linux-kernel - Re: [RFC PATCH 06/10] x86/entry: add CFI hint undwarf annotations

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Date:   Thu, 1 Jun 2017 09:23:58 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     X86 ML <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        live-patching@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Jiri Slaby <jslaby@...e.cz>, Ingo Molnar <mingo@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [RFC PATCH 06/10] x86/entry: add CFI hint undwarf annotations

On Thu, Jun 01, 2017 at 07:03:18AM -0700, Andy Lutomirski wrote:
> On Wed, May 31, 2017 at 10:44 PM, Josh Poimboeuf <jpoimboe@...hat.com> wrote:
> > Add CFI hint undwarf annotations to entry_64.S.  This will enable the
> > undwarf unwinder to unwind through any location in the entry code
> > including syscalls, interrupts, and exceptions.
> >
> > Signed-off-by: Josh Poimboeuf <jpoimboe@...hat.com>
> > ---
> >  arch/x86/entry/Makefile   |  1 -
> >  arch/x86/entry/calling.h  |  5 +++++
> >  arch/x86/entry/entry_64.S | 56 ++++++++++++++++++++++++++++++++++++++++++-----
> >  3 files changed, 55 insertions(+), 7 deletions(-)
> >
> > diff --git a/arch/x86/entry/Makefile b/arch/x86/entry/Makefile
> > index 9976fce..af28a8a 100644
> > --- a/arch/x86/entry/Makefile
> > +++ b/arch/x86/entry/Makefile
> > @@ -2,7 +2,6 @@
> >  # Makefile for the x86 low level entry code
> >  #
> >
> > -OBJECT_FILES_NON_STANDARD_entry_$(BITS).o   := y
> >  OBJECT_FILES_NON_STANDARD_entry_64_compat.o := y
> >
> >  CFLAGS_syscall_64.o            += $(call cc-option,-Wno-override-init,)
> > diff --git a/arch/x86/entry/calling.h b/arch/x86/entry/calling.h
> > index 05ed3d3..bbec02e 100644
> > --- a/arch/x86/entry/calling.h
> > +++ b/arch/x86/entry/calling.h
> > @@ -1,4 +1,6 @@
> >  #include <linux/jump_label.h>
> > +#include <asm/undwarf.h>
> > +
> >
> >  /*
> >
> 
> Just to make sure I understand this, if we unwind from...
> 
> > @@ -112,6 +114,7 @@ For 32-bit we have the following conventions - kernel is built with
> >         movq %rdx, 12*8+\offset(%rsp)
> >         movq %rsi, 13*8+\offset(%rsp)
> 
> ...here..., will objtool think that rdx and rsi (etc) still live in
> their respective regs, or will it find them in the on-stack data given
> by CFI_REGS?  If the former, how does undwarf deal with the
> corresponding pops?

It will find them in their respective registers, which is fine because
they haven't been clobbered yet.

> 
> >         movq %rdi, 14*8+\offset(%rsp)
> > +       CFI_REGS offset=\offset extra=0

And here it will find them on the stack.

> > @@ -414,6 +424,7 @@ ENTRY(ret_from_fork)
> >  2:
> >         movq    %rsp, %rdi
> >         call    syscall_return_slowpath /* returns with IRQs disabled */
> > +       CFI_REGS
> 
> I'm confused.  syscall_return_slowpath didn't change anything relevant
> to unwinding, right?  What's CFI_REGS here for?

Yes, you're right, this CFI_REGS should be right at the '2' label.

-- 
Josh