Re: [PATCH] sparc: Add support for seek and shorter read to /dev/mdesc

Sam Ravnborg
2014-07-16T19:04:42Z

Hi Kahlid.

On Wed, Jul 16, 2014 at 08:02:03AM -0600, Khalid Aziz wrote:
> /dev/mdesc on Linux does not support reading arbitrary number
> of bytes and seeking while /dev/mdesc on Solaris does. This
> causes tools that work on Solaris to break on Linux. This patch
> adds these two capabilities to /dev/mdesc.
>
> Signed-off-by: Khalid Aziz <khalid.aziz@oracle.com>
> ---
> arch/sparc/kernel/mdesc.c | 77 ++++++++++++++++++++++++++++++++++++++++-------
> 1 file changed, 66 insertions(+), 11 deletions(-)
>
> +/* mdesc_open() - Grab a reference to mdesc_handle when /dev/mdesc is
> + * opened. Hold this reference until /dev/mdesc is closed to ensure
> + * mdesc data structure is not released underneath us. Store the
> + * pointer to mdesc structure in private_data for read and seek to use
> + */
> +static int mdesc_open(struct inode *inode, struct file *file)
> {
> struct mdesc_handle *hp = mdesc_grab();
>
> if (!hp)
> return -ENODEV;
>
> + file->private_data = hp;
> + return 0;
> +}

Do we know the open/close always come in pairs?
I assume so - but there is no check fo this (at least on this level).

> +
> +static ssize_t mdesc_read(struct file *file, char __user *buf,
> + size_t len, loff_t *offp)
> +{
> + struct mdesc_handle *hp = file->private_data;
> + unsigned char *mdesc;
> + int err, bytes_left;
> +
> + if (*offp >= hp->handle_size)
> + return 0;
> + err = len;
> + bytes_left = hp->handle_size - *offp;
> + if (len > bytes_left)
> + err = bytes_left;
> + mdesc = (unsigned char *)&hp->mdesc;
> + mdesc += *offp;
> + if (copy_to_user(buf, mdesc, err))
> err = -EFAULT;
> - mdesc_release(hp);
> + else
> + *offp += err;
> +
> + return err;
> +}

When reading your code it is confusing to read that err is set to len,
and then maybe later set to an error value or a new len.

See the following refactoring of mdesc_read() that avoids the err local
variable resulting in more readable code.

static ssize_t mdesc_read(struct file *file, char __user *buf,
size_t len, loff_t *offp)
{
struct mdesc_handle *hp = file->private_data;
unsigned char *mdesc;
int bytes_left;

if (*offp >= hp->handle_size)
return 0;

bytes_left = hp->handle_size - *offp;
if (len > bytes_left)
len = bytes_left;

mdesc = (unsigned char *)&hp->mdesc;
mdesc += *offp;
if (!copy_to_user(buf, mdesc, len)) {
*offp += len;
return len;
} else {
return -EFAULT;
}
}

The above is IMO more readable.
>
> +static loff_t mdesc_llseek(struct file *file, loff_t offset, int whence)
> +{
> + struct mdesc_handle *hp;
> + int err;
> +
> + switch (whence) {
> + case SEEK_CUR:
> + offset += file->f_pos;
> + break;
> + case SEEK_SET:
> + break;
> + default:
> + return -EINVAL;
> + }
> +
> + err = offset;
> + hp = file->private_data;
> + if (offset > hp->handle_size)
> + err = -EINVAL;
> + else
> + file->f_pos = offset;
> return err;
> }
Same story here with err.

Sam