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Possible race in loop.ko

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  Possible race in loop.ko Anton Volkov <avolkov@ispras.ru> - 2017-07-28 18:00 +0200

#1698915 — Possible race in loop.ko

Hello.
While searching for races in Linux kernel I've come across 
drivers/block/loop.ko module. Here is the question that I came up with 
while analyzing results. Lines are given using the info from Linux v4.12.

In loop_init function additional initialization happens after a 
successful call to misc_register() (loop.c: line 1961). Consider the 
following case:

Thread 1:                 Thread 2:
loop_init()
   misc_register()         loop_control_ioctl
   part_shift = 0          -> loop_add
   if (max_part > 0) {          alloc_disk(1 << part_shift)
     part_shift =
          <greater than 0>
     ...
   }

In this case alloc_disk() will be called with 1 as a parameter although 
part_shift should have been greater than 0. Maybe it would be better to 
move the call to a misc_register() function a bit further down (at least 
so it could be after the part_shift initialization)?

Thank you for your time.

-- Anton Volkov
Linux Verification Center, ISPRAS
web: http://linuxtesting.org
e-mail: avolkov@ispras.ru

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