Messages in this thread | | | Date | Sat, 24 Aug 2013 14:51:39 -0700 | From | Guenter Roeck <> | Subject | Re: [PATCH] gpiolib: Fix crash when exporting non-existant gpio |
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On 08/24/2013 01:48 PM, danielfsantos@att.net wrote: > 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))
I think this triggers a WARN if someone requests an invalid gpio pin from userspace. Is this really a good idea ?
[ and then export_store and unexport_store complain again with pr_warn ]
May be a separate patch, but if the WARN is useful it might make sense to introduce gpio_to_desc_silent() which doesn't produce the WARN if it fails.
Looking further into the code, I suspect there may be some race condition where desc->chip is not (yet) set and export_store is called. So we will see a WARNING instead of a crash, as the underlying condition still exists.
> 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); > ... which in turn means we might see this one. If so, this code might replace an invalid memory access crash with a BUG crash. Is this really desirable, or should this better be a WARN ?
Guenter
> if (!try_module_get(chip->owner)) > goto done; >
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