kernel: Switch to a stack we control for the main task as soon as we leave early boot

This is probably pointless as the main task only exists up until the point where the Scheduler starts running, but some init code was running into a stack overflow due to the fact that we only had one bootloader-provided page of stack.

This should probably be changed afterwards.

kernel/src/arch/CPU.h

@@ -26,5 +26,7 @@ namespace CPU
     void get_stack_trace_at(Registers* regs, void (*callback)(u64, void*), void* arg);
     void print_stack_trace_at(Registers* regs);
 
+    [[noreturn]] void bootstrap_switch_stack(u64 stack, void* function);
+
     void pause();
 }

kernel/src/arch/x86_64/CPU.cpp

@@ -295,6 +295,15 @@ namespace CPU
             &frame_index);
     }
 
+    [[noreturn]] void bootstrap_switch_stack(u64 stack, void* function)
+    {
+        asm volatile("mov %0, %%rsp\n"
+                     "jmp *%1"
+                     :
+                     : "r"(stack), "r"(function));
+        __builtin_unreachable();
+    }
+
     void pause()
     {
         asm volatile("pause");

kernel/src/main.cpp

@@ -90,11 +90,19 @@ Result<void> init()
     return {};
 }
 
-extern "C" [[noreturn]] void _start()
+[[noreturn]] void init_wrapper()
 {
-    Init::check_magic();
-    Init::early_init();
     auto rc = init();
     if (rc.has_error()) kerrorln("Runtime error: %s", rc.error_string());
     CPU::idle_loop();
 }
+
+// FIXME: Add a guard page to make sure the stack doesn't end up in random kernel memory. Also reclaim this memory after
+// leaving the init task.
+extern "C" [[noreturn]] void _start()
+{
+    Init::check_magic();
+    Init::early_init();
+    Stack stack { MemoryManager::alloc_for_kernel(8, MMU::ReadWrite | MMU::NoExecute).value(), 8 * ARCH_PAGE_SIZE };
+    CPU::bootstrap_switch_stack(stack.top(), (void*)init_wrapper);
+}