Re: [PATCH 3/3] gpiolib: cdev: Cleanup kfifo_out() error handling

[Bartosz Golaszewski]

On Mon, May 27, 2024 at 1:55 PM Kent Gibson <warthog618@xxxxxxxxx> wrote:
>
> The handling of kfifo_out() errors in read functions obscures any error.
> The error condition should never occur but, while a ret is set to -EIO, it
> is subsequently ignored and the read functions instead return the number
> of bytes copied to that point, potentially masking the fact that any error
> occurred.
>
> Return -EIO in the case of a kfifo_out() error to make it clear something
> very odd is going on here.
>
> Signed-off-by: Kent Gibson <warthog618@xxxxxxxxx>
> ---
>  drivers/gpio/gpiolib-cdev.c | 47 +++++++++++++++++--------------------
>  1 file changed, 21 insertions(+), 26 deletions(-)
>
> diff --git a/drivers/gpio/gpiolib-cdev.c b/drivers/gpio/gpiolib-cdev.c
> index c7218c9f2c5e..6a986d7f1f2f 100644
> --- a/drivers/gpio/gpiolib-cdev.c
> +++ b/drivers/gpio/gpiolib-cdev.c
> @@ -1642,16 +1642,13 @@ static ssize_t linereq_read(struct file *file, char __user *buf,
>                                         return ret;
>                         }
>
> -                       ret = kfifo_out(&lr->events, &le, 1);
> -               }
> -               if (ret != 1) {
> -                       /*
> -                        * This should never happen - we were holding the
> -                        * lock from the moment we learned the fifo is no
> -                        * longer empty until now.
> -                        */
> -                       ret = -EIO;
> -                       break;
> +                       if (kfifo_out(&lr->events, &le, 1) != 1)
> +                               /*
> +                                * This should never happen - we hold the

I'm not a native speaker but this looks odd to me - shouldn't it be
"we held the lock from the moment..."?

> +                                * lock from the moment we learned the fifo
> +                                * is no longer empty until now.
> +                                */
> +                               return -EIO;

Since this is so unlikely maybe a WARN() would be justified here too?

Bart

>                 }
>
>                 if (copy_to_user(buf + bytes_read, &le, sizeof(le)))
> @@ -1995,16 +1992,13 @@ static ssize_t lineevent_read(struct file *file, char __user *buf,
>                                         return ret;
>                         }
>
> -                       ret = kfifo_out(&le->events, &ge, 1);
> -               }
> -               if (ret != 1) {
> -                       /*
> -                        * This should never happen - we were holding the lock
> -                        * from the moment we learned the fifo is no longer
> -                        * empty until now.
> -                        */
> -                       ret = -EIO;
> -                       break;
> +                       if (kfifo_out(&le->events, &ge, 1) != 1)
> +                               /*
> +                                * This should never happen - we hold the
> +                                * lock from the moment we learned the fifo
> +                                * is no longer empty until now.
> +                                */
> +                               return -EIO;
>                 }
>
>                 if (copy_to_user(buf + bytes_read, &ge, ge_size))
> @@ -2707,12 +2701,13 @@ static ssize_t lineinfo_watch_read(struct file *file, char __user *buf,
>                         if (count < event_size)
>                                 return -EINVAL;
>  #endif
> -                       ret = kfifo_out(&cdev->events, &event, 1);
> -               }
> -               if (ret != 1) {
> -                       ret = -EIO;
> -                       break;
> -                       /* We should never get here. See lineevent_read() */
> +                       if (kfifo_out(&cdev->events, &event, 1) != 1)
> +                               /*
> +                                * This should never happen - we hold the
> +                                * lock from the moment we learned the fifo
> +                                * is no longer empty until now.
> +                                */
> +                               return -EIO;
>                 }
>
>  #ifdef CONFIG_GPIO_CDEV_V1
> --
> 2.39.2
>