Re: [PATCH v2] iio: industrialio-core: Fix debugfs read

[Jonathan Cameron]

On Fri, 21 Feb 2020 14:06:55 +0200
Alexandru Tachici <alexandru.tachici@xxxxxxxxxx> wrote:

> Currently iio_debugfs_read_reg calls debugfs_reg_access
> every time it is ran. Reading the same hardware register
> multiple times during the same reading of a debugfs file
> can cause unintended effects.
> 
> For example for each: cat iio:device0/direct_reg_access
> the file_operations.read function will be called at least
> twice. First will return the full length of the string in
> bytes  and the second will return 0.
> 
> This patch makes iio_debugfs_read_reg to call debugfs_reg_access
> only when the user's buffer position (*ppos) is 0. (meaning
> it is the beginning of a new reading of the debugfs file).
> 
> Fixes: e553f182d55b ("staging: iio: core: Introduce debugfs support, add support for direct register access")
> Signed-off-by: Alexandru Ardelean <alexandru.ardelean@xxxxxxxxxx>
> Signed-off-by: Alexandru Tachici <alexandru.tachici@xxxxxxxxxx>

I'm going to take this one via the slow path to give it a bit of time
to soak in linux-next.   I think we are fine for unexpected side effects
but it's been there for a long time so we can take this slowly.

Applied to the togreg branch of iio.git and pushed out as testing for
the autobuilders to play with it.

Thanks,

Jonathan

> ---
>  drivers/iio/industrialio-core.c | 15 +++++++++++----
>  include/linux/iio/iio.h         |  2 ++
>  2 files changed, 13 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/iio/industrialio-core.c b/drivers/iio/industrialio-core.c
> index 65ff0d067018..c4d5104934fc 100644
> --- a/drivers/iio/industrialio-core.c
> +++ b/drivers/iio/industrialio-core.c
> @@ -301,11 +301,14 @@ static ssize_t iio_debugfs_read_reg(struct file *file, char __user *userbuf,
>  			      size_t count, loff_t *ppos)
>  {
>  	struct iio_dev *indio_dev = file->private_data;
> -	char buf[20];
>  	unsigned val = 0;
> -	ssize_t len;
>  	int ret;
>  
> +	if (*ppos > 0)
> +		return simple_read_from_buffer(userbuf, count, ppos,
> +					       indio_dev->read_buf,
> +					       indio_dev->read_buf_len);
> +
>  	ret = indio_dev->info->debugfs_reg_access(indio_dev,
>  						  indio_dev->cached_reg_addr,
>  						  0, &val);
> @@ -314,9 +317,13 @@ static ssize_t iio_debugfs_read_reg(struct file *file, char __user *userbuf,
>  		return ret;
>  	}
>  
> -	len = snprintf(buf, sizeof(buf), "0x%X\n", val);
> +	indio_dev->read_buf_len = snprintf(indio_dev->read_buf,
> +					   sizeof(indio_dev->read_buf),
> +					   "0x%X\n", val);
>  
> -	return simple_read_from_buffer(userbuf, count, ppos, buf, len);
> +	return simple_read_from_buffer(userbuf, count, ppos,
> +				       indio_dev->read_buf,
> +				       indio_dev->read_buf_len);
>  }
>  
>  static ssize_t iio_debugfs_write_reg(struct file *file,
> diff --git a/include/linux/iio/iio.h b/include/linux/iio/iio.h
> index 862ce0019eba..eed58ed2f368 100644
> --- a/include/linux/iio/iio.h
> +++ b/include/linux/iio/iio.h
> @@ -568,6 +568,8 @@ struct iio_dev {
>  #if defined(CONFIG_DEBUG_FS)
>  	struct dentry			*debugfs_dentry;
>  	unsigned			cached_reg_addr;
> +	char				read_buf[20];
> +	unsigned int			read_buf_len;
>  #endif
>  };
>