[PATCH v2] media: cec: extron: clear response tail before dispatch

[Pengpeng Hou]

The serial receive buffer is reused between responses. Terminating a
short response with one NUL leaves bytes from the previous response
behind it, while the dispatcher also checks fixed offsets beyond some
prefixes. Those checks can consume stale tail bytes.

Clear up to ten bytes at the end of each completed response, limited by
the remaining array capacity. This includes the terminator and covers
the dispatcher's fixed-offset tests without adding a separate length
condition to every prefix branch.

The issue was found by our static-analysis tool.

Fixes: 056f2821b631 ("media: cec: extron-da-hd-4k-plus: add the Extron DA HD 4K Plus CEC driver")
Assisted-by: gpt 5
Signed-off-by: Pengpeng Hou <hppiscas@xxxxxxx>
---
Changes since v1:
https://lore.kernel.org/all/20260830130356.159-1-pengpeng@xxxxxxxxxxx/
Use the bounded tail clearing suggested by Hans instead of per-prefix
guards; preserve the existing nonempty-line and receive-buffer limits.

 .../extron-da-hd-4k-plus/extron-da-hd-4k-plus.c    |    4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/drivers/media/cec/usb/extron-da-hd-4k-plus/extron-da-hd-4k-plus.c b/drivers/media/cec/usb/extron-da-hd-4k-plus/extron-da-hd-4k-plus.c
index 3c6ce6f3d93e..1f8ed7ff31e5 100644
--- a/drivers/media/cec/usb/extron-da-hd-4k-plus/extron-da-hd-4k-plus.c
+++ b/drivers/media/cec/usb/extron-da-hd-4k-plus/extron-da-hd-4k-plus.c
@@ -847,7 +847,9 @@ static irqreturn_t extron_interrupt(struct serio *serio, unsigned char data,
 			return IRQ_HANDLED;
 		memcpy(extron->data, extron->buf, extron->idx);
 		extron->len = extron->idx;
-		extron->data[extron->len] = 0;
+		/* Keep fixed-offset response tests from using stale tail bytes. */
+		memset(extron->data + extron->len, 0,
+		       min_t(size_t, 10, sizeof(extron->data) - extron->len));
 		if (debug)
 			dev_info(extron->dev, "received %s\n", extron->data);
 		extron->idx = 0;

base-commit: 518e5b794c06c0f0eb40df3e202274a66202c137
-- 
2.50.1 (Apple Git-155)