libbb: switch bb_ask_noecho() to "mallocing" string return API

function                                             old     new   delta
bb_ask_noecho                                        313     330     +17
get_cred_or_die                                      125     115     -10
passwd_main                                          995     958     -37
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 1/2 up/down: 17/-47)            Total: -30 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

libbb/bb_askpass.c

@@ -13,16 +13,9 @@ static void askpass_timeout(int UNUSED_PARAM ignore)
 {
 }
 
-char* FAST_FUNC bb_ask_noecho_stdin(const char *prompt)
+char* FAST_FUNC bb_ask_noecho(int fd, int timeout, const char *prompt)
 {
-	return bb_ask_noecho(STDIN_FILENO, 0, prompt);
-}
-char* FAST_FUNC bb_ask_noecho(const int fd, int timeout, const char *prompt)
-{
-	/* Was static char[BIGNUM] */
-	enum { sizeof_passwd = 128 };
-
-	char *passwd;
+#define MAX_LINE 0xfff
 	char *ret;
 	int i;
 	struct sigaction sa, oldsa;
@@ -37,7 +30,17 @@ char* FAST_FUNC bb_ask_noecho(const int fd, int timeout, const char *prompt)
 
 	tcgetattr(fd, &oldtio);
 	tio = oldtio;
-	/* Switch off echo */
+	/* Switch off echo. ECHOxyz meaning:
+	 * ECHO    echo input chars
+	 * ECHOE   echo BS-SP-BS on erase character
+	 * ECHOK   echo kill char specially, not as ^c (ECHOKE controls how exactly)
+	 * ECHOKE  erase all input via BS-SP-BS on kill char (else go to next line)
+	 * ECHOCTL Echo ctrl chars as ^c (else echo verbatim:
+	 *         e.g. up arrow emits "ESC-something" and thus moves cursor up!)
+	 * ECHONL  Echo NL even if ECHO is not set
+	 * ECHOPRT On erase, echo erased chars
+	 *         [qwe<BS><BS><BS> input looks like "qwe\ewq/" on screen]
+	 */
 	tio.c_lflag &= ~(ECHO|ECHOE|ECHOK|ECHONL);
 	tcsetattr(fd, TCSANOW, &tio);
 
@@ -51,20 +54,30 @@ char* FAST_FUNC bb_ask_noecho(const int fd, int timeout, const char *prompt)
 		alarm(timeout);
 	}
 
-	passwd = auto_string(xmalloc(sizeof_passwd));
-	ret = passwd;
+	ret = NULL;
 	i = 0;
 	while (1) {
-		int r = read(fd, &ret[i], 1);
+		int r;
+
+		/* User input is uber-slow, no need to optimize reallocs.
+		 * Grow it on every char.
+		 */
+		ret = xrealloc(ret, i + 2);
+		r = read(fd, &ret[i], 1);
+
 		if ((i == 0 && r == 0) /* EOF (^D) with no password */
 		 || r < 0 /* read is interrupted by timeout or ^C */
 		) {
+			ret[i] = '\0'; /* paranoia */
+			nuke_str(ret); /* paranoia */
+			free(ret);
 			ret = NULL;
 			break;
 		}
+
 		if (r == 0 /* EOF */
 		 || ret[i] == '\r' || ret[i] == '\n' /* EOL */
-		 || ++i == sizeof_passwd-1 /* line limit */
+		 || ++i == MAX_LINE /* line limit */
 		) {
 			ret[i] = '\0';
 			break;
@@ -80,3 +93,7 @@ char* FAST_FUNC bb_ask_noecho(const int fd, int timeout, const char *prompt)
 	fflush_all();
 	return ret;
 }
+char* FAST_FUNC bb_ask_noecho_stdin(const char *prompt)
+{
+	return bb_ask_noecho(STDIN_FILENO, 0, prompt);
+}