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

[danielfsantos@att.net]

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

[  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 | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/gpio/gpiolib.c b/drivers/gpio/gpiolib.c
index d6413b2..e547f75f8b 100644
--- a/drivers/gpio/gpiolib.c
+++ b/drivers/gpio/gpiolib.c
@@ -136,7 +136,7 @@ static struct gpio_desc *gpio_to_desc(unsigned gpio)
  */
 static int desc_to_gpio(const struct gpio_desc *desc)
 {
-	return desc->chip->base + gpio_chip_hwgpio(desc);
+	return desc->chip ?  desc->chip->base + gpio_chip_hwgpio(desc) : -1;
 }
 
 
-- 
1.8.1.5