apps: Add a new su utility

apps/Makefile

@@ -1,4 +1,4 @@
-APPS := init sym sh crash uname uptime hello ps ls args cat stat
+APPS := init sym sh crash uname uptime hello ps ls args cat stat su
 
 APPS_DIR := $(LUNA_ROOT)/apps
 APPS_SRC := $(APPS_DIR)/src

apps/src/su.c

@@ -0,0 +1,32 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <unistd.h>
+
+char* default_argv[] = {"/bin/sh", NULL};
+
+void run_program(char** argv)
+{
+    execv(argv[0], argv);
+
+    perror("execv");
+    exit(EXIT_FAILURE);
+}
+
+int main(int argc, char** argv)
+{
+    if (argc == 1)
+    {
+        fprintf(stderr, "Usage: %s [uid] [command | sh]\n", argv[0]);
+        return EXIT_FAILURE;
+    }
+
+    if (setuid(atoi(argv[1])) < 0)
+    {
+        perror("setuid");
+        return EXIT_FAILURE;
+    }
+
+    if (argc == 2) run_program(default_argv);
+    else
+        run_program(argv + 2);
+}