gpiolib: cdev: annotate struct linereq with __counted_by

Prepare for the coming implementation by GCC and Clang of the __counted_by
attribute. Flexible array members annotated with __counted_by can have
their accesses bounds-checked at run-time checking via CONFIG_UBSAN_BOUNDS
(for array indexing) and CONFIG_FORTIFY_SOURCE (for strcpy/memcpy-family
functions).

As found with Coccinelle[1], add __counted_by for struct linereq.
Additionally, since the element count member must be set before accessing
the annotated flexible array member, move its initialization earlier.

[1] https://github.com/kees/kernel-tools/blob/trunk/coccinelle/examples/counted_by.cocciSigned-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Andy Shevchenko <andy@kernel.org>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>

drivers/gpio/gpiolib-cdev.c

@@ -572,7 +572,7 @@ struct linereq {
 	DECLARE_KFIFO_PTR(events, struct gpio_v2_line_event);
 	atomic_t seqno;
 	struct mutex config_mutex;
-	struct line lines[];
+	struct line lines[] __counted_by(num_lines);
 };
 
 #define GPIO_V2_LINE_BIAS_FLAGS \
@@ -1656,6 +1656,7 @@ static int linereq_create(struct gpio_device *gdev, void __user *ip)
 	lr = kzalloc(struct_size(lr, lines, ulr.num_lines), GFP_KERNEL);
 	if (!lr)
 		return -ENOMEM;
+	lr->num_lines = ulr.num_lines;
 
 	lr->gdev = gpio_device_get(gdev);
 
@@ -1684,7 +1685,6 @@ static int linereq_create(struct gpio_device *gdev, void __user *ip)
 		lr->event_buffer_size = GPIO_V2_LINES_MAX * 16;
 
 	atomic_set(&lr->seqno, 0);
-	lr->num_lines = ulr.num_lines;
 
 	/* Request each GPIO */
 	for (i = 0; i < ulr.num_lines; i++) {