From afcb5a811ff3ab3969f09666535eb6018a160358 Mon Sep 17 00:00:00 2001
From: Geert Uytterhoeven <geert@linux-m68k.org>
Date: Tue, 19 Oct 2021 16:45:02 +0200
Subject: [PATCH] auxdisplay: img-ascii-lcd: Fix lock-up when displaying empty string
Git-commit: afcb5a811ff3ab3969f09666535eb6018a160358
Patch-mainline: v5.16-rc1
References: git-fixes stable-5.14.19

While writing an empty string to a device attribute is a no-op, and thus
does not need explicit safeguards, the user can still write a single
newline to an attribute file:

    echo > .../message

If that happens, img_ascii_lcd_display() trims the newline, yielding an
empty string, and causing an infinite loop in img_ascii_lcd_scroll().

Fix this by adding a check for empty strings.  Clear the display in case
one is encountered.

Fixes: 0cad855fbd083ee5 ("auxdisplay: img-ascii-lcd: driver for simple ASCII LCD displays")
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
Acked-by: Takashi Iwai <tiwai@suse.de>

---
 drivers/auxdisplay/img-ascii-lcd.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/drivers/auxdisplay/img-ascii-lcd.c b/drivers/auxdisplay/img-ascii-lcd.c
index 1cce409ce5ca..e33ce0151cdf 100644
--- a/drivers/auxdisplay/img-ascii-lcd.c
+++ b/drivers/auxdisplay/img-ascii-lcd.c
@@ -280,6 +280,16 @@ static int img_ascii_lcd_display(struct img_ascii_lcd_ctx *ctx,
 	if (msg[count - 1] == '\n')
 		count--;
 
+	if (!count) {
+		/* clear the LCD */
+		devm_kfree(&ctx->pdev->dev, ctx->message);
+		ctx->message = NULL;
+		ctx->message_len = 0;
+		memset(ctx->curr, ' ', ctx->cfg->num_chars);
+		ctx->cfg->update(ctx);
+		return 0;
+	}
+
 	new_msg = devm_kmalloc(&ctx->pdev->dev, count + 1, GFP_KERNEL);
 	if (!new_msg)
 		return -ENOMEM;
-- 
2.26.2