linux-kernel - Re: [RFC PATCH 2/9] x86/purgatory: Simplify stack handling

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Message-ID: <pgsqflimo7d2ozhpmdrmscx25mxs5lypcvwq6mgjs7uixsxvsl@5papnq46mwbs>
Date: Fri, 26 Apr 2024 21:32:52 +0000
From: Justin Stitt <justinstitt@...gle.com>
To: Nathan Chancellor <nathan@...nel.org>
Cc: Ard Biesheuvel <ardb+git@...gle.com>, linux-kernel@...r.kernel.org, 
	x86@...nel.org, Ard Biesheuvel <ardb@...nel.org>, Arnd Bergmann <arnd@...db.de>, 
	Eric Biederman <ebiederm@...ssion.com>, kexec@...ts.infradead.org, 
	Nick Desaulniers <ndesaulniers@...gle.com>, Kees Cook <keescook@...omium.org>, 
	Bill Wendling <morbo@...gle.com>, Masahiro Yamada <masahiroy@...nel.org>
Subject: Re: [RFC PATCH 2/9] x86/purgatory: Simplify stack handling

Hi,

On Wed, Apr 24, 2024 at 11:26:59AM -0700, Nathan Chancellor wrote:
> On Wed, Apr 24, 2024 at 05:53:12PM +0200, Ard Biesheuvel wrote:
> > From: Ard Biesheuvel <ardb@...nel.org>
> > 
> > The x86 purgatory, which does little more than verify a SHA-256 hash of
> > the loaded segments, currently uses three different stacks:
> > - one in .bss that is used to call the purgatory C code
> > - one in .rodata that is only used to switch to an updated code segment
> >   descriptor in the GDT
> > - one in .data, which allows it to be prepopulated from the kexec loader
> >   in theory, but this is not actually being taken advantage of.
> > 
> > Simplify this, by dropping the latter two stacks, as well as the loader
> > logic that programs RSP.
> > 
> > Both the stacks in .bss and .data are 4k aligned, but 16 byte alignment
> > is more than sufficient.
> > 
> > Signed-off-by: Ard Biesheuvel <ardb@...nel.org>
> > ---
> >  arch/x86/include/asm/kexec.h      |  1 -
> >  arch/x86/kernel/kexec-bzimage64.c |  8 --------
> >  arch/x86/purgatory/entry64.S      |  8 --------
> >  arch/x86/purgatory/setup-x86_64.S |  2 +-
> >  arch/x86/purgatory/stack.S        | 18 ------------------
> 
> This needs a small fix up to build.
> 
>   make[6]: *** No rule to make target 'arch/x86/purgatory/stack.o', needed by 'arch/x86/purgatory/purgatory.ro'.
> 

I was trying to reproduce this build failure, but to no avail. I am
curious what your build target / build command was.

It is clear that stack.S has been removed so your change makes sense, I
don't doubt that -- I just cannot get that specific error message you
encountered (what is a .ro file supposed to be, anyway?).

> diff --git a/arch/x86/purgatory/Makefile b/arch/x86/purgatory/Makefile
> index acc09799af2a..2b6b2fb033d6 100644
> --- a/arch/x86/purgatory/Makefile
> +++ b/arch/x86/purgatory/Makefile
> @@ -1,7 +1,7 @@
>  # SPDX-License-Identifier: GPL-2.0
>  OBJECT_FILES_NON_STANDARD := y
>  
> -purgatory-y := purgatory.o stack.o setup-x86_$(BITS).o sha256.o entry64.o string.o
> +purgatory-y := purgatory.o setup-x86_$(BITS).o sha256.o entry64.o string.o
>  
>  targets += $(purgatory-y)
>  PURGATORY_OBJS = $(addprefix $(obj)/,$(purgatory-y))

Thanks
Justin