kernel: Don't create a new kernel stack on exec()

The old one was not getting freed, creating a memory leak every exec(),
which can get huge over time.
Plus, there was no need for a new stack.
And we couldn't just free the old one, since sys_execve() runs on the old stack...

kernel/src/arch/x86_64/CPU.cpp

@@ -327,7 +327,7 @@ namespace CPU
     static void backtrace_impl(u64 base_pointer, void (*callback)(u64, void*), void* arg)
     {
         StackFrame* current_frame = (StackFrame*)base_pointer;
-        while (current_frame &&
+        while (current_frame && (u64)current_frame >= 0xFFFF'FFFF'8000'0000 &&
                MemoryManager::validate_access(current_frame, sizeof(*current_frame), MemoryManager::DEFAULT_ACCESS) &&
                current_frame->instruction)
         {

kernel/src/thread/ThreadImage.cpp

@@ -6,24 +6,20 @@
 
 static constexpr usize DEFAULT_USER_STACK_PAGES = 6;
 static constexpr usize DEFAULT_USER_STACK_SIZE = DEFAULT_USER_STACK_PAGES * ARCH_PAGE_SIZE;
+static constexpr u64 THREAD_STACK_BASE = 0x10000;
 
-static Result<void> create_stacks(Stack& user_stack, Stack& kernel_stack, AddressSpace* space)
+static Result<void> create_user_stack(Stack& user_stack, AddressSpace* space)
 {
-    const u64 THREAD_STACK_BASE = 0x10000;
-
-    if (!TRY(space->test_and_alloc_region(THREAD_STACK_BASE, DEFAULT_USER_STACK_PAGES, true))) return err(ENOMEM);
-
     TRY(MemoryManager::alloc_at_zeroed(THREAD_STACK_BASE, DEFAULT_USER_STACK_PAGES,
                                        MMU::ReadWrite | MMU::NoExecute | MMU::User));
 
-    auto guard = make_scope_guard([&] { MemoryManager::unmap_owned(THREAD_STACK_BASE, 4); });
+    auto guard = make_scope_guard([] { MemoryManager::unmap_owned(THREAD_STACK_BASE, DEFAULT_USER_STACK_PAGES); });
 
-    const u64 kernel_stack_base = TRY(MemoryManager::alloc_for_kernel(4, MMU::ReadWrite | MMU::NoExecute));
+    if (!TRY(space->test_and_alloc_region(THREAD_STACK_BASE, DEFAULT_USER_STACK_PAGES, true))) return err(ENOMEM);
 
     guard.deactivate();
 
     user_stack = { THREAD_STACK_BASE, DEFAULT_USER_STACK_SIZE };
-    kernel_stack = { kernel_stack_base, 4 * ARCH_PAGE_SIZE };
 
     return {};
 }
@@ -43,12 +39,10 @@ Result<OwnedPtr<ThreadImage>> ThreadImage::try_load_from_elf(SharedPtr<VFS::Inod
     const ELFData data = TRY(ELFLoader::load(inode, address_space.ptr()));
 
     Stack user_stack;
-    Stack kernel_stack;
-    TRY(create_stacks(user_stack, kernel_stack, address_space.ptr()));
+    TRY(create_user_stack(user_stack, address_space.ptr()));
 
     guard.deactivate();
 
-    image->m_kernel_stack = kernel_stack;
     image->m_user_stack = user_stack;
     image->m_loaded_image_data = data;
     image->m_address_space = move(address_space);
@@ -111,7 +105,7 @@ void ThreadImage::apply(Thread* thread)
 
     thread->set_ip(m_loaded_image_data.entry);
 
-    thread->kernel_stack = m_kernel_stack;
+    if (m_kernel_stack.bottom()) thread->kernel_stack = m_kernel_stack;
     thread->stack = m_user_stack;
     thread->set_sp(align_down<16>(m_sp));
 

libluna/include/luna/Stack.h

@@ -19,6 +19,6 @@ struct Stack
     }
 
   private:
-    u64 m_base;
-    usize m_bytes;
+    u64 m_base { 0 };
+    usize m_bytes { 0 };
 };