Kernel: Implement thread start trampoline for userspace

This is needed on i686 to set segment registers.

kernel/arch/x86_64/Thread.S

@@ -24,3 +24,29 @@ start_kernel_thread:
 	movq 24(%rsp), %rdi
 	movq 16(%rsp), %rsi
 	call *%rsi
+
+.global start_userspace_thread
+start_userspace_thread:
+	call get_start_kernel_thread_sp
+	movq %rax, %rsp
+
+	# STACK LAYOUT
+	#   entry
+	#   argc
+	#   argv
+	#   envp
+	#   userspace stack
+
+	call get_userspace_thread_stack_top
+
+	popq %rdx
+	popq %rsi
+	popq %rdi
+	popq %rcx
+
+	pushq $(0x20 | 3)
+	pushq %rax
+	pushq $0x202
+	pushq $(0x18 | 3)
+	pushq %rcx
+	iretq

kernel/kernel/Scheduler.cpp

@@ -29,11 +29,6 @@ namespace Kernel
 		return *s_instance;
 	}
 
-	extern "C" uintptr_t get_start_kernel_thread_sp()
-	{
-		return Scheduler::get().current_thread().kernel_stack_top() - 4 * sizeof(uintptr_t);
-	}
-
 	void Scheduler::start()
 	{
 		ASSERT(Processor::get_interrupt_state() == InterruptState::Disabled);

kernel/kernel/Thread.cpp

@@ -13,6 +13,7 @@ namespace Kernel
 {
 
 	extern "C" [[noreturn]] void start_kernel_thread();
+	extern "C" [[noreturn]] void start_userspace_thread();
 
 	extern "C" void signal_trampoline();
 
@@ -23,6 +24,16 @@ namespace Kernel
 		*(uintptr_t*)rsp = (uintptr_t)value;
 	}
 
+	extern "C" uintptr_t get_start_kernel_thread_sp()
+	{
+		return Thread::current().kernel_stack_top() - 4 * sizeof(uintptr_t);
+	}
+
+	extern "C" uintptr_t get_userspace_thread_stack_top()
+	{
+		return Thread::current().userspace_stack_top() - 4 * sizeof(uintptr_t);
+	}
+
 	static pid_t s_next_tid = 1;
 
 	BAN::ErrorOr<Thread*> Thread::create_kernel(entry_t entry, void* data, Process* process)
@@ -197,19 +208,19 @@ namespace Kernel
 		ASSERT(userspace_info.entry);
 
 		// Initialize stack for returning
-		PageTable::with_fast_page(process().page_table().physical_address_of(userspace_stack_top() - PAGE_SIZE), [&] {
+		PageTable::with_fast_page(process().page_table().physical_address_of(kernel_stack_top() - PAGE_SIZE), [&] {
 			uintptr_t sp = PageTable::fast_page() + PAGE_SIZE;
-			write_to_stack(sp, nullptr);
+			write_to_stack(sp, userspace_info.entry);
 			write_to_stack(sp, userspace_info.argc);
 			write_to_stack(sp, userspace_info.argv);
 			write_to_stack(sp, userspace_info.envp);
 		});
 
-		m_interrupt_stack.ip = userspace_info.entry;
-		m_interrupt_stack.cs = 0x18 | 3;
-		m_interrupt_stack.flags = 0x202;
-		m_interrupt_stack.sp = userspace_stack_top() - 4 * sizeof(uintptr_t);
-		m_interrupt_stack.ss = 0x20 | 3;
+		m_interrupt_stack.ip = reinterpret_cast<vaddr_t>(start_userspace_thread);;
+		m_interrupt_stack.cs = 0x08;
+		m_interrupt_stack.flags = 0x002;
+		m_interrupt_stack.sp = kernel_stack_top() - 4 * sizeof(uintptr_t);
+		m_interrupt_stack.ss = 0x10;
 
 		memset(&m_interrupt_registers, 0, sizeof(InterruptRegisters));
 	}

libc/arch/x86_64/crt0.S

@@ -2,6 +2,11 @@
 
 .global _start
 _start:
+	pushq $0
+	pushq %rdi
+	pushq %rsi
+	pushq %rdx
+
 	# STACK LAYOUT
 	#   null
 	#   argc