[PATCH 2/2] 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.

Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
---
 drivers/gpio/gpiolib.c | 40 +++++++++++++++++++++++-----------------
 1 file changed, 23 insertions(+), 17 deletions(-)

diff --git a/drivers/gpio/gpiolib.c b/drivers/gpio/gpiolib.c
index 1baeb6778ec6..8a15b8f6b50e 100644
--- a/drivers/gpio/gpiolib.c
+++ b/drivers/gpio/gpiolib.c
@@ -4147,27 +4147,33 @@ 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