Add an initial fork() implementation

kernel/include/sys/Syscall.h

@@ -18,6 +18,7 @@
 #define SYS_fcntl 13
 #define SYS_mprotect 14
 #define SYS_clock 15
+#define SYS_fork 16
 
 namespace Syscall
 {
@@ -41,4 +42,5 @@ void sys_seek(Context* context, int fd, long offset, int whence);
 void sys_exec(Context* context, const char* pathname);
 void sys_fcntl(Context* context, int fd, int command, uintptr_t arg);
 void sys_mprotect(Context* context, void* address, size_t size, int prot);
-void sys_clock(Context* context);
+void sys_clock(Context* context);
+void sys_fork(Context* context);

kernel/src/sys/Syscall.cpp

@@ -27,6 +27,7 @@ void Syscall::entry(Context* context)
     case SYS_fcntl: sys_fcntl(context, (int)context->rdi, (int)context->rsi, context->rdx); break;
     case SYS_mprotect: sys_mprotect(context, (void*)context->rdi, context->rsi, (int)context->rdx); break;
     case SYS_clock: sys_clock(context); break;
+    case SYS_fork: sys_fork(context); break;
     default: context->rax = -ENOSYS; break;
     }
     VMM::exit_syscall_context();

kernel/src/sys/exec.cpp

@@ -11,12 +11,43 @@
 #include "sys/Syscall.h"
 #include "sys/elf/ELFLoader.h"
 #include "thread/Scheduler.h"
+#include "utils/Addresses.h"
+
+void sys_fork(Context* context) // FIXME: Even though both processes's address spaces are the same in content, writing
+                                // to one should not affect the other.
+{
+    kinfoln("fork(): attempting fork");
+
+    Task* parent = Scheduler::current_task();
+
+    Task* child = Scheduler::create_user_task();
+
+    memcpy(&child->regs, &parent->regs, sizeof(Context));
+    if (parent->floating_saved)
+    {
+        memcpy(child->floating_region, parent->floating_region, sizeof(parent->floating_region));
+        child->floating_saved = true;
+    }
+    for (int i = 0; i < TASK_MAX_FDS; i++) { child->files[i] = parent->files[i]; }
+
+    size_t stack_bytes = get_top_of_stack(parent->allocated_stack, TASK_PAGES_IN_STACK) - parent->regs.rsp;
+
+    child->regs.rsp -= stack_bytes;
+
+    memcpy((void*)child->regs.rsp, (void*)parent->regs.rsp, stack_bytes);
+
+    child->address_space = parent->address_space.clone();
+
+    child->regs.rax = 0;
+    context->rax = child->id;
+
+    kinfoln("fork(): forked parent %d into child %d", parent->id, child->id);
+
+    return;
+}
 
 void sys_exec(Context* context, const char* pathname)
 {
-    /*context->rax = -ENOSYS; // FIXME: Make exec() work under separate address spaces.
-    return;*/
-
     char* kpathname = Syscall::strdup_from_user(pathname);
     if (!kpathname)
     {