linux-kernel - [PATCH 0/5] replace magic numbers in GDT descriptors

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Date: Tue, 19 Dec 2023 16:11:55 +0100
From: Vegard Nossum <vegard.nossum@...cle.com>
To: Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
        Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org
Cc: "H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org,
        Brian Gerst <brgerst@...il.com>, Peter Zijlstra <peterz@...radead.org>,
        Linus Torvalds <torvalds@...uxfoundation.org>,
        Vegard Nossum <vegard.nossum@...cle.com>
Subject: [PATCH 0/5] replace magic numbers in GDT descriptors

Linus suggested replacing the magic numbers in the GDT descriptors
using preprocessor macros [1].

[1] https://lore.kernel.org/all/CAHk-=wib5XLebuEra7y2YH96wxdk=8vJnA8XoVq0FExpzVvN=Q@mail.gmail.com/

For patch 3 ("x86: replace magic numbers in GDT descriptors, part 2") I've
verified it to the best of my abilities on 32-bit and 64-bit by ensuring
that the object files are bitwise identical before and after applying the
patch and ensuring that all the object files were rebuilt on at least one
of the two configs:

32-bit:
  arch/x86/boot/pm.o -- no change
  arch/x86/kernel/apm_32.o -- no change
  arch/x86/kernel/cpu/common.o -- no change
  arch/x86/kernel/head64.o -- not built for 32
  arch/x86/kernel/setup_percpu.o -- no change
  arch/x86/platform/pvh/head.o -- not built for 32
  arch/x86/realmode/rm/reboot.o -- no change
  drivers/firmware/efi/libstub/x86-5lvl.o -- not built for 32
  drivers/pnp/pnpbios/bioscalls.o -- no change

64-bit:
  arch/x86/boot/pm.o -- no change
  arch/x86/kernel/apm_32.o -- not built for 64
  arch/x86/kernel/cpu/common.o -- no change
  arch/x86/kernel/head64.o -- no change
  arch/x86/kernel/setup_percpu.o -- no change
  arch/x86/platform/pvh/head.o -- no change
  arch/x86/realmode/rm/reboot.o -- no change
  drivers/firmware/efi/libstub/x86-5lvl.o -- no change
  drivers/pnp/pnpbios/bioscalls.o -- not built for 64

Patches 2+3 can be squashed to a single commit, but I've submitted them
separately because it makes verifying correctness easier.

I've done basic boot testing on both 32-bit and 64-bit with all of the
patches.

Vegard Nossum (5):
  x86: provide new infrastructure for GDT descriptors
  x86: replace magic numbers in GDT descriptors, part 1
  x86: replace magic numbers in GDT descriptors, part 2
  x86: always set A (accessed) flag in GDT descriptors
  x86: add DB flag to 32-bit percpu GDT entry

 arch/x86/boot/pm.c                      |  7 +--
 arch/x86/include/asm/desc_defs.h        | 68 ++++++++++++++++++++++---
 arch/x86/kernel/apm_32.c                |  2 +-
 arch/x86/kernel/cpu/common.c            | 50 ++++++++----------
 arch/x86/kernel/head64.c                |  6 +--
 arch/x86/kernel/setup_percpu.c          |  4 +-
 arch/x86/platform/pvh/head.S            |  7 +--
 arch/x86/realmode/rm/reboot.S           |  3 +-
 drivers/firmware/efi/libstub/x86-5lvl.c |  4 +-
 drivers/pnp/pnpbios/bioscalls.c         |  2 +-
 10 files changed, 100 insertions(+), 53 deletions(-)

-- 
2.34.1

This is the script I used for verifying no binary changes (pass rev of
patch 3 to build as the only argumnet):

#! /bin/bash

set -e
set -u

rev=$(git rev-parse $1)

# get the paths that changed
paths="$(git diff --name-only $rev^- | grep '\.[cS]$' | sed 's/\.[cS]$/.o/')"

build() {
	id=$1

	# build without patch and save result as <path>.pre-<id>
	git checkout -d $rev^
	rm -f $paths
	make -j128
	for p in $paths
	do
		if [ -e $p ]
		then
			mv -f $p $p.pre-$id
		fi
	done

	# build with patch and save result as <path>.post-<id>
	git checkout -d $rev
	rm -f $paths
	make -j128
	for p in $paths
	do
		if [ -e $p ]
		then
			mv -f $p $p.post-$id
		fi
	done
}

# build i386
make defconfig
scripts/config --disable 64BIT
scripts/config --disable MODULES
# for arch/x86/kernel/apm_32.o
scripts/config --enable APM
# for drivers/pnp/pnpbios/bioscalls.o
scripts/config --enable ISA --enable PNP --enable PNPBIOS
make olddefconfig
build 32

# build x86_64
make defconfig
scripts/config --disable MODULES
# for arch/x86/platform/pvh/head.o
scripts/config --enable XEN --enable XEN_PVHVM --enable PVH
make olddefconfig
build 64

echo
echo results:
echo

for id in 32 64
do
	echo $id:

	for p in $paths
	do
		if [ -e $p.pre-$id ] && [ -e $p.post-$id ]
		then
			if cmp -s $p.pre-$id $p.post-$id
			then
				echo $p -- no change
			else
				echo $p -- differences:
				diff -U3 <(objdump -dr $p.pre-$id | tail -n +3) <(objdump -dr $p.post-$id | tail -n +3) || true
			fi
		else
			echo $p -- not built for $id
		fi
	done

	echo
done