linux-kernel - Re: [PATCH v3 2/2] gpiolib: protect the GPIO device against being dropped while in use by user-space

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Date:   Wed, 30 Nov 2022 07:57:26 +0800
From:   Kent Gibson <warthog618@...il.com>
To:     Bartosz Golaszewski <brgl@...ev.pl>
Cc:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Linus Walleij <linus.walleij@...aro.org>,
        linux-gpio@...r.kernel.org, linux-kernel@...r.kernel.org,
        Bartosz Golaszewski <bartosz.golaszewski@...aro.org>
Subject: Re: [PATCH v3 2/2] gpiolib: protect the GPIO device against being
 dropped while in use by user-space

On Tue, Nov 29, 2022 at 06:53:26PM +0200, Andy Shevchenko wrote:
> On Tue, Nov 29, 2022 at 01:35:53PM +0100, Bartosz Golaszewski wrote:
> > From: Bartosz Golaszewski <bartosz.golaszewski@...aro.org>
> > 
> > While any of the GPIO cdev syscalls is in progress, the kernel can call
> > gpiochip_remove() (for instance, when a USB GPIO expander is disconnected)
> > which will set gdev->chip to NULL after which any subsequent access will
> > cause a crash.
> > 
> > To avoid that: use an RW-semaphore in which the syscalls take it for
> > reading (so that we don't needlessly prohibit the user-space from calling
> > syscalls simultaneously) while gpiochip_remove() takes it for writing so
> > that it can only happen once all syscalls return.
> 
> ...
> 
> I would do
> 
> typedef __poll_t (*poll_fn)(struct file *, struct poll_table_struct *);
> 
> and so on and use that one in the respective parameters.
> 
> BUT. Since it's a fix, up to you which one to choose.
> 

FWIW, the typedef looks cleaner to me too.

> > +static __poll_t call_poll_locked(struct file *file,
> > +				 struct poll_table_struct *wait,
> > +				 struct gpio_device *gdev,
> > +				 __poll_t (*func)(struct file *,
> > +						  struct poll_table_struct *))
> > +{
> > +	__poll_t ret;
> > +
> > +	down_read(&gdev->sem);
> > +	ret = func(file, wait);
> > +	up_read(&gdev->sem);
> > +
> > +	return ret;
> > +}
> 
> ...
> 
> > +	down_write(&gdev->sem);
> 
> + Blank line?
> 

Agreed.

> >  	/* FIXME: should the legacy sysfs handling be moved to gpio_device? */
> >  	gpiochip_sysfs_unregister(gdev);
> >  	gpiochip_free_hogs(gc);
> 
> ...
> 
> >  	gcdev_unregister(gdev);
> 
> + Blank line ?
> 

Disagree with this one though.
The comment prior to the gcdev_unregister() appears to apply to the block,
so the following lines should remain grouped.

Other than those nits, the series looks good to me.

Reviewed-by: Kent Gibson <warthog618@...il.com>

Cheers,
Kent.

> > +	up_write(&gdev->sem);
> >  	put_device(&gdev->dev);
> 
> -- 
> With Best Regards,
> Andy Shevchenko
> 
>