From fb7dcf0f401cad2fb124044df8104747c008a2ed Mon Sep 17 00:00:00 2001
From: Artemis Tosini <me@artem.ist>
Date: Wed, 7 Aug 2024 02:28:18 +0000
Subject: [PATCH] Move kernel offset into one constant

---
 src/abi.rs    | 11 +++++++++++
 src/boot.rs   | 15 +++++++--------
 src/main.rs   | 14 ++++++--------
 src/object.rs |  4 ++--
 4 files changed, 26 insertions(+), 18 deletions(-)

diff --git a/src/abi.rs b/src/abi.rs
index 35cb6f0..1b87c1f 100644
--- a/src/abi.rs
+++ b/src/abi.rs
@@ -14,6 +14,17 @@ fn align_slice(unaligned: &mut [u8], alignment: usize) -> &mut [u8] {
     &mut unaligned[offset..]
 }
 
+// x86_64 FreeBSD kernels leave 2MiB between KERNBASE and first load,
+// but expect us to still be relative to KERNBASE
+#[cfg(target_arch = "x86_64")]
+pub const KERN_START_OFFSET: usize = 1 << 21;
+#[cfg(not(target_arch = "x86_64"))]
+pub const KERN_START_OFFSET: usize = 0;
+
+pub fn relative_addr(addr: *const u8, kern_base: *const u8) -> usize {
+    (addr as usize) - (kern_base as usize) + KERN_START_OFFSET
+}
+
 /// Serialize object into a buffer for FreeBSD
 pub trait Serialize {
     /// Output buffer must be aligned to this size
diff --git a/src/boot.rs b/src/boot.rs
index 978d0be..b7aa423 100644
--- a/src/boot.rs
+++ b/src/boot.rs
@@ -20,7 +20,10 @@ mod x86_64 {
     };
 
     use crate::{
-        abi::{EFIFramebufferParams, ModuleInfoItem, ModuleInfoType, ModuleInfoValue, Serialize},
+        abi::{
+            relative_addr, EFIFramebufferParams, ModuleInfoItem, ModuleInfoType, ModuleInfoValue,
+            Serialize, KERN_START_OFFSET,
+        },
         object::Kernel,
         staging::allocate_code,
     };
@@ -81,10 +84,6 @@ trampoline_size:
         free_ptr: *const u8,
         staging: &mut [u8],
     ) {
-        // FreeBSD x86_64 makes stuff relative to KERNBASE,
-        // 2MiB before the first mapping (KERNSTART)
-        let kernel_base = unsafe { kernel_start.byte_offset(-(1 << 21)) };
-
         let misc_alloc = allocate_code(
             system_table.boot_services(),
             core::mem::size_of::<MiscBuf>(),
@@ -112,7 +111,7 @@ trampoline_size:
 
         // Map the kernel to 2GiB from the top of RAM with 2MiB pages
         for (idx, pte) in misc_buf.pt2_upper.iter_mut().enumerate() {
-            let addr = kernel_base as usize + (idx << 21);
+            let addr = kernel_start as usize + (idx << 21) - KERN_START_OFFSET;
             pte.set_addr(
                 PhysAddr::new(addr as u64),
                 PageTableFlags::PRESENT | PageTableFlags::HUGE_PAGE | PageTableFlags::WRITABLE,
@@ -150,8 +149,8 @@ trampoline_size:
             trampoline_func(
                 &misc_buf.stack[PAGE_SIZE - 8] as *const u8,
                 &misc_buf.pt4 as *const PageTable,
-                modinfo_base.byte_offset_from(kernel_base) as usize,
-                free_ptr.byte_offset_from(kernel_base) as usize,
+                relative_addr(modinfo_base, kernel_start),
+                relative_addr(free_ptr, kernel_start),
                 entry_addr,
             );
         }
diff --git a/src/main.rs b/src/main.rs
index cb6b5b1..8d408b2 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -9,7 +9,9 @@ mod staging;
 
 extern crate alloc;
 
-use abi::{Environment, Howto, ModuleInfoItem, ModuleInfoType, ModuleInfoValue, Serialize};
+use abi::{
+    relative_addr, Environment, Howto, ModuleInfoItem, ModuleInfoType, ModuleInfoValue, Serialize,
+};
 use alloc::{collections::btree_map::BTreeMap, string::ToString};
 use hardware::add_acpi_environ;
 use log::info;
@@ -67,17 +69,13 @@ fn main(image_handle: Handle, mut system_table: SystemTable<Boot>) -> Status {
     let alloc_size = kernel.alloc_size() + environment.alloc_size() + modinfo_size;
     let staging = staging::allocate_code(system_table.boot_services(), alloc_size);
 
-    let (kernel_base, next) = kernel.serialize_unaligned_base(staging);
+    let (kern_base, next) = kernel.serialize_unaligned_base(staging);
 
     let (environ_base, next) = environment.serialize_unaligned_base(next);
 
     kernel_modinfo.push(abi::ModuleInfoItem {
         tag: ModuleInfoType::Environment,
-        value: ModuleInfoValue::Size(
-            (environ_base as usize)
-                .wrapping_sub(kernel_base as usize)
-                .wrapping_add(1 << 21),
-        ),
+        value: ModuleInfoValue::Size(relative_addr(environ_base, kern_base)),
     });
 
     kernel_modinfo.push(abi::ModuleInfoItem {
@@ -105,7 +103,7 @@ fn main(image_handle: Handle, mut system_table: SystemTable<Boot>) -> Status {
     boot::boot_kernel(
         system_table,
         kernel,
-        kernel_base,
+        kern_base,
         modinfo_base,
         free_ptr,
         next,
diff --git a/src/object.rs b/src/object.rs
index 7aaaf54..9b386d5 100644
--- a/src/object.rs
+++ b/src/object.rs
@@ -1,6 +1,6 @@
 extern crate alloc;
 
-use crate::abi::{ModuleInfoItem, ModuleInfoType, ModuleInfoValue, Serialize};
+use crate::abi::{ModuleInfoItem, ModuleInfoType, ModuleInfoValue, Serialize, KERN_START_OFFSET};
 use alloc::vec::Vec;
 use goblin::elf::{program_header, section_header, Elf, SectionHeader};
 use log::{debug, info};
@@ -198,7 +198,7 @@ impl<'a> Module for Kernel<'a> {
         ];
 
         if self.symbol_headers.is_some() {
-            let start_symbols = self.size_load_segments() + (1 << 21);
+            let start_symbols = self.size_load_segments() + KERN_START_OFFSET;
             let end_symbols = start_symbols + self.size_symbols();
 
             items.push(ModuleInfoItem {