From 672e36f401b8e6d5ce473c904e2863ddd4545331 Mon Sep 17 00:00:00 2001
From: bjorn3 <17426603+bjorn3@users.noreply.github.com>
Date: Thu, 14 Mar 2024 21:22:13 +0100
Subject: [PATCH] Force the zero page of the kernel to be part of a segment

This will be necessary for limine and likely multiboot support. See the
comment in the linker scripts.
---
 linkers/aarch64.ld | 15 ++++++++++++++-
 linkers/i686.ld    | 12 ++++++++++++
 linkers/x86_64.ld  | 12 ++++++++++++
 3 files changed, 38 insertions(+), 1 deletion(-)

diff --git a/linkers/aarch64.ld b/linkers/aarch64.ld
index cce4c9edc9..e3e7b80c66 100644
--- a/linkers/aarch64.ld
+++ b/linkers/aarch64.ld
@@ -7,6 +7,19 @@ SECTIONS {
     . = KERNEL_OFFSET;
 
     . += SIZEOF_HEADERS;
+
+    /* Force the zero page to be part of a segment by creating a
+     * dummy section in the zero page.
+     * Limine will map the segment with the lowest vaddr value at
+     * 0xFFFFFFFF80000000 even if the segment has a higher vaddr.
+     * As such without the zero page being part of a segment, the
+     * kernel would be loaded at an offset from the expected
+     * location. As the redox kernel is not currently relocatable,
+     * this would result in a crash. A similar issue likely exists
+     * with multiboot/multiboot2 and the paddr of the segment.
+     */
+    .dummy ALIGN(8) : AT(ADDR(.dummy) - KERNEL_OFFSET) {}
+
     . = ALIGN(4096);
 
     .text : AT(ADDR(.text) - KERNEL_OFFSET) {
@@ -21,7 +34,7 @@ SECTIONS {
         __text_end = .;
     }
 
-	.rodata : AT(ADDR(.rodata) - KERNEL_OFFSET) {
+    .rodata : AT(ADDR(.rodata) - KERNEL_OFFSET) {
         __rodata_start = .;
         *(.rodata*)
 	. = ALIGN(4096);
diff --git a/linkers/i686.ld b/linkers/i686.ld
index 0405e100a2..f7eb9c4295 100644
--- a/linkers/i686.ld
+++ b/linkers/i686.ld
@@ -8,6 +8,18 @@ SECTIONS {
 
     . += SIZEOF_HEADERS;
 
+    /* Force the zero page to be part of a segment by creating a
+     * dummy section in the zero page.
+     * Limine will map the segment with the lowest vaddr value at
+     * 0xFFFFFFFF80000000 even if the segment has a higher vaddr.
+     * As such without the zero page being part of a segment, the
+     * kernel would be loaded at an offset from the expected
+     * location. As the redox kernel is not currently relocatable,
+     * this would result in a crash. A similar issue likely exists
+     * with multiboot/multiboot2 and the paddr of the segment.
+     */
+    .dummy : AT(ADDR(.dummy) - KERNEL_OFFSET) {}
+
     .text ALIGN(4K) : AT(ADDR(.text) - KERNEL_OFFSET) {
         __text_start = .;
         *(.text*)
diff --git a/linkers/x86_64.ld b/linkers/x86_64.ld
index c6c04349d9..6610509aef 100644
--- a/linkers/x86_64.ld
+++ b/linkers/x86_64.ld
@@ -8,6 +8,18 @@ SECTIONS {
 
     . += SIZEOF_HEADERS;
 
+    /* Force the zero page to be part of a segment by creating a
+     * dummy section in the zero page.
+     * Limine will map the segment with the lowest vaddr value at
+     * 0xFFFFFFFF80000000 even if the segment has a higher vaddr.
+     * As such without the zero page being part of a segment, the
+     * kernel would be loaded at an offset from the expected
+     * location. As the redox kernel is not currently relocatable,
+     * this would result in a crash. A similar issue likely exists
+     * with multiboot/multiboot2 and the paddr of the segment.
+     */
+    .dummy : AT(ADDR(.dummy) - KERNEL_OFFSET) {}
+
     .text ALIGN(4K) : AT(ADDR(.text) - KERNEL_OFFSET) {
         __text_start = .;
         *(.text*)