linux-kernel - [PATCH 1/1] tty: n_tty: Fix buffer offsets when lookahead is used

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Date: Tue, 14 May 2024 17:04:29 +0300
From: Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>
To: Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Jiri Slaby <jirislaby@...nel.org>,
	Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>,
	Andy Shevchenko <andy.shevchenko@...il.com>,
	linux-kernel@...r.kernel.org,
	linux-serial@...r.kernel.org
Cc: Vadym Krevs <vkrevs@...oo.com>,
	stable@...r.kernel.org
Subject: [PATCH 1/1] 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@...oo.com>
Fixes: 6bb6fa6908eb ("tty: Implement lookahead to process XON/XOFF timely")
Closes: https://bugzilla.kernel.org/show_bug.cgi?id=218834
Tested-by: Vadym Krevs <vkrevs@...oo.com>
Cc: stable@...r.kernel.org
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>
---
 drivers/tty/n_tty.c | 22 ++++++++++++++++------
 1 file changed, 16 insertions(+), 6 deletions(-)

diff --git a/drivers/tty/n_tty.c b/drivers/tty/n_tty.c
index f252d0b5a434..5e9ca4376d68 100644
--- a/drivers/tty/n_tty.c
+++ b/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)
-- 
2.39.2