Fix stack pointer initialization

The last page of stack memory was not used before because we initialized the stack pointer with the start address of the inclusive end page of the stack. This commit fixes this by initializing the stack pointer with the exact configured address, aligned to a 16-byte boundary.

Author: phil-opp

common/src/lib.rs

@@ -210,11 +210,10 @@ where
         16,
         &mut used_entries,
     );
+    let stack_end_addr = stack_start_addr + config.kernel_stack_size;
+
     let stack_start: Page = Page::containing_address(stack_start_addr);
-    let stack_end = {
-        let end_addr = stack_start_addr + config.kernel_stack_size;
-        Page::containing_address(end_addr - 1u64)
-    };
+    let stack_end = Page::containing_address(stack_end_addr - 1u64);
     for page in Page::range_inclusive(stack_start, stack_end) {
         let frame = frame_allocator
             .allocate_frame()
@@ -387,7 +386,10 @@ where
     Mappings {
         framebuffer: framebuffer_virt_addr,
         entry_point,
-        stack_end,
+        // Use the configured stack size, even if it's not page aligned. However, we
+        // need to align it down to the next 16-byte boundary because the System V
+        // ABI requires a 16-byte stack alignment.
+        stack_top: stack_end_addr.align_down(16u8),
         used_entries,
         physical_memory_offset,
         recursive_index,
@@ -404,8 +406,8 @@ where
 pub struct Mappings {
     /// The entry point address of the kernel.
     pub entry_point: VirtAddr,
-    /// The stack end page of the kernel.
-    pub stack_end: Page,
+    /// The (exclusive) end address of the kernel stack.
+    pub stack_top: VirtAddr,
     /// Keeps track of used entries in the level 4 page table, useful for finding a free
     /// virtual memory when needed.
     pub used_entries: UsedLevel4Entries,
@@ -556,7 +558,7 @@ pub fn switch_to_kernel(
     } = page_tables;
     let addresses = Addresses {
         page_table: kernel_level_4_frame,
-        stack_top: mappings.stack_end.start_address(),
+        stack_top: mappings.stack_top,
         entry_point: mappings.entry_point,
         boot_info,
     };