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From:   Geert Uytterhoeven <geert@linux-m68k.org>
To:     Robin van der Gracht <robin@protonic.nl>,
        Rob Herring <robh+dt@kernel.org>,
        Miguel Ojeda <ojeda@kernel.org>,
        Paul Burton <paulburton@kernel.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Pavel Machek <pavel@ucw.cz>
Cc:     devicetree@vger.kernel.org, linux-leds@vger.kernel.org,
        linux-mips@vger.kernel.org, linux-kernel@vger.kernel.org,
        Geert Uytterhoeven <geert@linux-m68k.org>
Subject: [PATCH v2 03/18] auxdisplay: img-ascii-lcd: Fix lock-up when displaying empty string
Date:   Fri, 25 Jun 2021 14:58:47 +0200
Message-Id: <20210625125902.1162428-4-geert@linux-m68k.org>
In-Reply-To: <20210625125902.1162428-1-geert@linux-m68k.org>
References: <20210625125902.1162428-1-geert@linux-m68k.org>
MIME-Version: 1.0
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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>
---
Untested with img-ascii-lcd, but triggered with my initial version of
linedisp.

v2:
  - No changes.
---
 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 1cce409ce5cacbc8..e33ce0151cdfd150 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;
-- 
2.25.1