Respect user-defined exit signal in clone() and clone3()

When calling clone() and clone3(), the user is allowed to specify a
signal to be sent to the parent process on exit. Respect this value by
storing it in the Process struct and sending the signal on exit.

Add a test as well to verify that the signal is properly delivered to
the parent.

kernel/src/process/clone.rs

@@ -345,6 +345,10 @@ fn clone_child_process(
         process_builder.build()?
     };
 
+    if let Some(sig) = clone_args.exit_signal {
+        child.set_exit_signal(sig);
+    };
+
     // Deals with clone flags
     let child_thread = thread_table::get_thread(child_tid).unwrap();
     let child_posix_thread = child_thread.as_posix_thread().unwrap();

kernel/src/process/exit.rs

@@ -5,7 +5,7 @@ use crate::{
     prelude::*,
     process::{
         posix_thread::{do_exit, PosixThreadExt},
-        signal::{constants::SIGCHLD, signals::kernel::KernelSignal},
+        signal::signals::kernel::KernelSignal,
     },
     thread::Thread,
 };
@@ -60,10 +60,11 @@ pub fn do_exit_group(term_status: TermStatus) {
     let parent = current.parent().lock().process();
     if let Some(parent) = parent.upgrade() {
         // Notify parent
-        let signal = KernelSignal::new(SIGCHLD);
-        parent.enqueue_signal(signal);
+        if let Some(signal) = current.exit_signal().map(KernelSignal::new) {
+            parent.enqueue_signal(signal);
+        };
         parent.children_wait_queue().wake_all();
-    }
+    };
 }
 
 const INIT_PROCESS_PID: Pid = 1;

kernel/src/process/process/mod.rs

@@ -101,6 +101,9 @@ pub struct Process {
     /// The signal that the process should receive when parent process exits.
     parent_death_signal: AtomicSigNum,
 
+    /// The signal that should be sent to the parent when this process exits.
+    exit_signal: AtomicSigNum,
+
     /// A profiling clock measures the user CPU time and kernel CPU time of the current process.
     prof_clock: Arc<ProfClock>,
 
@@ -218,6 +221,7 @@ impl Process {
             umask,
             sig_dispositions,
             parent_death_signal: AtomicSigNum::new_empty(),
+            exit_signal: AtomicSigNum::new_empty(),
             resource_limits: Mutex::new(resource_limits),
             nice: Atomic::new(nice),
             timer_manager: PosixTimerManager::new(&prof_clock, process_ref),
@@ -690,6 +694,14 @@ impl Process {
         self.parent_death_signal.as_sig_num()
     }
 
+    pub fn set_exit_signal(&self, sig_num: SigNum) {
+        self.exit_signal.set(sig_num);
+    }
+
+    pub fn exit_signal(&self) -> Option<SigNum> {
+        self.exit_signal.as_sig_num()
+    }
+
     // ******************* Status ********************
 
     fn set_runnable(&self) {