Re: [PATCH] driver core: Make it safe to use get_device() if the reference count is zero
From: Jason Yan
Date: Thu Dec 14 2017 - 04:31:56 EST
On 2017/12/14 16:56, Greg KH wrote:
On Thu, Dec 14, 2017 at 04:27:56PM +0800, Jason Yan wrote:
On 2017/12/14 16:10, Greg KH wrote:
On Thu, Dec 14, 2017 at 03:56:46PM +0800, Jason Yan wrote:
On 2017/12/14 15:42, Greg KH wrote:
On Thu, Dec 14, 2017 at 11:39:36AM +0800, Jason Yan wrote:
Some driviers may have the chance to increase a reference count that
has dropped to zero when using get_device() because of their design.
Then those drivers are broken :)
We have met such a issue with scsi:
https://www.spinics.net/lists/linux-scsi/msg115295.html
The scsi core will keep the scsi device object in the host list after
it has been deleted and the iterator can still find it. All of the
places where need iterating have to check the state of the scsi device
and this makes a lot of code redundancy and complexity.
Provide a safe mechanism in get_device() by using
kobject_get_unless_zero().
Suggested-by: Bart Van Assche <bart.vanassche@xxxxxxx>
Signed-off-by: Jason Yan <yanaijie@xxxxxxxxxx>
CC: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
CC: Bart Van Assche <bart.vanassche@xxxxxxx>
CC: Ewan D. Milne <emilne@xxxxxxxxxx>
CC: James E.J. Bottomley <jejb@xxxxxxxxxxxxxxxxxx>
CC: Christoph Hellwig <hch@xxxxxx>
---
drivers/base/core.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/drivers/base/core.c b/drivers/base/core.c
index 12ebd05..cc74810 100644
--- a/drivers/base/core.c
+++ b/drivers/base/core.c
@@ -1916,7 +1916,7 @@ EXPORT_SYMBOL_GPL(device_register);
*/
struct device *get_device(struct device *dev)
{
- return dev ? kobj_to_dev(kobject_get(&dev->kobj)) : NULL;
+ return dev && kobject_get_unless_zero(&dev->kobj) ? dev : NULL;
I really don't want to do this, the bus the device is on should prevent
this from happening.
Also, once that reference count drops to zero, the memory should be
freed, so you really have a stale pointer here, and this code would fail
if you had slab debugging enabled anyway.
Actually I don't want this either. But the design of scsi core will leave
the scsi device on the host list after it is deleted, and it can be
found later and the refcount have a very big chance to increase from
zero again. And after a lot of discussion it seems that the scsi layer
is difficult to change the situation in the near future.
Keeping a 'struct device' reference counted chunk of memory on a list
that has a different lifetime rule from that device itself, is crazy.
The lifetime rule is the same. That device itself will delete from the
host list in the destructor, scsi_device_dev_release_usercontext(), and
the memory will be freed after that. That's why this issue came out.
And yes, I remember how all of this came about, but I really don't have
the time to work on it myself...
So I don't even think this fixes the issue you think it fixes :)
This issue is very easy to reproduce on my machine and I have tested the
patch and it really fixes the issue.
Even with slab debugging enabled? If so, what is keeping that memory
from being freed once the reference count drops to 0?
As above, the memory is not freed yet when we increasing the refconunt from
zero, so it's nothing to do with slab debugging enabled or not. If
we want to free it, we have to grab host lock first to delete it from
the list, so if we are grabing the host lock, we can increase the
refcount safely from 0 to 1.
Wait, what? Once that refcount goes to 0, the release callback will be
called, and the memory had better be freed. Any pointer you might still
have to the structure is now invalid. The fact that you are somehow
still keeping that pointer around is not ok, and slab debugging should
have caused the memory to be overwritten and garbage would result if you
tried to make this call again.
I don't know why scsi have this design. Anyone who is familiar with the
history of this design can explain this?
thanks,
greg k-h
.