[PATCH] gpiolib: Fix potential NULL pointer dereference in gpiod_get_label()

[Prabhakar <prabhakar.csengg@xxxxxxxxx>]

From: Lad Prabhakar <prabhakar.mahadev-lad.rj@xxxxxxxxxxxxxx>

In `gpiod_get_label()`, it is possible that `srcu_dereference_check()` may
return a NULL pointer, leading to a scenario where `label->str` is accessed
without verifying if `label` itself is NULL.

This patch adds a proper NULL check for `label` before accessing
`label->str`. The check for `label->str != NULL` is removed because
`label->str` can never be NULL if `label` is not NULL.

This fixes the issue where the label name was being printed as `(efault)`
when dumping the sysfs GPIO file when `label == NULL`.

Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@xxxxxxxxxxxxxx>
---
Output before this patch:
root@smarc-rzg2l:~# cat /sys/kernel/debug/gpio | grep P2_1
 gpio-529 (P2_1                |(efault)            ) in  hi IRQ

Output after this patch:
root@smarc-rzg2l:~# cat /sys/kernel/debug/gpio | grep P2_1
 gpio-529 (P2_1                |interrupt           ) in  hi IRQ
---
 drivers/gpio/gpiolib.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/gpio/gpiolib.c b/drivers/gpio/gpiolib.c
index c6afbf434366..d5952ab7752c 100644
--- a/drivers/gpio/gpiolib.c
+++ b/drivers/gpio/gpiolib.c
@@ -114,12 +114,12 @@ const char *gpiod_get_label(struct gpio_desc *desc)
 				srcu_read_lock_held(&desc->gdev->desc_srcu));
 
 	if (test_bit(FLAG_USED_AS_IRQ, &flags))
-		return label->str ?: "interrupt";
+		return label ? label->str : "interrupt";
 
 	if (!test_bit(FLAG_REQUESTED, &flags))
 		return NULL;
 
-	return label->str;
+	return label ? label->str : NULL;
 }
 
 static void desc_free_label(struct rcu_head *rh)
-- 
2.43.0