Add a is_in_handler function to validate being in an interrupt handler

kernel/include/interrupts/Interrupts.h

@@ -1,7 +1,11 @@
 #pragma once
+#include "interrupts/SavedContext.h"
 
 namespace Interrupts
 {
     void enable();
     void disable();
+
+    bool is_in_handler();
+    void return_from_handler(SavedContext* context);
 }

kernel/src/interrupts/InterruptEntry.asm

@@ -37,6 +37,12 @@ unused:
 
 extern common_handler
 
+section .bss
+global __is_in_interrupt_handler
+__is_in_interrupt_handler:
+    resb 1
+
+section .text
 global asm_common
 asm_common:
     cld
@@ -66,12 +72,18 @@ asm_common:
     mov r8, cr2
     push r8
 
+    mov BYTE [__is_in_interrupt_handler], 1
+
     mov rdi, rsp
 
     call common_handler
 
+global _asm_interrupt_exit
+_asm_interrupt_exit:
     add rsp, 8
 
+    mov BYTE [__is_in_interrupt_handler], 0
+
     pop r8
     mov ds, r8
     mov es, r8

kernel/src/interrupts/Interrupts.cpp

@@ -8,4 +8,18 @@ void Interrupts::disable()
 void Interrupts::enable()
 {
     asm volatile("sti");
+}
+
+extern char __is_in_interrupt_handler;
+bool Interrupts::is_in_handler()
+{
+    return __is_in_interrupt_handler;
+}
+
+void Interrupts::return_from_handler(SavedContext* context)
+{
+    asm volatile("mov %0, %%rsp\n"
+                 "jmp _asm_interrupt_exit"
+                 :
+                 : "r"(context));
 }