linux-kernel - Re: [PATCH] ipc/sem.c: synchronize semop and semctl with IPC

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Date:	Tue, 01 Oct 2013 06:22:30 +0200
From:	Manfred Spraul <manfred@...orfullife.com>
To:	Davidlohr Bueso <davidlohr@...com>
CC:	Davidlohr Bueso <davidlohr.bueso@...com>,
	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mike Galbraith <efault@....de>
Subject: Re: [PATCH] ipc/sem.c: synchronize semop and semctl with IPC_RMID

Hi Davidlohr,

On 09/30/2013 07:54 PM, Davidlohr Bueso wrote:
> Hi Manfred,
>
> On Mon, 2013-09-30 at 11:13 +0200, Manfred Spraul wrote:
>> After acquiring the semlock spinlock, the operations must test that the
>> array is still valid.
>>
>> - semctl() and exit_sem() would walk stale linked lists (ugly, but should
>>    be ok: all lists are empty)
>>
>> - semtimedop() would sleep forever - and if woken up due to a signal -
>>    access memory after free.
> Yep, that was next on my list - I had a patch for semtimedop() but was
> waiting to rebase it on top of your previous changes. Anyway thanks for
> sending this.
>
>> The patch standardizes the tests for .deleted, so that all tests in one
>> function leave the function with the same approach.
>>
>> Right now, it's a mixture of "goto cleanup", some cleanup and then
>> "goto further_cleanup" and all cleanup+"return -EIDRM" - that makes the
>> review much harder.
>>
>> Davidlohr: Could you please review the patch?
>> I did some stress test, but probably I didn't hit exactly the modified
>> lines.
> This shouldn't affect performance, if that's what you mean.
All goto's must go to the correct target, free everything, unlock 
everything, do not unlock twice, ...

>   One more
> read in the critical region won't make any difference. The patch looks
> good, just one doubt below.
>
>
>> Signed-off-by: Manfred Spraul <manfred@...orfullife.com>
>> ---
>>   ipc/sem.c | 43 ++++++++++++++++++++++++++++++-------------
>>   1 file changed, 30 insertions(+), 13 deletions(-)
>>
>> diff --git a/ipc/sem.c b/ipc/sem.c
>> index 19c8b98..a2fa795 100644
>> --- a/ipc/sem.c
>> +++ b/ipc/sem.c
>> @@ -1229,6 +1229,12 @@ static int semctl_setval(struct ipc_namespace *ns, int semid, int semnum,
>>   
>>   	sem_lock(sma, NULL, -1);
>>   
>> +	if (sma->sem_perm.deleted) {
>> +		sem_unlock(sma, -1);
>> +		rcu_read_unlock();
>> +		return -EIDRM;
>> +	}
>> +
>>   	curr = &sma->sem_base[semnum];
>>   
>>   	ipc_assert_locked_object(&sma->sem_perm);
>> @@ -1285,10 +1291,8 @@ static int semctl_main(struct ipc_namespace *ns, int semid, int semnum,
>>   		sem_lock(sma, NULL, -1);
>>   		if(nsems > SEMMSL_FAST) {
>>   			if (!ipc_rcu_getref(sma)) {
>> -				sem_unlock(sma, -1);
>> -				rcu_read_unlock();
>>   				err = -EIDRM;
>> -				goto out_free;
>> +				goto out_unlock;
>>   			}
>>   			sem_unlock(sma, -1);
>>   			rcu_read_unlock();
>> @@ -1301,10 +1305,13 @@ static int semctl_main(struct ipc_namespace *ns, int semid, int semnum,
>>   			rcu_read_lock();
>>   			sem_lock_and_putref(sma);
>>   			if (sma->sem_perm.deleted) {
>> -				sem_unlock(sma, -1);
>> -				rcu_read_unlock();
>>   				err = -EIDRM;
^^^^^^^^^^^^^^^^^^ check if nsems > SEMMSL_FAST
>> -				goto out_free;
>> +				goto out_unlock;
>> +			}
>> +		} else {
>> +			if (sma->sem_perm.deleted) {
>> +				err = -EIDRM;
>> +				goto out_unlock;
>>   			}
> I'm a bit lost here. Why should we only check the existence of the sem
> if nsems <= SEMMSL_FAST? Shouldn't the same should apply either way?
It is checked in both branches:
- the check for "nsems > SEMMSL_FAST" was always there, due to the 
kmalloc, the lock is dropped.

--
     Manfred
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