Kernel: Preserve signal mask across fork(2) and execve(2)

A child created via fork(2) inherits a copy of its parent's signal
mask; the signal mask is preserved across execve(2).

Kernel/Syscalls/execve.cpp

@@ -523,7 +523,7 @@ ErrorOr<void> Process::do_exec(NonnullRefPtr<OpenFileDescription> main_program_d
     for (auto& property : m_coredump_properties)
         property = {};
 
-    auto current_thread = Thread::current();
+    auto* current_thread = Thread::current();
     current_thread->clear_signals();
 
     clear_futex_queues_on_exec();

Kernel/Syscalls/fork.cpp

@@ -43,6 +43,9 @@ ErrorOr<FlatPtr> Process::sys$fork(RegisterState& regs)
     dbgln_if(FORK_DEBUG, "fork: child={}", child);
     child->address_space().set_enforces_syscall_regions(address_space().enforces_syscall_regions());
 
+    // A child created via fork(2) inherits a copy of its parent's signal mask
+    child_first_thread->update_signal_mask(Thread::current()->signal_mask());
+
 #if ARCH(I386)
     auto& child_regs = child_first_thread->m_regs;
     child_regs.eax = 0; // fork() returns 0 in the child :^)

Kernel/Thread.cpp

@@ -681,7 +681,7 @@ u32 Thread::signal_mask_block(sigset_t signal_set, bool block)
 void Thread::clear_signals()
 {
     SpinlockLocker lock(g_scheduler_lock);
-    m_signal_mask = 0;
+    // The signal mask is preserved across execve(2).
     m_pending_signals = 0;
     m_have_any_unmasked_pending_signals.store(false, AK::memory_order_release);
     m_signal_action_data.fill({});