[PATCH] gpiolib: Fix crash when exporting non-existant gpio

[danielfsantos]

I got this on an RPi and I can't find anything specific to that.
Besides, it's clearly wrong to try to access desc->chip when we have
just tested that it may be NULL at drivers/gpio/gpiolib.c:1409:

	chip = desc->chip;
	if (chip == NULL)
		goto done;

....

done:
	if (status)
		pr_debug("_gpio_request: gpio-%d (%s) status %d\n",
			 desc_to_gpio(desc), label ? : "?", status);

To reproduce, just pick an invalid gpio nubmer and:

echo -n 248 > /sys/class/gpio/export

However, I wasn't able to reproduce it on my laptop, maybe because I
don't have any real gpio chips there, not sure.  More info on RPi bug
report: https://github.com/raspberrypi/linux/issues/364

This fix makes sure that gpio_to_desc() only returns non-NULL if the
specified gpio really has a chip, and not just if it's within the ranged
of gpios for the arch.

[  222.961384] Unable to handle kernel NULL pointer dereference at
virtual address 00000044
[  222.969486] pgd = d97d0000
[  222.972190] [00000044] *pgd=1aaca831, *pte=00000000, *ppte=00000000
[  222.978483] Internal error: Oops: 17 [#1] PREEMPT ARM
---
 drivers/gpio/gpiolib.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/gpio/gpiolib.c b/drivers/gpio/gpiolib.c
index d6413b2..db7c6bb 100644
--- a/drivers/gpio/gpiolib.c
+++ b/drivers/gpio/gpiolib.c
@@ -123,7 +123,8 @@ static int gpio_chip_hwgpio(const struct gpio_desc *desc)
  */
 static struct gpio_desc *gpio_to_desc(unsigned gpio)
 {
-	if (WARN(!gpio_is_valid(gpio), "invalid GPIO %d\n", gpio))
+	if (WARN(!gpio_is_valid(gpio) || !gpio_desc[gpio].chip,
+			"invalid GPIO %d\n", gpio))
 		return NULL;
 	else
 		return &gpio_desc[gpio];
@@ -1406,8 +1407,7 @@ static int gpiod_request(struct gpio_desc *desc, const char *label)
 	spin_lock_irqsave(&gpio_lock, flags);
 
 	chip = desc->chip;
-	if (chip == NULL)
-		goto done;
+	BUG_ON(!chip);
 
 	if (!try_module_get(chip->owner))
 		goto done;
-- 
1.8.1.5

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