ELFLoader works inside the kernel!

We load /_hello.o which just prints out a simple message.
It executes inside the kernel itself, so no fancy userspace process
or anything, but this is still so cool!

Kernel/_hello.cpp

@@ -0,0 +1,7 @@
+extern "C" int puts(const char*);
+
+extern "C" int elf_entry()
+{
+    puts("Home, where you are supposed to be...");
+    return 0;
+}

Kernel/_test.cpp

@@ -2,7 +2,7 @@
 
 using namespace Userspace;
 
-int elf_entry()
+extern "C" int elf_entry()
 {
     int fd = open("/Banner.txt");
     char buf[2048];

Kernel/init.cpp

@@ -23,6 +23,7 @@
 #include <VirtualFileSystem/FileHandle.h>
 #include <AK/OwnPtr.h>
 #include "MemoryManager.h"
+#include <ELFLoader/ELFLoader.h>
 
 #if 0
 /* Keyboard LED disco task ;^) */
@@ -82,6 +83,8 @@ static void user_kprintf_main()
     DO_SYSCALL_A1(0x4000, 0);
     kprintf("This should not work!\n");
     HANG;
+    for (;;) {
+    }
 }
 
 system_t system;
@@ -179,6 +182,25 @@ void init()
     }
 #endif
 
+    {
+        auto testExecutable = vfs->open("/_hello.o");
+        ASSERT(testExecutable);
+        auto testExecutableData = testExecutable->readEntireFile();
+        ASSERT(testExecutableData);
+        
+        ExecSpace space;
+        space.loadELF(move(testExecutableData));
+        auto* elf_entry = space.symbolPtr("elf_entry");
+        ASSERT(elf_entry);
+
+        typedef int (*MainFunctionPtr)(void);
+        kprintf("elf_entry: %p\n", elf_entry);
+        int rc = reinterpret_cast<MainFunctionPtr>(elf_entry)();
+        kprintf("it returned %d\n", rc);
+
+        HANG;
+    }
+
     // The idle task will spend its eternity here for now.
     for (;;) {
         asm("hlt");