[LibOS] Do not try to send signal 0 in kill()/tkill()

Previously, Graphene didn't handle special case of kill()/tkill()
with signal equal to zero. This signal is used to check for existence
of processes and threads. However, Graphene tried to send/append it
which led to segfaults. This commit correctly handles signal equal
to zero and improves LibOS tests to verify this fix.

LibOS/shim/src/sys/shim_sigaction.c

@@ -245,6 +245,13 @@ static int _signal_one_thread(struct shim_thread* thread, void* _arg) {
             BUG();
     }
 
+    if (!arg->sig) {
+        /* special case of sig == 0: don't really send signal but simply report success */
+        arg->sent = true;
+        ret = 1;
+        goto out;
+    }
+
     /* Appending the signal to the whole process. */
     if (!arg->sent) {
         siginfo_t info = {
@@ -393,6 +400,13 @@ int do_kill_thread(IDTYPE sender, IDTYPE tgid, IDTYPE tid, int sig, bool use_ipc
 
         if (thread->in_vm) {
             if (!tgid || thread->tgid == tgid) {
+                if (!sig) {
+                    /* special case of sig == 0: don't really send signal but report success */
+                    unlock(&thread->lock);
+                    put_thread(thread);
+                    return 0;
+                }
+
                 siginfo_t info = {
                     .si_signo = sig,
                     .si_pid   = sender,

LibOS/shim/test/regression/sigaction_per_process.c

@@ -80,6 +80,12 @@ int main() {
 
     printf("parent tid: %d\n", tid);
 
+    /* the below dummy tkill (no signal is sent) is for sanity */
+    if (tkill(tid, /*sig=*/0)) {
+        fprintf(stderr, "tkill(sig=0) failed: %m\n");
+        return 1;
+    }
+
     if (tkill(tid, SIGTERM)) {
         fprintf(stderr, "tkill failed: %m\n");
         return 1;

LibOS/shim/test/regression/signal_multithread.c

@@ -51,6 +51,12 @@ int main() {
 
     wait_for(1);
 
+    /* the below dummy kill (no signal is sent) is for sanity */
+    if (kill(getpid(), /*sig=*/0)) {
+        fprintf(stderr, "kill(sig=0) failed: %m\n");
+        return 1;
+    }
+
     if (kill(getpid(), SIGTERM)) {
         fprintf(stderr, "kill failed: %m\n");
         return 1;