su: Read password from /dev/tty instead of stdin

apps/su.cpp

@@ -1,3 +1,4 @@
+#include <fcntl.h>
 #include <os/ArgumentParser.h>
 #include <pwd.h>
 #include <signal.h>
@@ -7,10 +8,11 @@
 #include <unistd.h>
 
 static struct termios orig;
+static int fd = -1;
 
 void restore_terminal()
 {
-    tcsetattr(STDIN_FILENO, TCSANOW, &orig);
+    tcsetattr(fd, TCSANOW, &orig);
 }
 
 void signal_handler(int signo)
@@ -21,21 +23,25 @@ void signal_handler(int signo)
 
 char* getpass()
 {
-    if (!isatty(STDIN_FILENO))
+    FILE* f = fopen("/dev/tty", "r");
+    if (!f)
     {
-        // FIXME: Just read from /dev/tty (the controlling terminal). Problem: that doesn't exist yet.
+        perror("Failed to open /dev/tty");
-        fprintf(stderr, "error: password must be read from a terminal!");
         return nullptr;
     }
 
-    tcsetpgrp(STDIN_FILENO, getpgid(0));
+    fd = fileno(f);
+
+    tcsetpgrp(fd, getpgid(0));
 
     fputs("Password: ", stdout);
     fflush(stdout);
 
-    if (tcgetattr(STDIN_FILENO, &orig) < 0)
+    if (tcgetattr(fd, &orig) < 0)
     {
         perror("tcgetattr");
+        fclose(f);
+        fd = -1;
         return nullptr;
     }
 
@@ -51,18 +57,23 @@ char* getpass()
 
     struct termios tc = orig;
     tc.c_lflag &= ~ECHO;
-    if (tcsetattr(STDIN_FILENO, TCSANOW, &tc) < 0)
+    if (tcsetattr(fd, TCSANOW, &tc) < 0)
     {
         perror("tcsetattr");
+        fclose(f);
+        fd = -1;
         return nullptr;
     }
 
     static char buf[BUFSIZ];
-    char* rc = fgets(buf, sizeof(buf), stdin);
+    char* rc = fgets(buf, sizeof(buf), f);
 
     restore_terminal();
     putchar('\n');
 
+    fclose(f);
+    fd = -1;
+
     if (!rc)
     {
         perror("fgets");