su: Make it setuid root, ask for a password, and accept a username

It asks for a password only if you are not root, obviously.

apps/Makefile

@@ -17,6 +17,7 @@ build: $(REAL_APPS)
 install: $(REAL_APPS)
 	@mkdir -p $(LUNA_ROOT)/initrd/bin
 	cp $(REAL_APPS) $(LUNA_ROOT)/initrd/bin
+	@chmod a+s $(LUNA_ROOT)/initrd/bin/su
 
 clean:
 	rm -f $(APPS_BIN)/*

apps/src/su.c

@@ -1,9 +1,10 @@
+#include <errno.h>
+#include <pwd.h>
 #include <stdio.h>
 #include <stdlib.h>
+#include <string.h>
 #include <unistd.h>
 
-char* default_argv[] = {"/bin/sh", NULL};
-
 void run_program(char** argv)
 {
     execv(argv[0], argv);
@@ -12,26 +13,64 @@ void run_program(char** argv)
     exit(EXIT_FAILURE);
 }
 
+void strip_newline(char* str)
+{
+    size_t len = strlen(str);
+    if (str[len - 1] == '\n') str[len - 1] = 0;
+}
+
 int main(int argc, char** argv)
 {
     if (argc == 1)
     {
-        fprintf(stderr, "Usage: %s [uid] [command | sh]\n", argv[0]);
+        fprintf(stderr, "Usage: %s [username] [command | login shell]\n", argv[0]);
         return EXIT_FAILURE;
     }
 
-    if (setuid(atoi(argv[1])) < 0)
+    if (getuid() != 0)
+    {
+        fprintf(stderr, "%s must be setuid root", argv[0]);
+        return EXIT_FAILURE;
+    }
+
+    struct passwd* user = getpwnam(argv[1]);
+
+    if (!user)
+    {
+        if (errno) perror("getpwnam");
+        else
+            fprintf(stderr, "Unknown user %s\n", argv[1]);
+        return EXIT_FAILURE;
+    }
+
+    if (getuid() != geteuid()) // we were started from a non-root user
+    {
+        printf("Password: ");
+        char buf[BUFSIZ];
+        fgets(buf, BUFSIZ, stdin);
+        strip_newline(buf);
+        putchar('\n');
+        if (strcmp(buf, user->pw_passwd) != 0)
+        {
+            fprintf(stderr, "Invalid password\n");
+            return EXIT_FAILURE;
+        }
+    }
+
+    if (setuid(user->pw_uid) < 0)
     {
         perror("setuid");
         return EXIT_FAILURE;
     }
 
-    if (setgid(atoi(argv[1])) < 0)
+    if (setgid(user->pw_gid) < 0)
     {
         perror("setgid");
         return EXIT_FAILURE;
     }
 
+    char* default_argv[] = {user->pw_shell, NULL};
+
     if (argc == 2) run_program(default_argv);
     else
         run_program(argv + 2);