namhyung@...il.com>,
	Bernd Petrovitsch <bernd@...rovitsch.priv.at>,
	Chris J Arges <chris.j.arges@...onical.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jiri Slaby <jslaby@...e.cz>,
	Arnaldo Carvalho de Melo <acme@...nel.org>
Subject: Re: [PATCH v15 21/25] x86/asm: Create stack frames in rwsem functions

On Tue, Jan 12, 2016 at 01:41:58PM +0100, Borislav Petkov wrote:
> On Fri, Dec 18, 2015 at 06:39:35AM -0600, Josh Poimboeuf wrote:
> > rwsem.S has several callable non-leaf functions which don't honor
> > CONFIG_FRAME_POINTER, which can result in bad stack traces.
> > 
> > Create stack frames for them when CONFIG_FRAME_POINTER is enabled.
> > 
> > Signed-off-by: Josh Poimboeuf <jpoimboe@...hat.com>
> > ---
> >  arch/x86/lib/rwsem.S | 11 ++++++++++-
> >  1 file changed, 10 insertions(+), 1 deletion(-)
> > 
> > diff --git a/arch/x86/lib/rwsem.S b/arch/x86/lib/rwsem.S
> > index 40027db..be110ef 100644
> > --- a/arch/x86/lib/rwsem.S
> > +++ b/arch/x86/lib/rwsem.S
> > @@ -15,6 +15,7 @@
> >  
> >  #include <linux/linkage.h>
> >  #include <asm/alternative-asm.h>
> > +#include <asm/frame.h>
> >  
> >  #define __ASM_HALF_REG(reg)	__ASM_SEL(reg, e##reg)
> >  #define __ASM_HALF_SIZE(inst)	__ASM_SEL(inst##w, inst##l)
> > @@ -84,24 +85,29 @@
> >  
> >  /* Fix up special calling conventions */
> >  ENTRY(call_rwsem_down_read_failed)
> > +	FRAME_BEGIN
> 
> Remind me again, please, why aren't we hiding those
> FRAME_BEGIN/FRAME_END macros in the ENTRY/ENDPROC ones?

Ingo made a similar suggestion a while back:

  https://lkml.kernel.org/r/20150717194307.GA26757@gmail.com

But the frame stuff can't be folded into ENTRY/ENDPROC because we don't
need to create a stack frame for *all* functions, but rather only for
non-leaf functions.

So then we considered something like:

  FUNCTION_ENTRY(func)
  FUNCTION_RETURN(func)

for non-leaf functions, and:

  LEAF_FUNCTION_ENTRY(func)
  LEAF_FUNCTION_RETURN(func)

for leaf functions.

But that was too inflexible for the case where a function ends with a
jump instead of a return.

> >  	save_common_regs
> >  	__ASM_SIZE(push,) %__ASM_REG(dx)
> >  	movq %rax,%rdi
> >  	call rwsem_down_read_failed
> >  	__ASM_SIZE(pop,) %__ASM_REG(dx)
> >  	restore_common_regs
> > +	FRAME_END
> >  	ret
> >  ENDPROC(call_rwsem_down_read_failed)

-- 
Josh