2014-10-19 00:11:59

by Martin Kepplinger

Subject: lguest: lguest_user.c is open() called by the user?

hi

Just a question for understanding: open() is not implemented in
lguest_user.c's miscdevice. The miscdevice core, in this case, does
_not_ set file->private_data on a user's open() call. Is open() called
by the user here? and do you here _depend_ on file->private_data being
NULL after open()? (could you even?)

Would the following force to NULL be necessary if the miscdevice core
_would_ set private_data?


diff --git a/drivers/lguest/lguest_user.c b/drivers/lguest/lguest_user.c
index 4263f4c..3d472ea 100644
--- a/drivers/lguest/lguest_user.c
+++ b/drivers/lguest/lguest_user.c
@@ -497,6 +497,12 @@ static int close(struct inode *inode, struct file
*file)
return 0;
}

+static int open(struct inode *inode, struct file *file)
+{
+ /* assuming the miscdevice core sets private_data on open() */
+ file->private_data = NULL;
+}
+
/*L:000
* Welcome to our journey through the Launcher!
*
@@ -514,6 +520,7 @@ static int close(struct inode *inode, struct file *file)
*/
static const struct file_operations lguest_fops = {
.owner = THIS_MODULE,
+ .open = open,
.release = close,
.write = write,
.read = read,
--

thanks
martin

2014-10-20 00:35:46

by Rusty Russell

Subject: Re: lguest: lguest_user.c is open() called by the user?

Martin Kepplinger <[email protected]> writes:
> hi
>
> Just a question for understanding: open() is not implemented in
> lguest_user.c's miscdevice. The miscdevice core, in this case, does
> _not_ set file->private_data on a user's open() call. Is open() called
> by the user here? and do you here _depend_ on file->private_data being
> NULL after open()? (could you even?)
>
> Would the following force to NULL be necessary if the miscdevice core
> _would_ set private_data?

Hi Martin!

Yes, the private_data is NULL on a new file. See
get_empty_filp in fs/file_table.c, which does kmem_cache_zalloc
(zeroing all the contents).

So this isn't necessary here.

Thanks!
Rusty.

> diff --git a/drivers/lguest/lguest_user.c b/drivers/lguest/lguest_user.c
> index 4263f4c..3d472ea 100644
> --- a/drivers/lguest/lguest_user.c
> +++ b/drivers/lguest/lguest_user.c
> @@ -497,6 +497,12 @@ static int close(struct inode *inode, struct file
> *file)
> return 0;
> }
>
> +static int open(struct inode *inode, struct file *file)
> +{
> + /* assuming the miscdevice core sets private_data on open() */
> + file->private_data = NULL;
> +}
> +
> /*L:000
> * Welcome to our journey through the Launcher!
> *
> @@ -514,6 +520,7 @@ static int close(struct inode *inode, struct file *file)
> */
> static const struct file_operations lguest_fops = {
> .owner = THIS_MODULE,
> + .open = open,
> .release = close,
> .write = write,
> .read = read,
> --
>
> thanks
> martin
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