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

From: Andy Lutomirski
Date: Thu Jun 01 2017 - 10:03:44 EST


On Wed, May 31, 2017 at 10:44 PM, Josh Poimboeuf <jpoimboe@xxxxxxxxxx> 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@xxxxxxxxxx>
> ---
> 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?

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

> @@ -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?