auxdisplay: img-ascii-lcd: Fix lock-up when displaying empty string

[ Upstream commit afcb5a811ff3ab3969f09666535eb6018a160358 ]

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>
Signed-off-by: Sasha Levin <sashal@kernel.org>

diff --git a/drivers/auxdisplay/img-ascii-lcd.c b/drivers/auxdisplay/img-ascii-lcd.c
index 6e8eaa7fe7a6ff7adcda5d660db0e6e5a07f744a..b5f849d2f7623bb946e2d6f3f7f347536fcb188b 100644 (file)
--- a/drivers/auxdisplay/img-ascii-lcd.c
+++ b/drivers/auxdisplay/img-ascii-lcd.c
@@ -283,6 +283,16 @@ 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;