Using __kuser_cmpxchg

[Matthieu CASTET]

Andrew Haley wrote:
> Nicolas Pitre writes:
>  > However, on a non SMP machine, the only thing that can break user
>  > space atomicity is a processor exception.  When an exception occurs
>  > because of an IRQ or a page fault, the kernel may well decide to
>  > schedule another user process or thread unexpectedly.
>  > 
>  > So we cheat a bit by simply performing the load+compare+store with 
>  > plain standard instructions as follows:
>  > 
>  >         teq     ip, ip                  @ set Z flag
>  >         ldr     ip, [r2]                @ load current val
>  >         add     r3, r2, #1              @ prepare store ptr
>  >         teqeq   ip, r0                  @ compare with oldval if still allowed
>  >         streq   r1, [r3, #-1]!          @ store newval if still allowed
>  >         subs    r0, r2, r3              @ if r2 == r3 the str occured
>  > 
>  > But there is a twist such that if ever an exception occurs during that 
>  > sequence, the Z flag is cleared by the exception handler before that 
>  > sequence is resumed and the store won't occur.
> 
> Ok, please stop there.  I don't understand this at all: if an
> exception occurs surely we'll save and restore the flags.  How is it
> possible for an exception to corrupt the user space Z flag?
> 
Look in arch/arm/kernel/entry-armv.S the usr_entry macro.

There are special code in the exception handler to detect if we come 
from __kuser_cmpxchg code and clear the Z flags.


Matthieu