linux-kernel - Re: [PATCH 6/6] x86: entry_64.S: use more understandable constants

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Date:	Mon, 23 Feb 2015 16:37:14 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	Denys Vlasenko <dvlasenk@...hat.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Borislav Petkov <bp@...en8.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	X86 ML <x86@...nel.org>, Alexei Starovoitov <ast@...mgrid.com>,
	Will Drewry <wad@...omium.org>,
	Kees Cook <keescook@...omium.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 6/6] x86: entry_64.S: use more understandable constants

On Mon, Feb 23, 2015 at 4:12 PM, Denys Vlasenko <dvlasenk@...hat.com> wrote:
> Constants such as SS+8 or SS+8-RIP are mysterious.
> In most cases, SS+8 is just meant to be SIZEOF_PTREGS,
> SS+8-RIP is RIP's offset in iret frame.
>
> This patch changes some of these constants to be less mysterious.
>
> No code changes (verified with objdump).

Applied.

>
> Signed-off-by: Denys Vlasenko <dvlasenk@...hat.com>
> CC: Linus Torvalds <torvalds@...ux-foundation.org>
> CC: Oleg Nesterov <oleg@...hat.com>
> CC: Borislav Petkov <bp@...en8.de>
> CC: "H. Peter Anvin" <hpa@...or.com>
> CC: Andy Lutomirski <luto@...capital.net>
> CC: Frederic Weisbecker <fweisbec@...il.com>
> CC: X86 ML <x86@...nel.org>
> CC: Alexei Starovoitov <ast@...mgrid.com>
> CC: Will Drewry <wad@...omium.org>
> CC: Kees Cook <keescook@...omium.org>
> CC: linux-kernel@...r.kernel.org
> ---
>  arch/x86/include/asm/calling.h |  2 ++
>  arch/x86/kernel/entry_64.S     | 28 ++++++++++++++++------------
>  2 files changed, 18 insertions(+), 12 deletions(-)
>
> diff --git a/arch/x86/include/asm/calling.h b/arch/x86/include/asm/calling.h
> index 4a7ceb9..3374235 100644
> --- a/arch/x86/include/asm/calling.h
> +++ b/arch/x86/include/asm/calling.h
> @@ -88,6 +88,8 @@ For 32-bit we have the following conventions - kernel is built with
>  #define RSP            19*8
>  #define SS             20*8
>
> +#define SIZEOF_PTREGS  21*8
> +
>         .macro ALLOC_PT_GPREGS_ON_STACK addskip=0
>         subq    $15*8+\addskip, %rsp
>         CFI_ADJUST_CFA_OFFSET 15*8+\addskip
> diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S
> index c628a1b..2fa9e59 100644
> --- a/arch/x86/kernel/entry_64.S
> +++ b/arch/x86/kernel/entry_64.S
> @@ -164,12 +164,12 @@ ENDPROC(native_usergs_sysret64)
>   * initial frame state for interrupts (and exceptions without error code)
>   */
>         .macro INTR_FRAME start=1 offset=0
> -       EMPTY_FRAME \start, SS+8+\offset-RIP
> -       /*CFI_REL_OFFSET ss, SS+\offset-RIP*/
> -       CFI_REL_OFFSET rsp, RSP+\offset-RIP
> -       /*CFI_REL_OFFSET rflags, EFLAGS+\offset-RIP*/
> -       /*CFI_REL_OFFSET cs, CS+\offset-RIP*/
> -       CFI_REL_OFFSET rip, RIP+\offset-RIP
> +       EMPTY_FRAME \start, 5*8+\offset
> +       /*CFI_REL_OFFSET ss, 4*8+\offset*/
> +       CFI_REL_OFFSET rsp, 3*8+\offset
> +       /*CFI_REL_OFFSET rflags, 2*8+\offset*/
> +       /*CFI_REL_OFFSET cs, 1*8+\offset*/
> +       CFI_REL_OFFSET rip, 0*8+\offset
>         .endm
>
>  /*
> @@ -177,7 +177,7 @@ ENDPROC(native_usergs_sysret64)
>   * with vector already pushed)
>   */
>         .macro XCPT_FRAME start=1 offset=0
> -       INTR_FRAME \start, RIP+\offset-ORIG_RAX
> +       INTR_FRAME \start, 1*8+\offset
>         .endm
>
>  /*
> @@ -644,10 +644,14 @@ END(interrupt)
>         cmovzq PER_CPU_VAR(irq_stack_ptr),%rsp
>         CFI_DEF_CFA_REGISTER    rsi
>         pushq %rsi
> +       /*
> +        * For debugger:
> +        * "CFA (Current Frame Address) is the value on stack + offset"
> +        */
>         CFI_ESCAPE      0x0f /* DW_CFA_def_cfa_expression */, 6, \
> -                       0x77 /* DW_OP_breg7 */, 0, \
> +                       0x77 /* DW_OP_breg7 (rsp) */, 0, \
>                         0x06 /* DW_OP_deref */, \
> -                       0x08 /* DW_OP_const1u */, SS+8-RBP, \
> +                       0x08 /* DW_OP_const1u */, SIZEOF_PTREGS-RBP, \
>                         0x22 /* DW_OP_plus */
>         /* We entered an interrupt context - irqs are off: */
>         TRACE_IRQS_OFF
> @@ -673,7 +677,7 @@ ret_from_intr:
>
>         /* Restore saved previous stack */
>         popq %rsi
> -       CFI_DEF_CFA rsi,SS+8-RBP        /* reg/off reset after def_cfa_expr */
> +       CFI_DEF_CFA rsi,SIZEOF_PTREGS-RBP /* reg/off reset after def_cfa_expr */
>         /* return code expects complete pt_regs - adjust rsp accordingly: */
>         leaq -RBP(%rsi),%rsp
>         CFI_DEF_CFA_REGISTER    rsp
> @@ -1539,7 +1543,7 @@ first_nmi:
>         .rept 5
>         pushq_cfi 11*8(%rsp)
>         .endr
> -       CFI_DEF_CFA_OFFSET SS+8-RIP
> +       CFI_DEF_CFA_OFFSET 5*8
>
>         /* Everything up to here is safe from nested NMIs */
>
> @@ -1567,7 +1571,7 @@ repeat_nmi:
>         pushq_cfi -6*8(%rsp)
>         .endr
>         subq $(5*8), %rsp
> -       CFI_DEF_CFA_OFFSET SS+8-RIP
> +       CFI_DEF_CFA_OFFSET 5*8
>  end_repeat_nmi:
>
>         /*
> --
> 1.8.1.4
>



-- 
Andy Lutomirski
AMA Capital Management, LLC
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