libc: Add support for fork()

That doesn't mean it works.
Why doesn't it work??

Oh well...

apps/src/init.c

@@ -141,9 +141,13 @@ int main()
 
     fclose(new_stderr);
 
-    execv("/bin/sym", NULL);
+    pid_t child = fork();
+    if (child == 0)
+    {
+        msleep(500);
+        printf("I am the child, who is my parent?\n");
+    }
+    else { printf("My child is %ld!", child); }
 
-    perror("execv"); // If we're here, execv failed
-
-    return 1;
+    return 0;
 }

kernel/src/trace/StackTracer.cpp

@@ -1,5 +1,6 @@
 #include "trace/StackTracer.h"
 #include "memory/Memory.h"
+#include "memory/VMM.h"
 #include "std/stdio.h"
 #include "trace/Resolve.h"
 
@@ -20,6 +21,11 @@ typedef struct stackframe
 
 void StackTracer::trace()
 {
+    if (!VMM::is_using_kernel_address_space())
+    {
+        VMM::switch_back_to_kernel_address_space();
+        VMM::apply_address_space();
+    }
     stackframe* frame = (stackframe*)m_base_pointer;
     while (frame && frame->instruction)
     {
@@ -27,6 +33,7 @@ void StackTracer::trace()
         get_symbol_name(frame->instruction, symbol_name, sizeof(symbol_name));
         printf("%lx: %s\n", frame->instruction, symbol_name);
         frame = frame->next;
+        if (VMM::get_physical((uint64_t)frame) == (uint64_t)-1) return;
     }
 }
 

libs/libc/include/luna/syscall.h

@@ -17,6 +17,7 @@
 #define SYS_fcntl 13
 #define SYS_mprotect 14
 #define SYS_clock 15
+#define SYS_fork 16
 
 #ifndef __want_syscalls
 #ifdef __cplusplus

libs/libc/include/unistd.h

@@ -22,7 +22,8 @@ extern "C"
     int execve(const char*, char* const[], char* const[]);
     /* Not implemented. */
     int execvp(const char*, char* const[]);
-    /* Not implemented. */
+
+    /* Creates an identical copy of the current process and returns its process ID. */
     pid_t fork(void);
 
     /* Terminates the program with the status code status. */

libs/libc/src/unistd.cpp

@@ -19,9 +19,10 @@ extern "C"
     {
         NOT_IMPLEMENTED("execvp");
     }
+
     pid_t fork(void)
     {
-        NOT_IMPLEMENTED("fork");
+        return syscall(SYS_fork);
     }
 
     long syscall(long number, ...)
@@ -34,6 +35,7 @@ extern "C"
         {
         case SYS_clock:
         case SYS_yield:
+        case SYS_fork:
         case SYS_gettid: result = __luna_syscall0(number); break;
         case SYS_exit:
         case SYS_close: