apps: Add a new session program which manages user login

apps/Makefile

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

apps/src/init.c

@@ -41,7 +41,13 @@ int main()
 {
     if (getpid() != 1)
     {
-        fprintf(stderr, "init should be started as PID 1\n");
+        fprintf(stderr, "init must be started as PID 1\n");
+        return 1;
+    }
+
+    if (getuid() != 0)
+    {
+        fprintf(stderr, "init must be started as root\n");
         return 1;
     }
 
@@ -57,7 +63,7 @@ int main()
     }
     if (child == 0)
     {
-        char* argv[] = {"/bin/sh", NULL};
+        char* argv[] = {"/bin/session", NULL};
         execv(argv[0], argv);
         perror("execv");
         return 1;

apps/src/session.c

@@ -0,0 +1,109 @@
+#include <errno.h>
+#include <pwd.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/wait.h>
+#include <unistd.h>
+
+static char* echoing_fgets(char* buf, size_t size, FILE* stream)
+{
+    char* s = buf;
+    memset(buf, 0, size);
+    while (size)
+    {
+        int c = fgetc(stream);
+        if (c == EOF)
+        {
+            if (ferror(stream)) return NULL;
+            if (feof(stream))
+            {
+                if (s != buf) return s;
+                else
+                    return NULL;
+            };
+        }
+        size--;
+        *buf = (char)c;
+        buf++;
+        *buf = 0;
+        putchar((char)c);
+        if ((char)c == '\n') return s;
+    }
+    return s;
+}
+
+static void strip_newline(char* str)
+{
+    size_t len = strlen(str);
+    if (str[len - 1] == '\n') str[len - 1] = 0;
+}
+
+static void login_as(struct passwd* user)
+{
+    pid_t child = fork();
+    if (child < 0)
+    {
+        perror("fork");
+        return;
+    }
+    if (child == 0)
+    {
+        setuid(user->pw_uid);
+        setgid(user->pw_gid);
+        char* argv[] = {user->pw_shell, NULL};
+        execv(argv[0], argv);
+        perror("execv");
+        exit(EXIT_FAILURE);
+    }
+    wait(NULL);
+}
+
+static int login()
+{
+    printf("Username: ");
+    char username[BUFSIZ];
+    echoing_fgets(username, BUFSIZ, stdin);
+    strip_newline(username);
+    if (strcmp("exit", username) == 0) return 1;
+    struct passwd* user = getpwnam(username);
+    if (!user)
+    {
+        if (errno) perror("getpwnam");
+        else
+            printf("Unknown user %s\n", username);
+        return 0;
+    }
+    printf("Password: ");
+    char password[BUFSIZ];
+    fgets(password, BUFSIZ, stdin);
+    strip_newline(password);
+    puts("\n");
+    if (strcmp(user->pw_passwd, password) == 0)
+    {
+        login_as(user);
+        puts("logout\n");
+    }
+    else
+        puts("Invalid password.\n");
+    return 0;
+}
+
+int main(int argc, char** argv)
+{
+    if (getuid() != 0)
+    {
+        fprintf(stderr,
+                "%s must be run as root.\nYou are probably looking for the 'su' command, which lets you switch users "
+                "once logged in.\n",
+                argv[0]);
+        return EXIT_FAILURE;
+    }
+
+    for (;;)
+    {
+        if (login()) return EXIT_SUCCESS;
+    }
+
+    (void)argc;
+}