kernel: Separate a thread's page directory into two

The self directory, and the active directory. The active directory is the one the thread is currently using,
and the self directory is the one the thread owns.

This lets us keep track of both, which fixes ext2 executables crashing the system.

kernel/src/arch/x86_64/disk/ATA.cpp

@@ -146,7 +146,7 @@ namespace ATA
     {
         if (drive == m_current_drive) return;
 
-        u8 value = (drive << 4) | 0xa0;
+        u8 value = (u8)(drive << 4) | 0xa0;
         write_register(Register::DriveSelect, value);
 
         delay_400ns();

kernel/src/sys/exec.cpp

@@ -70,7 +70,7 @@ Result<u64> sys_execve(Registers* regs, SyscallArgs args)
     kdbgln("exec: attempting to replace current image with %s", path.chars());
 #endif
 
-    auto guard = make_scope_guard([current] { MMU::switch_page_directory(current->directory); });
+    auto guard = make_scope_guard([current] { MMU::switch_page_directory(current->self_directory); });
 
     auto image = TRY(ThreadImage::try_load_from_elf(inode));
 
@@ -99,7 +99,7 @@ Result<u64> sys_execve(Registers* regs, SyscallArgs args)
         }
     }
 
-    MMU::delete_userspace_page_directory(current->directory);
+    MMU::delete_userspace_page_directory(current->self_directory);
 
     if (VFS::is_setuid(inode)) current->auth.euid = current->auth.suid = inode->uid();
     if (VFS::is_setgid(inode)) current->auth.egid = current->auth.sgid = inode->gid();
@@ -108,7 +108,7 @@ Result<u64> sys_execve(Registers* regs, SyscallArgs args)
 
     image->apply(current);
 
-    MMU::switch_page_directory(current->directory);
+    MMU::switch_page_directory(current->self_directory);
 
     current->set_arguments(user_argc, user_argv, user_envc, user_envp);
 
@@ -123,7 +123,7 @@ Result<u64> sys_fork(Registers* regs, SyscallArgs)
 {
     auto current = Scheduler::current();
 
-    auto guard = make_scope_guard([current] { MMU::switch_page_directory(current->directory); });
+    auto guard = make_scope_guard([current] { MMU::switch_page_directory(current->self_directory); });
 
     memcpy(&current->regs, regs, sizeof(*regs));
 

kernel/src/thread/Scheduler.cpp

@@ -191,7 +191,7 @@ namespace Scheduler
             }
         }
 
-        if (!thread->is_kernel) MMU::delete_userspace_page_directory(thread->directory);
+        if (!thread->is_kernel) MMU::delete_userspace_page_directory(thread->self_directory);
 
         delete thread;
 
@@ -236,8 +236,8 @@ namespace Scheduler
             switch_context(old_thread, new_thread, regs);
             if (!old_thread->is_kernel) old_thread->fp_data.save();
             if (MMU::get_page_directory() != MMU::kernel_page_directory())
-                old_thread->directory = MMU::get_page_directory();
-            if (new_thread->directory) MMU::switch_page_directory(new_thread->directory);
+                old_thread->active_directory = MMU::get_page_directory();
+            if (new_thread->active_directory) MMU::switch_page_directory(new_thread->active_directory);
             if (!new_thread->is_kernel)
             {
                 CPU::switch_kernel_stack(new_thread->kernel_stack.top());

kernel/src/thread/Thread.h

@@ -97,7 +97,8 @@ struct Thread : public LinkedListNode<Thread>
     Thread* parent { nullptr };
     Option<pid_t> child_being_waited_for = {};
 
-    PageDirectory* directory;
+    PageDirectory* self_directory;
+    PageDirectory* active_directory { nullptr };
 
     [[noreturn]] void exit_and_signal_parent(u8 status);
 

kernel/src/thread/ThreadImage.cpp

@@ -69,7 +69,7 @@ Result<OwnedPtr<ThreadImage>> ThreadImage::clone_from_thread(Thread* parent)
 
     auto vm_allocator = TRY(parent->vm_allocator->clone());
 
-    auto new_directory = TRY(MMU::clone_userspace_page_directory(parent->directory));
+    auto new_directory = TRY(MMU::clone_userspace_page_directory(parent->self_directory));
 
     const ELFData data = { .entry = parent->ip() };
 
@@ -124,7 +124,8 @@ void ThreadImage::apply(Thread* thread)
     thread->stack = m_user_stack;
     thread->set_sp(align_down<16>(m_sp));
 
-    thread->directory = m_directory;
+    thread->self_directory = m_directory;
+    thread->active_directory = m_directory;
 
     thread->vm_allocator = move(m_vm_allocator);
 }