linux-kernel - Re: [PATCH] iio: buffer: return 0 for buffer getfd ioctl handler

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Date:   Mon, 29 Mar 2021 13:33:24 +0100
From:   Jonathan Cameron <jic23@...nel.org>
To:     Alexandru Ardelean <aardelean@...iqon.com>
Cc:     linux-iio@...r.kernel.org, linux-kernel@...r.kernel.org,
        Lars-Peter Clausen <lars@...afoo.de>
Subject: Re: [PATCH] iio: buffer: return 0 for buffer getfd ioctl handler

On Mon, 22 Mar 2021 10:41:35 +0200
Alexandru Ardelean <aardelean@...iqon.com> wrote:

> As Lars pointed out, we could either return the FD vs memcpy-ing it to the
> userspace data object.
> 
> However, this comment exposed a bug. We should return 0 or negative from
> these ioctl() handlers. Because an ioctl() handler can also return
> IIO_IOCTL_UNHANDLED (which is positive 1), which means that the ioctl()
> handler doesn't support this ioctl number. Positive 1 could also be a valid
> FD number in some corner cases.
> 
> The reason we did this is to be able to differentiate between an error
> code and an unsupported ioctl number; for unsupported ioctl numbers, the
> main loop should keep going.
> 
> Maybe we should change this to a higher negative number, to avoid such
> cases when/if we add more ioctl() handlers.

That sounds like a sensible follow up plan to me - perhaps we could
use EOPNOTSUPP for this directly but add special handling for that in
the core so it doesn't worry about it as long as someone supports the ioctl.

> 
> Cc: Lars-Peter Clausen <lars@...afoo.de>
> Fixes: f73f7f4da5818 ("iio: buffer: add ioctl() to support opening extra buffers for IIO device")
> Signed-off-by: Alexandru Ardelean <aardelean@...iqon.com>

Applied to the togreg branch of iio.git and pushed out as testing.

Thanks,

Jonathan
> ---
>  drivers/iio/industrialio-buffer.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/iio/industrialio-buffer.c b/drivers/iio/industrialio-buffer.c
> index ee5aab9d4a23..d7a15c9bb0cd 100644
> --- a/drivers/iio/industrialio-buffer.c
> +++ b/drivers/iio/industrialio-buffer.c
> @@ -1442,7 +1442,7 @@ static long iio_device_buffer_getfd(struct iio_dev *indio_dev, unsigned long arg
>  		goto error_free_ib;
>  	}
>  
> -	return fd;
> +	return 0;
>  
>  error_free_ib:
>  	kfree(ib);