From a85b624c0698873155668ff91fc8edc39fb1c33e Mon Sep 17 00:00:00 2001
From: "Chloe M." <chloe@aptel.org>
Date: Thu, 20 Aug 2026 08:23:06 +0000
Subject: boot/amd64: mbr: Verify RDAR image

Signed-off-by: Chloe M. <chloe@aptel.org>
---
 boot/arch/amd64/mbr/mbr.S | 49 +++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 49 insertions(+)

(limited to 'boot/arch')

diff --git a/boot/arch/amd64/mbr/mbr.S b/boot/arch/amd64/mbr/mbr.S
index ee58525..e6c84fa 100644
--- a/boot/arch/amd64/mbr/mbr.S
+++ b/boot/arch/amd64/mbr/mbr.S
@@ -2,6 +2,10 @@
 #define BOOT_SEG       0x7C0    /* Segment we reside on startup */
 #define BOOT_RELOC_SEG 0x7A0    /* Relocation segment */
 
+/* Constants */
+#define RDAR_MAGIC ">RDAR!"
+#define RDAR_MAGIC_LEN 6
+
     .code16
 _start:
     /* Normalize CS + other selectors */
@@ -29,6 +33,10 @@ _start:
     /* Jump to our relocated self */
     ljmp $BOOT_RELOC_SEG,$reloc
 reloc:
+    /* Load %DS with our new segment */
+    mov $BOOT_RELOC_SEG, %ax
+    mov %ax, %ds
+
     /* Save the drive number */
     mov %dl, drive_num
 
@@ -44,10 +52,49 @@ reloc:
     movl $1, dap_lba_low
     call disk_read_lba
 
+    /* Get stage2 */
+    call find_stage2
+
 1:  cli
     hlt
     jmp 1b
 
+/*
+ * Locate stage2 on the disk
+ *
+ * TODO: This only verifies the magic number is correct, this should
+ *       be completed.
+ */
+find_stage2:
+    push %ds
+    push %es
+
+    /* Verify that magic is correct */
+    mov $BOOT_SEG, %ax
+    mov %ax, %ds
+    xor %si, %si
+    mov $BOOT_RELOC_SEG, %ax
+    mov %ax, %es
+    mov $rdar_magic, %di
+    mov $RDAR_MAGIC_LEN, %cx
+    rep cmpsb
+    jne .bad_magic
+
+    pop %es
+    pop %ds
+    ret
+.bad_magic:
+    /* Restore %DS */
+    mov $BOOT_RELOC_SEG, %ax
+    mov %ax, %ds
+
+    mov $bad_magic_msg, %si
+    call putstr
+1:  cli
+    hlt
+    jmp 1b
+
+
 /*
  * Read sectors from the disk
  */
@@ -109,7 +156,9 @@ dap_lba_high:
 
 drive_num: .byte 0x00
 boot_msg: .ascii "booting\r\n\0"
+bad_magic_msg: .ascii "bad RDAR magic\r\n\0"
 disk_err_msg: .ascii "\r\n!!!disk error!!!\r\n\0"
+rdar_magic: .ascii RDAR_MAGIC
 
     .org 510
     .byte 0x55
-- 
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