problematic rc9 futex changes.

problematic rc9 futex changes.

[Dave Jones]

c1e2f0eaf015fb: "futex: Avoid violating the 10th rule of futex" seems to
make up a few new rules to violate.

Coverity picked up these two problems in the same code:


First it or's a value with stack garbage.

_______________________________________________________________________________________________________
*** CID 1427826:  Uninitialized variables  (UNINIT)
/kernel/futex.c: 2316 in fixup_pi_state_owner()
2310     
2311     	raw_spin_lock_irq(&pi_state->pi_mutex.wait_lock);
2312     
2313     	oldowner = pi_state->owner;
2314     	/* Owner died? */
2315     	if (!pi_state->owner)
>>>     CID 1427826:  Uninitialized variables  (UNINIT)
>>>     Using uninitialized value "newtid".
2316     		newtid |= FUTEX_OWNER_DIED;
2317     
2318     	/*
2319     	 * We are here because either:
2320     	 *
2321     	 *  - we stole the lock and pi_state->owner needs updating to reflect

Then it notices that value is never read from before it's written
anyway.

*** CID 1427824:  Code maintainability issues  (UNUSED_VALUE)
/kernel/futex.c: 2316 in fixup_pi_state_owner()
2310     
2311     	raw_spin_lock_irq(&pi_state->pi_mutex.wait_lock);
2312     
2313     	oldowner = pi_state->owner;
2314     	/* Owner died? */
2315     	if (!pi_state->owner)
>>>     CID 1427824:  Code maintainability issues  (UNUSED_VALUE)
>>>     Assigning value from "newtid | 0x40000000U" to "newtid" here, but that stored value is overwritten before it can be used.
2316     		newtid |= FUTEX_OWNER_DIED;
2317     
2318     	/*
2319     	 * We are here because either:
2320     	 *
2321     	 *  - we stole the lock and pi_state->owner needs updating to reflect


(The next reference of newtid being..

2369         newtid = task_pid_vnr(newowner) | FUTEX_WAITERS;


	Dave

Re: problematic rc9 futex changes.

[Peter Zijlstra]

On Tue, Jan 23, 2018 at 12:34:46AM -0500, Dave Jones wrote:
> c1e2f0eaf015fb: "futex: Avoid violating the 10th rule of futex" seems to
> make up a few new rules to violate.
> 
> Coverity picked up these two problems in the same code:
> 

Yeah, Geert also spotted it:

  https://lkml.kernel.org/r/20180122103947.GD2228@hirez.programming.kicks-ass.net

I've been running the robustpi tests from glibc but have so far failed
to actually trigger the bug. I think I'll just write up a Changelog and
post the fix from the above link.