Re: [linux-usb-devel] [PATCH] back out sysfs reference count change

Alan Stern
2004-03-31T15:11:41Z

On Wed, 31 Mar 2004, Maneesh Soni wrote:

> For convenience I will explain the race here..
>
> cpu 0 cpu 1
> kobject_unregister() sysfs_open_file()
> kobject_del() check_perm()
> sysfs_remove_dir() :
> (dentry remains alive due to ref. taken :
> on the way to sysfs_open_file) :
> kobject_put() :
> kobject_cleanup() kobject_get(->d_fsdata)
>
> cpu 1 could end up referring to a freed kobject through dentry->d_fsdata or
> starts spitting Badness in kobject_get at lib/kobject.c:429. For triggering
> this race try running these two loops simultaneously on SMP
>
> # while true; do insmod drivers/net/dummy.ko; rmmod dummy; done
> # while true; do find /sys/class/net | xargs cat; done
>
> Probably it can be solved by making sure that when sysfs file is
> opened/read/written some _race_ free check is done and fail if kobject if gone.
>
> Maneesh

Here's a suggestion. At the start of check_perm() grab the dentry
semaphore, then check whether d_fsdata is NULL, if it isn't then do the
kobject_get(), then unlock the semaphore.

Alan Stern