Implement fork()!

This is quite cool! The syscall entry point plumbs the register dump
down to sys$fork(), which uses it to set up the child process's TSS
in order to resume execution right after the int 0x80 fork() call. :^)

This works pretty well, although there is some problem with the kernel
alias mappings used to clone the parent process's regions. If I disable
the MM::release_page_directory() code, there's no problem. Probably there's
a premature freeing of a physical page somehow.

Userland/.gitignore

@@ -16,3 +16,4 @@ tst
 mm
 kill
 tty
+ft

Userland/Makefile

@@ -14,6 +14,7 @@ OBJS = \
        tst.o \
        mm.o \
        kill.o \
+       ft.o \
        tty.o
 
 APPS = \
@@ -32,6 +33,7 @@ APPS = \
        tst \
        mm \
        kill \
+       ft \
        tty
 
 ARCH_FLAGS =
@@ -91,6 +93,9 @@ clear: clear.o
 tst: tst.o
 	$(LD) -o $@ $(LDFLAGS) $< ../LibC/LibC.a
 
+ft: ft.o
+	$(LD) -o $@ $(LDFLAGS) $< ../LibC/LibC.a
+
 mm: mm.o
 	$(LD) -o $@ $(LDFLAGS) $< ../LibC/LibC.a
 

Userland/ft.cpp

@@ -0,0 +1,14 @@
+#include <stdio.h>
+#include <unistd.h>
+
+int main(int argc, char** argv)
+{
+    printf("Testing fork()...\n");
+    pid_t pid = fork();
+    if (!pid) {
+        printf("child, pid=%d\n", getpid());
+    } else {
+        printf("parent, child pid=%d\n", pid);
+    }
+    return 0;
+}

Userland/sh.cpp

@@ -32,6 +32,13 @@ static int sh_pwd(int, const char**)
     return 0;
 }
 
+static int sh_fork(int, const char**)
+{
+    pid_t pid = fork();
+    printf("getpid()=%d, fork()=%d\n", getpid(), pid);
+    return 0;
+}
+
 static int sh_exit(int, const char**)
 {
     printf("Good-bye!\n");
@@ -94,6 +101,11 @@ static bool handle_builtin(int argc, const char** argv, int& retval)
         retval = sh_exit(argc, argv);
         return true;
     }
+
+    if (!strcmp(argv[0], "fork")) {
+        retval = sh_fork(argc, argv);
+        return true;
+    }
     return false;
 }