[PATCH 3/3] gpiolib: pin GPIO devices in place during descriptor lookup

[Bartosz Golaszewski <brgl@bgdev.pl>]

From: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>

There's time between when we locate the relevant descriptor during
lookup and when we actually take the reference to its parent GPIO
device where - if the GPIO device in question is removed - we'll end up
with a dangling pointer to freed memory. Make sure devices cannot be
removed until we hold a new reference to the device.

In order to not overcomplicate the code with locking and unlocking
variants of the same functions, just use the nesting feature of RW
semaphores in gpio_device_find().

Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
---
 drivers/gpio/gpiolib.c | 42 ++++++++++++++++++++++++------------------
 1 file changed, 24 insertions(+), 18 deletions(-)

diff --git a/drivers/gpio/gpiolib.c b/drivers/gpio/gpiolib.c
index 4c93cf73a826..972f33bc1a72 100644
--- a/drivers/gpio/gpiolib.c
+++ b/drivers/gpio/gpiolib.c
@@ -1114,7 +1114,7 @@ struct gpio_device *gpio_device_find(void *data,
 	 */
 	might_sleep();
 
-	guard(rwsem_read)(&gpio_devices_sem);
+	guard(rwsem_read_nested)(&gpio_devices_sem, SINGLE_DEPTH_NESTING);
 
 	list_for_each_entry(gdev, &gpio_devices, list) {
 		if (gdev->chip && match(gdev->chip, data))
@@ -4134,27 +4134,33 @@ static struct gpio_desc *gpiod_find_and_request(struct device *consumer,
 	struct gpio_desc *desc;
 	int ret;
 
-	desc = gpiod_find_by_fwnode(fwnode, consumer, con_id, idx, &flags, &lookupflags);
-	if (gpiod_not_found(desc) && platform_lookup_allowed) {
+	scoped_guard(rwsem_read, &gpio_devices_sem) {
+		desc = gpiod_find_by_fwnode(fwnode, consumer, con_id, idx,
+					    &flags, &lookupflags);
+		if (gpiod_not_found(desc) && platform_lookup_allowed) {
+			/*
+			 * Either we are not using DT or ACPI, or their lookup
+			 * did not return a result. In that case, use platform
+			 * lookup as a fallback.
+			 */
+			dev_dbg(consumer,
+				"using lookup tables for GPIO lookup\n");
+			desc = gpiod_find(consumer, con_id, idx, &lookupflags);
+		}
+
+		if (IS_ERR(desc)) {
+			dev_dbg(consumer, "No GPIO consumer %s found\n",
+				con_id);
+			return desc;
+		}
+
 		/*
-		 * Either we are not using DT or ACPI, or their lookup did not
-		 * return a result. In that case, use platform lookup as a
-		 * fallback.
+		 * If a connection label was passed use that, else attempt to
+		 * use the device name as label
 		 */
-		dev_dbg(consumer, "using lookup tables for GPIO lookup\n");
-		desc = gpiod_find(consumer, con_id, idx, &lookupflags);
+		ret = gpiod_request(desc, label);
 	}
 
-	if (IS_ERR(desc)) {
-		dev_dbg(consumer, "No GPIO consumer %s found\n", con_id);
-		return desc;
-	}
-
-	/*
-	 * If a connection label was passed use that, else attempt to use
-	 * the device name as label
-	 */
-	ret = gpiod_request(desc, label);
 	if (ret) {
 		if (!(ret == -EBUSY && flags & GPIOD_FLAGS_BIT_NONEXCLUSIVE))
 			return ERR_PTR(ret);
-- 
2.40.1