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)

LibC/signal.cpp

@@ -22,5 +22,11 @@ sighandler_t signal(int signum, sighandler_t handler)
     return old_handler;
 }
 
+int sigaction(int signum, const struct sigaction* act, struct sigaction* old_act)
+{
+    int rc = Syscall::invoke(Syscall::Sigaction, (dword)signum, (dword)act, (dword)old_act);
+    __RETURN_WITH_ERRNO(rc, rc, -1);
+}
+
 }
 

LibC/signal.h

@@ -11,8 +11,10 @@ typedef uint32_t sigset_t;
 typedef void siginfo_t;
 
 struct sigaction {
-    void (*sa_handler)(int);
-    void (*sa_sigaction)(int, siginfo_t*, void*);
+    union {
+        void (*sa_handler)(int);
+        void (*sa_sigaction)(int, siginfo_t*, void*);
+    };
     sigset_t sa_mask;
     int sa_flags;
     void (*sa_restorer)(void);
@@ -20,11 +22,16 @@ struct sigaction {
 
 int kill(pid_t, int sig);
 sighandler_t signal(int sig, sighandler_t);
+int sigaction(int signum, const struct sigaction* act, struct sigaction* old_act);
 
 #define SIG_DFL ((__sighandler_t)0)
 #define SIG_ERR ((__sighandler_t)-1)
 #define SIG_IGN ((__sighandler_t)1)
 
+#define SA_NOCLDSTOP 1
+#define SA_NOCLDWAIT 2
+#define SA_SIGINFO 4
+
 #define SIG_BLOCK 0
 #define SIG_UNBLOCK 1
 #define SIG_SETMASK 2

LibC/stdlib.h

@@ -5,15 +5,15 @@
 
 __BEGIN_DECLS
 
-void* malloc(size_t);
+void* malloc(size_t) __MALLOC;
 void free(void*);
 void* calloc(size_t nmemb, size_t);
 void* realloc(void *ptr, size_t);
 char* getenv(const char* name);
 int atoi(const char*);
 
-void exit(int status);
-void abort();
+void exit(int status) __NORETURN;
+void abort() __NORETURN;
 
 __END_DECLS
 

LibC/sys/cdefs.h

@@ -4,6 +4,7 @@
 
 #define ALWAYS_INLINE inline __attribute__ ((always_inline))
 #define __NORETURN __attribute__ ((noreturn))
+#define __MALLOC __attribute__ ((malloc))
 
 #ifdef __cplusplus
 #define __BEGIN_DECLS extern "C" {