Re: [PATCH v3 2/2] gpiolib: check the 'ngpios' property in core gpiolib code

On Thu, Nov 25, 2021 at 11:10:08AM +0100, Bartosz Golaszewski wrote:

On Wed, Nov 24, 2021 at 3:47 PM Andy Shevchenko
[off-list ref] wrote:

quoted

On Wed, Nov 24, 2021 at 01:28:50PM +0100, Bartosz Golaszewski wrote:

...

quoted

quoted

+             ret = device_property_read_u32(&gdev->dev, "ngpios", &ngpios);
+             if (ret) {
+                     if (ret == -ENODATA) {
+                             chip_err(gc, "tried to insert a GPIO chip with zero lines\n");
+                             ret = -EINVAL;
+                     }
+
+                     goto err_free_descs;
+             }

And if the property returns 0 in ngpios?

What about the modified suggestion from previous version:

        if (gc->ngpio == 0) {
                ret = device_property_read_u32(&gdev->dev, "ngpios", &ngpios);

The comment is a good idea but other than that - it's overcomplicating things.

I don't think so. It is plain and self-explaining each step. See at the end of
the message how.

quoted

                if (ret == -ENODATA)
                        ngpios = 0;
                else if (ret)
                        return ret;

You still need to goto err_free_descs here.

Right, this doesn't affect the main point / logic here.

quoted

                gc->ngpio = ngpios;
        }

        if (gc->ngpio == 0) {

Why check that again? We already know the driver set it to 0, we
checked it a couple lines before. If we can't get the setting from the
properties then it won't be non 0 here right?

No, it's not right. The check is needed to tell that properties supplied 0.

quoted

                chip_err(gc, "tried to insert a GPIO chip with zero lines\n");
                ret = -EINVAL;
                goto err_free_descs;
        }

?

quoted

+             gc->ngpio = ngpios;
      }

      if (gc->ngpio > FASTPATH_NGPIO)

I suggest the following:

It's buggy as submitted version (I actually haven't found any difference in
the code, but comments).

You see, I propose less changes and straight forward logic:

1. Check if the supplied ->ngpio equal to 0
2. If so, try device properties
2.1. If there is no property found, make sure we a) don't use uninitialized
     variable, b) we don't change ->ngpio, so it stays 0
2.2. If there is an error, return it as is to the caller
2.3. Assign ->ngpio by value from property (which very well may be 0!)
3. Check ->ngpio for 0 again, if so, issue a message and return -EINVAL to
the user.

We have three places where ->ngpio can be 0, all of them I covered.

-- 
With Best Regards,
Andy Shevchenko