linux-kernel - Re: [PATCH] x86: make 32-bit "emergency stack" better documented

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Date:	Mon, 9 Mar 2015 06:43:24 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Denys Vlasenko <dvlasenk@...hat.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...nel.org>, Borislav Petkov <bp@...en8.de>,
	"H. Peter Anvin" <hpa@...or.com>, Oleg Nesterov <oleg@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Alexei Starovoitov <ast@...mgrid.com>,
	Will Drewry <wad@...omium.org>,
	Kees Cook <keescook@...omium.org>, X86 ML <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86: make 32-bit "emergency stack" better documented

On Mon, Mar 9, 2015 at 6:28 AM, Denys Vlasenko <dvlasenk@...hat.com> wrote:
> Before the patch, tss.stack field was not referenced anywhere.
> It was used only by setting sysenter's stack to point after
> last byte of tss, thus the trailing field, stack[64], was used.
> But grep would not know it. You can comment it out, compile,
> and kernel will even run until an unlucky NMI corrupts
> iomap[] (which is also not easily detectable).
>
> This patch changes code so the purpose and usage of this field
> is not mysterious anymore, and can be easily grepped for.
>
> This does change generated code, for a subtle reason:
> since tss_struct is ____cacheline_aligned, there happen to be
> 5 longs of padding at the end. Old code was using the padding too;
> new code will strictly use only SYSENTER_stack[].
>
> Signed-off-by: Denys Vlasenko <dvlasenk@...hat.com>
> CC: Linus Torvalds <torvalds@...ux-foundation.org>
> CC: Steven Rostedt <rostedt@...dmis.org>
> CC: Ingo Molnar <mingo@...nel.org>
> CC: Borislav Petkov <bp@...en8.de>
> CC: "H. Peter Anvin" <hpa@...or.com>
> CC: Andy Lutomirski <luto@...capital.net>
> CC: Oleg Nesterov <oleg@...hat.com>
> CC: Frederic Weisbecker <fweisbec@...il.com>
> CC: Alexei Starovoitov <ast@...mgrid.com>
> CC: Will Drewry <wad@...omium.org>
> CC: Kees Cook <keescook@...omium.org>
> CC: x86@...nel.org
> CC: linux-kernel@...r.kernel.org
>
> ---
>  arch/x86/include/asm/processor.h | 5 +++--
>  arch/x86/kernel/asm-offsets_32.c | 2 +-
>  arch/x86/kernel/cpu/common.c     | 2 +-
>  3 files changed, 5 insertions(+), 4 deletions(-)
>
> diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
> index 48a61c1..4172241 100644
> --- a/arch/x86/include/asm/processor.h
> +++ b/arch/x86/include/asm/processor.h
> @@ -276,9 +276,10 @@ struct tss_struct {
>         unsigned long           io_bitmap[IO_BITMAP_LONGS + 1];
>
>         /*
> -        * .. and then another 0x100 bytes for the emergency kernel stack:
> +        * and then space for temporary SYSENTER stack:
>          */
> -       unsigned long           stack[64];
> +       unsigned long           SYSENTER_stack[63];
> +       unsigned long           SYSENTER_stack_last_word;
>
>  } ____cacheline_aligned;
>
> diff --git a/arch/x86/kernel/asm-offsets_32.c b/arch/x86/kernel/asm-offsets_32.c
> index 3b3b9d3..4a3cbd0 100644
> --- a/arch/x86/kernel/asm-offsets_32.c
> +++ b/arch/x86/kernel/asm-offsets_32.c
> @@ -68,7 +68,7 @@ void foo(void)
>
>         /* Offset from the sysenter stack to tss.sp0 */
>         DEFINE(TSS_sysenter_sp0, offsetof(struct tss_struct, x86_tss.sp0) -
> -                sizeof(struct tss_struct));
> +               (offsetof(struct tss_struct, SYSENTER_stack_last_word) + 4));

Is there no way to do this using sizeof to avoid having a "last_word" member.

--Andy
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