tty: n_tty: Fix buffer offsets when lookahead is used

JIRA: https://issues.redhat.com/browse/RHEL-127716

commit b19ab7e
Author: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Date:   Tue May 14 17:04:29 2024 +0300

    tty: n_tty: Fix buffer offsets when lookahead is used

    When lookahead has "consumed" some characters (la_count > 0),
    n_tty_receive_buf_standard() and n_tty_receive_buf_closing() for
    characters beyond the la_count are given wrong cp/fp offsets which
    leads to duplicating and losing some characters.

    If la_count > 0, correct buffer pointers and make count consistent too
    (the latter is not strictly necessary to fix the issue but seems more
    logical to adjust all variables immediately to keep state consistent).

    Reported-by: Vadym Krevs <vkrevs@yahoo.com>
    Fixes: 6bb6fa6 ("tty: Implement lookahead to process XON/XOFF timely")
    Closes: https://bugzilla.kernel.org/show_bug.cgi?id=218834
    Tested-by: Vadym Krevs <vkrevs@yahoo.com>
    Cc: stable@vger.kernel.org
    Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
    Link: https://lore.kernel.org/r/20240514140429.12087-1-ilpo.jarvinen@linux.intel.com
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Signed-off-by: Radostin Stoyanov <rstoyano@redhat.com>

Author: rst0git

drivers/tty/n_tty.c

@@ -1619,15 +1619,25 @@ static void __receive_buf(struct tty_struct *tty, const u8 *cp, const u8 *fp,
 	else if (ldata->raw || (L_EXTPROC(tty) && !preops))
 		n_tty_receive_buf_raw(tty, cp, fp, count);
 	else if (tty->closing && !L_EXTPROC(tty)) {
-		if (la_count > 0)
+		if (la_count > 0) {
 			n_tty_receive_buf_closing(tty, cp, fp, la_count, true);
-		if (count > la_count)
-			n_tty_receive_buf_closing(tty, cp, fp, count - la_count, false);
+			cp += la_count;
+			if (fp)
+				fp += la_count;
+			count -= la_count;
+		}
+		if (count > 0)
+			n_tty_receive_buf_closing(tty, cp, fp, count, false);
 	} else {
-		if (la_count > 0)
+		if (la_count > 0) {
 			n_tty_receive_buf_standard(tty, cp, fp, la_count, true);
-		if (count > la_count)
-			n_tty_receive_buf_standard(tty, cp, fp, count - la_count, false);
+			cp += la_count;
+			if (fp)
+				fp += la_count;
+			count -= la_count;
+		}
+		if (count > 0)
+			n_tty_receive_buf_standard(tty, cp, fp, count, false);
 
 		flush_echoes(tty);
 		if (tty->ops->flush_chars)