Re: [PATCH] ARC: Improve cmpxchng syscall implementation

[Vineet Gupta]

On 03/21/2018 04:54 AM, Alexey Brodkin wrote:
> /*
> > >    	 * This is only for old cores lacking LLOCK/SCOND, which by defintion
> > > @@ -60,23 +62,48 @@ SYSCALL_DEFINE3(arc_usr_cmpxchg, int *, uaddr, int, expected, int, new)
> > >    	/* Z indicates to userspace if operation succeded */
> > >    	regs->status32 &= ~STATUS_Z_MASK;
> > >    
> > > -	if (!access_ok(VERIFY_WRITE, uaddr, sizeof(int)))
> > > -		return -EFAULT;
> > > +	ret = access_ok(VERIFY_WRITE, uaddr, sizeof(*uaddr));
> > > +	if (!ret)
> > > +		goto fail;
> > >    
> > > +again:
> > >    	preempt_disable();
> > >    
> > > -	if (__get_user(uval, uaddr))
> > > -		goto done;
> > > -
> > > -	if (uval == expected) {
> > > -		if (!__put_user(new, uaddr))
> > > +	ret = __get_user(val, uaddr);
> > > +	if (ret == -EFAULT) {
> >
> > Lets see if this warrants adding complexity ! This implies that TLB entry with
> > Read permissions didn't exist for reading the var and page fault handler could not
> > wire up even a zero page due to preempt_disable, meaning it was something not
> > touched by userspace already - sort of uninitialized variable in user code.
> Ok I completely missed the fact that fast path TLB miss handler is being
> executed even if we have preemption disabled. So given the mapping exist
> we do not need to retry with enabled preemption.
>
> Still maybe I'm a bit paranoid here but IMHO it's good to be ready for a corner-case
> when the pointer is completely bogus and there's no mapping for him.
> I understand that today we only expect this syscall to be used from libc's
> internals but as long as syscall exists nobody stops anybody from using it
> directly without libc. So maybe instead of doing get_user_pages_fast() just
> send a SIGSEGV to the process? At least user will realize there's some problem
> at earlier stage.

if the pointer is bogus, we currently return -EFAULT, is that not enough ! I'm 
fine if u want to change that to segv.

> > Otherwise it is extremely unlikely to start with a TLB entry with Read
> > permissions, followed by syscall Trap only to find the entry missing, unless a
> > global TLB flush came from other cores, right in the middle. But this syscall is
> > not guaranteed to work with SMP anyways, so lets ignore any SMP misdoings here.
> Well but that's exactly the situation I was debugging: we start from data from read-only
> page and on attempt to write back modified value COW machinery gets involved.

No exactly your situation. In your case the TLB entry *did* exist with Read 
permission. What I was pointing to is that case where it woudl vanish between user 
reading the backing page and making a syscall !

> > Now in case it was *an* uninitialized var, do we have to guarantee any well
> > defined semantics for the kernel emulation of cmpxchg ? IMO it should be fine to
> > return 0 or -EFAULT etc. Infact -EFAULT is better as it will force a retry loop on
> > user side, given the typical cmpxchg usage pattern.
> The problem is libc only expects to get a value read from memory.
> And in theory expected value might be -14 which is basically -EFAULT.
> I'm not talking about 0 at all because in some cases that's exactly what
> user-space expects.
>
> So if we read unexpected value then we'll just return it without even attempting
> to write.
>
> If we read expected data but fail to write then we'll send a SIGSEGV and
> return whatever... let it be -EFAULT - anyways the app will be killed on exit from
> this syscall.

I'm not sure what you mean here. I'm fine with adding segv kill semantics, but 
don't think complexity for get_user is worth it !

-Vineet