[PATCH 0/3] char: misc: assign file->private_data to miscdevice on open

[PATCH 0/3] char: misc: assign file->private_data to miscdevice on open

[Martin Kepplinger]

This slightly changes the behaviour of miscdevice drivers at an open()
syscall. Not only if the driver happens to implement an open() fop itself,
but now just always file->private_data points to struct miscdevice, when
a user opens the device file.

This call for review once more: if anybody has an hour to kill:

Look for the only dangerous situation where a miscdevice driver _depends_ on
file->private_data being NULL when _not_ implementing an open() routine
itself. I didn't find anything.

The one section I was unsure about should be ok, see
http://marc.info/?l=linux-kernel&m=141376535132316&w=2

Bonus: one could look for drivers that _do_ implement open()
and do the (in any case) redundant work themselves.

Martin Kepplinger (3):
  misc: always assign miscdevice to file->private_data in open()
  fbdev: pxa3xx-gcu: remove redundant implementation of open()
  char: misc: document behaviour of open()

 drivers/char/misc.c              |    6 ++++--
 drivers/video/fbdev/pxa3xx-gcu.c |   10 ----------
 2 files changed, 4 insertions(+), 12 deletions(-)

-- 
1.7.10.4

[PATCH 1/3] misc: always assign miscdevice to file->private_data in open()

[Martin Kepplinger]

As of now, a miscdevice driver has to provide an implementation of
the open() file operation if it wants to have misc_open() assign a
pointer to struct miscdevice to file->private_data for other file
operations to use (given the user calls open()).

This leads to situations where a miscdevice driver that doesn't need
internal operations during open() has to implement open() that only
returns immediately, in order to use the data in private_data in other
fops.

This provides consistent behaviour for miscdevice developers and will
always provide the pointer in private_data. A driver's open() fop would,
of course, just overwrite it, when using private_data itself.

Signed-off-by: Martin Kepplinger <martink@posteo.de>
---
 drivers/char/misc.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/char/misc.c b/drivers/char/misc.c
index ffa97d2..205ad4c 100644
--- a/drivers/char/misc.c
+++ b/drivers/char/misc.c
@@ -142,8 +142,8 @@ static int misc_open(struct inode * inode, struct file * file)
 
 	err = 0;
 	replace_fops(file, new_fops);
+	file->private_data = c;
 	if (file->f_op->open) {
-		file->private_data = c;
 		err = file->f_op->open(inode,file);
 	}
 fail:
-- 
1.7.10.4

[PATCH 2/3] fbdev: pxa3xx-gcu: remove redundant implementation of open()

[Martin Kepplinger]

the miscdevice core now does the work in any case.

Signed-off-by: Martin Kepplinger <martink@posteo.de>
---
 drivers/video/fbdev/pxa3xx-gcu.c |   10 ----------
 1 file changed, 10 deletions(-)

diff --git a/drivers/video/fbdev/pxa3xx-gcu.c b/drivers/video/fbdev/pxa3xx-gcu.c
index 4df3657..7678a94 100644
--- a/drivers/video/fbdev/pxa3xx-gcu.c
+++ b/drivers/video/fbdev/pxa3xx-gcu.c
@@ -373,15 +373,6 @@ static inline struct pxa3xx_gcu_priv *to_pxa3xx_gcu_priv(struct file *file)
 	return container_of(dev, struct pxa3xx_gcu_priv, misc_dev);
 }
 
-/*
- * provide an empty .open callback, so the core sets file->private_data
- * for us.
- */
-static int pxa3xx_gcu_open(struct inode *inode, struct file *file)
-{
-	return 0;
-}
-
 static ssize_t
 pxa3xx_gcu_write(struct file *file, const char *buff,
 		 size_t count, loff_t *offp)
@@ -580,7 +571,6 @@ pxa3xx_gcu_free_buffers(struct device *dev,
 
 static const struct file_operations pxa3xx_gcu_miscdev_fops = {
 	.owner =		THIS_MODULE,
-	.open =			pxa3xx_gcu_open,
 	.write =		pxa3xx_gcu_write,
 	.unlocked_ioctl =	pxa3xx_gcu_ioctl,
 	.mmap =			pxa3xx_gcu_mmap,
-- 
1.7.10.4

[PATCH 3/3] char: misc: document behaviour of open()

[Martin Kepplinger]

an open syscall now assignes file->private_data to a pointer to the
miscdevice structure. This reminds driver developers not to duplicate
code if they need this.

Signed-off-by: Martin Kepplinger <martink@posteo.de>
---
 drivers/char/misc.c |    4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/drivers/char/misc.c b/drivers/char/misc.c
index 205ad4c..69a08a6 100644
--- a/drivers/char/misc.c
+++ b/drivers/char/misc.c
@@ -169,7 +169,9 @@ static const struct file_operations misc_fops = {
  *	the minor number requested is used.
  *
  *	The structure passed is linked into the kernel and may not be
- *	destroyed until it has been unregistered.
+ *	destroyed until it has been unregistered. By default, an open()
+ *	syscall to the device sets file->private_data to point to the
+ *	structure.
  *
  *	A zero is returned on success and a negative errno code for
  *	failure.
-- 
1.7.10.4