linux-kernel - Re: [PATCH v4 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 13:05:30 +0100
From:   Bartosz Golaszewski <brgl@...ev.pl>
To:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc:     Kent Gibson <warthog618@...il.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 v4 2/2] gpiolib: protect the GPIO device against being
 dropped while in use by user-space

On Wed, Nov 30, 2022 at 1:02 PM Andy Shevchenko
<andriy.shevchenko@...ux.intel.com> wrote:
>
> On Wed, Nov 30, 2022 at 10:05:56AM +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.
>
> Bikeshedding below and one question.
> (As per tag I'm fine with this version anyway)
>
> ...
>
> > +typedef __poll_t (*poll_fn)(struct file *, struct poll_table_struct *);
> > +typedef long (*ioctl_fn)(struct file *, unsigned int, unsigned long);
>
> > +typedef ssize_t (*read_fn)(struct file *, char __user *,
> > +                        size_t count, loff_t *);
>
> <bikeshedding>
> It's only 84 is on a single line.
>
> Dunno if it's better to have typedef followed by wrapper pairs rather than
> all typedefs and wrappers grouped.
> </bikeshedding>
>
> > +static __poll_t call_poll_locked(struct file *file,
> > +                              struct poll_table_struct *wait,
> > +                              struct gpio_device *gdev, poll_fn func)
> > +{
> > +     __poll_t ret;
>
> > +     down_read(&gdev->sem);
>
> Thinking more about this, wouldn't be better to actually
>
>         ret = down_read_trylock(&gdev->sem);
>         if (ret)
>                 return ret;
>
> ?

You mean as in: try to take the lock, but if we're already removing
the device (as the down_write() can only happen in gpiochip_remove()),
then die right away? Smart! Yeah, I'll do it this way.

For the rest: I like my version better honestly.

Bart