Input: atmel_mxt_ts - check mem_size before calculating config memory size

In mxt_update_cfg(), the driver calculates the memory size needed to store
the configuration as data->mem_size - cfg.start_ofs. If data->mem_size is
less than or equal to cfg.start_ofs, this calculation will underflow or
result in a zero-size buffer, neither of which is valid for a configuration
update.

Add a check to return -EINVAL if data->mem_size is too small. While at it,
change the types of start_ofs and mem_size in struct mxt_cfg to u16 to
match the device address space.

Assisted-by: Gemini:gemini-3.1-pro
Link: https://patch.msgid.link/20260504185448.4055973-2-dmitry.torokhov@gmail.com
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>

Author: dtor

drivers/input/touchscreen/atmel_mxt_ts.c

@@ -275,8 +275,8 @@ struct mxt_cfg {
 	off_t raw_pos;
 
 	u8 *mem;
-	size_t mem_size;
-	int start_ofs;
+	u16 mem_size;
+	u16 start_ofs;
 
 	struct mxt_info info;
 };
@@ -1627,6 +1627,13 @@ static int mxt_update_cfg(struct mxt_data *data, const struct firmware *fw)
 	cfg.start_ofs = MXT_OBJECT_START +
 			data->info->object_num * sizeof(struct mxt_object) +
 			MXT_INFO_CHECKSUM_SIZE;
+
+	if (data->mem_size <= cfg.start_ofs) {
+		dev_err(dev, "Memory size too small: %u < %u\n",
+			data->mem_size, cfg.start_ofs);
+		return -EINVAL;
+	}
+
 	cfg.mem_size = data->mem_size - cfg.start_ofs;
 
 	u8 *mem_buf __free(kfree) = cfg.mem = kzalloc(cfg.mem_size, GFP_KERNEL);