On Tue, 22 May 2018, Uwe Kleine-König wrote:

[adding linux-rtc ML and Alexandre to Cc:]

Hello,

On Tue, May 22, 2018 at 02:09:36PM +0200, Jiri Kosina wrote:

quoted

On Mon, 21 May 2018, Uwe Kleine-König wrote:

quoted

quoted

The race looks like that (thanks Jiri):

CPU0:                                CPU1:
sys_load_module()
 do_init_module()
  do_one_initcall()
   cmos_do_probe()
    rtc_device_register()
     __register_chrdev()
     cdev->owner = struct module*
                                     open("/dev/rtc0")
    rtc_device_unregister()
  module_put()
  free_module()
   module_free(mod->module_core)
   /* struct module *module is now
      freed */
                                      chrdev_open()
                                       spin_lock(cdev_lock)
                                       cdev_get()
                                        try_module_get()
                                         module_is_live()
                                         /* dereferences already
                                            freed struct module* */

[Context: For a patch to rtc-pcf2127.c Alexandre Belloni asked not to
fail after rtc_device_register successfully finished and pointed to this
reasoning as explaination.]

If there is really such a race then (I hope) there is
something in the cdev code that needs fixing. According to my
understanding, when rtc_device_unregister returned, the cdev is gone and
so chrdev_open is supposed to fail.

Oh wow, hello back to 4 years ago!

:-)

quoted

Looking at the current code, I don't think there is no such race any more, 
as the last thing cmos_do_probe() -> __rtc_register_device() does that can 
potentially fail is the chardev creation itself.

OK, so you agree that it's also save to do something in a driver's probe
after rtc_device_register() and call rtc_device_unregister() in the error
path, right? 

Hmm, not really; that's what the code apparently did 4 years ago (judging 
from the scenario in the old mail, I of course forgot all the details), 
but doesn't do it any more.

Looking at the current code, if you call rtc_device_unregister() in the 
probe path, where is the guarantee that cdev_get() will not derefernce 
already freed struct module*?

Thanks,

-- 
Jiri Kosina
SUSE Labs