Re: [PATCH] ipc/sem.c: synchronize semop and semctl with IPC_RMID

[Manfred Spraul]

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@xxxxxxxxxxxxxxxx>
> > ---
> >   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
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/