sulogin: allow system maintenance login if root password is empty

The current password checking is unable to distinguish between the user
entering an empty password or pressing Control-D. As a result, an empty
password always results in normal startup.

We modify bb_ask to return NULL if Control-D is pressed without entering
a password. The sulogin applet is then modified to only proceed to
normal startup if bb_ask returns NULL. This covers EOF with no password,
interrupt by timeout and ^C.

Signed-off-by: Jonathan Liu <net147@gmail.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

2 files changed, 5 insertions, 3 deletions

diff --git a/libbb/bb_askpass.c b/libbb/bb_askpass.c
index fe2b50677..77c1bcd95 100644
--- a/libbb/bb_askpass.c
+++ b/libbb/bb_askpass.c
@@ -65,7 +65,9 @@ char* FAST_FUNC bb_ask(const int fd, int timeout, const char *prompt)
         i = 0;
         while (1) {
                 int r = read(fd, &ret[i], 1);
-                if (r < 0) {
+                if ((i == 0 && r == 0) /* EOF (^D) with no password */
+                 || r < 0
+                ) {
                         /* read is interrupted by timeout or ^C */
                         ret = NULL;
                         break;
diff --git a/loginutils/sulogin.c b/loginutils/sulogin.c
index f79802a4d..65e638489 100644
--- a/loginutils/sulogin.c
+++ b/loginutils/sulogin.c
@@ -83,8 +83,8 @@ int sulogin_main(int argc UNUSED_PARAM, char **argv)
                 cp = bb_ask(STDIN_FILENO, timeout,
                                 "Give root password for system maintenance\n"
                                 "(or type Control-D for normal startup):");
-
-                if (!cp || !*cp) {
+                if (!cp) {
+                        /* ^D, ^C, timeout, or read error */
                         bb_info_msg("Normal startup");
                         return 0;
                 }