In cyapa_update_fw_store(), if count is zero, the write to
fw_name[count-1] underflows the array.

Presumably that's not possible in normal operation, as its caller
sysfs_kf_write() already checks for zero.  Regardless it's a good idea
to check for the error explicitly and avoid undefined behavior.

Fixes the following warning with an UBSAN kernel:

  vmlinux.o: warning: objtool: .text.cyapa_update_fw_store: unexpected end of section

Cc: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202503171547.LlCTJLQL-lkp@intel.com/
Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
---
 drivers/input/mouse/cyapa.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/input/mouse/cyapa.c b/drivers/input/mouse/cyapa.c
index 2f2d925a55d7..00c87c0532a6 100644
--- a/drivers/input/mouse/cyapa.c
+++ b/drivers/input/mouse/cyapa.c
@@ -1080,8 +1080,8 @@ static ssize_t cyapa_update_fw_store(struct device *dev,
 	char fw_name[NAME_MAX];
 	int ret, error;
 
-	if (count >= NAME_MAX) {
-		dev_err(dev, "File name too long\n");
+	if (!count || count >= NAME_MAX) {
+		dev_err(dev, "Bad file name size\n");
 		return -EINVAL;
 	}
 
-- 
2.48.1