Debug output intercept demo

Author: thrimbor

samples/hello/Makefile

@@ -1,5 +1,5 @@
 XBE_TITLE = hello
 GEN_XISO = $(XBE_TITLE).iso
-SRCS = $(CURDIR)/main.c
+SRCS = $(CURDIR)/main.c $(CURDIR)/int.s
 NXDK_DIR = $(CURDIR)/../..
 include $(NXDK_DIR)/Makefile

samples/hello/int.s

@@ -0,0 +1,19 @@
+.globl _dbg_output_isr_stub
+_dbg_output_isr_stub:
+    pushl %ebp
+    pushl %edi
+    pushl %esi
+    pushl %edx
+    pushl %ecx
+    pushl %ebx
+    pushl %eax
+    call _dbg_output
+    popl %eax
+    popl %ebx
+    popl %ecx
+    popl %edx
+    popl %esi
+    popl %edi
+    popl %ebp
+.extern dbg_output_original_isr_addr
+    jmp *_dbg_output_original_isr_addr

samples/hello/main.c

@@ -2,12 +2,80 @@
 #include <hal/video.h>
 #include <windows.h>
 
+struct idtr_t
+{
+    uint16_t limit;
+    uint32_t base;
+} __attribute__((packed));
+
+struct idt_descriptor_t
+{
+	uint16_t offset_low;
+	uint16_t selector;
+    uint8_t zero;
+	uint8_t type_attr;
+	uint16_t offset_high;
+} __attribute__((packed));
+
+void get_idtr (struct idtr_t *idtr)
+{
+    asm ("sidt %0" : "=m" (*idtr));
+}
+
+uint32_t dbg_output_original_isr_addr;
+extern char dbg_output_isr_stub;
+
+void hook_int_2d ()
+{
+    struct idtr_t idtr;
+    struct idt_descriptor_t *entries;
+    uint32_t new_isr = (uint32_t)&dbg_output_isr_stub;
+
+    get_idtr(&idtr);
+    entries = (struct idt_descriptor_t *)idtr.base;
+
+    uint32_t old_isr = (((uint32_t)entries[0x2d].offset_high) << 16) | entries[0x2d].offset_low;
+    dbg_output_original_isr_addr = old_isr;
+    entries[0x2d].offset_low = ((uint32_t)new_isr) & 0xffff;
+    entries[0x2d].offset_high = (((uint32_t)new_isr) >> 16) & 0xffff;
+}
+
+void unhook_int_2d ()
+{
+    struct idtr_t idtr;
+    struct idt_descriptor_t *entries;
+
+    get_idtr(&idtr);
+    entries = (struct idt_descriptor_t *)idtr.base;
+    entries[0x2d].offset_low = dbg_output_original_isr_addr & 0xffff;
+    entries[0x2d].offset_high = (dbg_output_original_isr_addr >> 16) & 0xffff;
+}
+
+__cdecl void dbg_output (uint32_t eax, uint32_t ebx, uint32_t ecx, uint32_t edx, uint32_t esi, uint32_t edi, uint32_t ebp)
+{
+    if (eax == 1) {
+        PANSI_STRING s = (PANSI_STRING)ecx;
+        debugPrint("%s\n", s->Buffer);
+    }
+}
+
 int main(void)
 {
     XVideoSetMode(640, 480, 32, REFRESH_DEFAULT);
 
+    hook_int_2d();
+
+    // This will be visible on screen
+    OutputDebugString("test\n");
+
+    unhook_int_2d();
+
+    // This will not be visible on screen
+    OutputDebugString("test2\n");
+
+    debugPrint("tests done\n");
+
     while(1) {
-        debugPrint("Hello nxdk!\n");
         Sleep(2000);
     }