Subject: Re: [PATCH] powerpc: Don't clear larx reservation on system call exit

From: Anton Blanchard <[email-masked]>
Date: Mon, 15 Feb 2010 15:06:57 +1100

Hi Ben,

> Well, the main issue here is leaking kernel reservations into userspace,
> and thus the question of whether it is a big deal or not. There's also
> an issue I can see with signals.
> 
> The risk with kernel reservations leaking into userspace is a problem on
> some processors that do not compare the reservation address locally
> (only for snoops), thus userspace code doing lwarx/syscall/stwcx. might
> end up with a suceeding stwcx. despite the fact that the original
> reservation was long lost. 

Yeah that was my primary concern. Right now these things fail 100%, so
no one is relying on it. The worry is if people start writing their own
crazy low level system call + locking stubs that might work most of the
time (if we remove the stwcx in syscall exit).

> At this stage it becomes an ABI problem, ie, whether we define the
> behaviour of a lwarx/stwcx. accross a syscall as defined or not.
> 
> The other problem I see is that signal handlers would have to be made
> very careful not to leave dangling reservations since the return from
> the syscall is a syscall, unless we add code specifically to this (and
> set_context too I'd say) to clear reservations.
> 
> IE. You could have something like:
> 
> lwarx, <interrupt>, signal handler, sigreturn, stwcx.
> 
> In the above case, the reservation would be cleared by the return from
> the interrupt, but the signal handler might leave a dangling one, which
> sigreturn might fail to clear (in practice, our current implementation
> of sys_sigreturn() will probably clear any reservation as a side effect
> of restore_sigmask() spinlock or set_thread_flag() but it sounds a bit
> fragile to rely on unless it's well documented). 

Good point, I hadn't thought of signals and I agree we'd need to clear the
reservation in the sigreturn path.

Anton