Add some basic signal support.

It only works for sending a signal to a process that's in userspace code.

We implement reception by synthesizing a PUSHA+PUSHF in the receiving process
(operating on values in the TSS.)
The TSS CS:EIP is then rerouted to the signal handler and a tiny return
trampoline is constructed in a dedicated region in the receiving process.

Also hacked up /bin/kill to be able to send arbitrary signals (kill -N PID)

Userland/cat.cpp

@@ -1,7 +1,9 @@
-#include <LibC/stdio.h>
-#include <LibC/unistd.h>
-#include <LibC/errno.h>
-#include <LibC/string.h>
+#include <stdio.h>
+#include <unistd.h>
+#include <errno.h>
+#include <string.h>
+#include <stdlib.h>
+#include <assert.h>
 
 int main(int argc, char** argv)
 {

Userland/kill.cpp

@@ -1,6 +1,7 @@
-#include <LibC/unistd.h>
-#include <LibC/stdio.h>
-#include <LibC/signal.h>
+#include <unistd.h>
+#include <stdio.h>
+#include <signal.h>
+#include <stdlib.h>
 #include <AK/String.h>
 
 static unsigned parseUInt(const String& str, bool& ok)
@@ -18,20 +19,36 @@ static unsigned parseUInt(const String& str, bool& ok)
     return value;
 }
 
+static void print_usage_and_exit()
+{
+    printf("usage: kill [-signal] <PID>\n");
+    exit(1);
+}
+
 int main(int argc, char** argv)
 {
-    if (argc < 2) {
-        printf("usage: kill <PID>\n");
-        return 1;
-    }
+    if (argc != 2 && argc != 3)
+        print_usage_and_exit();
     bool ok;
-    unsigned value = parseUInt(argv[1], ok);
+    unsigned signum = SIGTERM;
+    int pid_argi = 1;
+    if (argc == 3) {
+        pid_argi = 2;
+        if (argv[1][0] != '-')
+            print_usage_and_exit();
+        signum = parseUInt(&argv[1][1], ok);
+        if (!ok) {
+            printf("%s is not a valid signal number\n", &argv[1][1]);
+            return 2;
+        }
+    }
+    unsigned pid = parseUInt(argv[pid_argi], ok);
     if (!ok) {
-        printf("%s is not a valid PID\n", argv[1]);
-        return 2;
+        printf("%s is not a valid PID\n", argv[pid_argi]);
+        return 3;
     }
 
-    kill((pid_t)value, SIGKILL);
+    kill((pid_t)pid, signum);
     return 0;
 }
 

Userland/sh.cpp

@@ -6,6 +6,7 @@
 #include <LibC/stdlib.h>
 #include <LibC/utsname.h>
 #include <LibC/pwd.h>
+#include <signal.h>
 #include <AK/FileSystemPath.h>
 
 struct GlobalState {
@@ -32,6 +33,29 @@ static int sh_pwd(int, const char**)
     return 0;
 }
 
+void did_receive_signal(int signum)
+{
+    printf("\nMy word, I've received a signal with number %d\n", signum);
+    //exit(0);
+}
+
+static int sh_busy(int, const char**)
+{
+    struct sigaction sa;
+    sa.sa_handler = did_receive_signal;
+    sa.sa_flags = 0;
+    sa.sa_mask = 0;
+    sa.sa_restorer = nullptr;
+    int rc = sigaction(SIGUSR1, &sa, nullptr);
+    assert(rc == 0);
+    printf("listening for SIGUSR1 while looping in userspace...\n");
+    for (;;) {
+        for (volatile int i = 0; i < 100000; ++i)
+            ;
+    }
+    return 0;
+}
+
 static int sh_fork(int, const char**)
 {
     pid_t pid = fork();
@@ -147,6 +171,10 @@ static bool handle_builtin(int argc, const char** argv, int& retval)
         retval = sh_fef(argc, argv);
         return true;
     }
+    if (!strcmp(argv[0], "busy")) {
+        retval = sh_busy(argc, argv);
+        return true;
+    }
     if (!strcmp(argv[0], "wt")) {
         retval = sh_wt(argc, argv);
         return true;