linux-kernel - Re: [PATCH] x86/smap: Fix the smap

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Date:   Thu, 17 Sep 2020 09:32:32 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Borislav Petkov <bp@...en8.de>
Cc:     David Laight <David.Laight@...lab.com>,
        Andrew Cooper <andrew.cooper3@...rix.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Andy Lutomirski <luto@...nel.org>,
        Bill Wendling <morbo@...gle.com>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Greg Thelen <gthelen@...gle.com>,
        John Sperbeck <jsperbeck@...gle.com>,
        "# 3.4.x" <stable@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>
Subject: Re: [PATCH] x86/smap: Fix the smap_save() asm

On Thu, Sep 17, 2020 at 7:39 AM Borislav Petkov <bp@...en8.de> wrote:
>
> On Thu, Sep 17, 2020 at 02:25:50PM +0000, David Laight wrote:
> > I actually wonder if there is any code that really benefits from
> > the red-zone.
>
> The kernel has been without a red zone since 2002 at least:
>
>   commit 47f16da277d10ef9494f3e9da2a9113bb22bcd75
>   Author: Andi Kleen <ak@....de>
>   Date:   Tue Feb 12 20:17:35 2002 -0800
>
>       [PATCH] x86_64 merge: arch + asm
>
>       This adds the x86_64 arch and asm directories and a Documentation/x86_64.
>
>   ...
>   +CFLAGS += $(shell if $(CC) -mno-red-zone -S -o /dev/null -xc /dev/null >/dev/null 2>&1; then echo "-mno-red-zone"; fi )
>
>
> Also, from the ABI doc:
>
> "A.2.2 Stack Layout
>
> The Linux kernel may align the end of the input argument area to a
> 8, instead of 16, byte boundary. It does not honor the red zone (see
> section 3.2.2) and therefore this area is not allowed to be used by
> kernel code. Kernel code should be compiled by GCC with the option
> -mno-red-zone."
>
> so forget the red zone.
>
> --
> Regards/Gruss,
>     Boris.
>
> https://people.kernel.org/tglx/notes-about-netiquette

Regardless of anything that any docs may or may not say, the kernel
*can't* use a redzone -- an interrupt would overwrite it.