Messages in this thread Patch in this message | | | Date | Wed, 18 Dec 2013 15:34:43 -0200 | From | Rafael Aquini <> | Subject | Re: [PATCH v2] ipc: introduce ipc_valid_object() helper to sort out IPC_RMID races |
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On Wed, Dec 18, 2013 at 07:46:27AM -0800, Davidlohr Bueso wrote: > On Wed, 2013-12-18 at 10:51 -0200, Rafael Aquini wrote: > > On Wed, Dec 18, 2013 at 01:11:29PM +0100, Manfred Spraul wrote: > > > On 12/18/2013 12:28 AM, Rafael Aquini wrote: > > > >After the locking semantics for the SysV IPC API got improved, a couple of > > > >IPC_RMID race windows were opened because we ended up dropping the > > > >'kern_ipc_perm.deleted' check performed way down in ipc_lock(). > > > >The spotted races got sorted out by re-introducing the old test within > > > >the racy critical sections. > > > > > > > >This patch introduces ipc_valid_object() to consolidate the way we cope with > > > >IPC_RMID races by using the same abstraction across the API implementation. > > > > > > > >Signed-off-by: Rafael Aquini <aquini@redhat.com> > > > >Acked-by: Rik van Riel <riel@redhat.com> > > > >Acked-by: Greg Thelen <gthelen@google.com> > > > >--- > > > >Changelog: > > > >* v2: > > > > - drop assert_spin_locked() from ipc_valid_object() for less overhead > > > a) sysv ipc is lockless whereever possible, without writing to any > > > shared cachelines. > > > Therefore my first reaction was: No, please leave the assert in. It > > > will help us to catch bugs. > > > > > > b) then I noticed: the assert would be a bug, the comment in front > > > of ipc_valid_object() that the caller must hold _perm.lock is wrong: > > > >@@ -1846,7 +1846,7 @@ SYSCALL_DEFINE4(semtimedop, int, semid, struct sembuf __user *, tsops, > > > > error = -EIDRM; > > > > locknum = sem_lock(sma, sops, nsops); > > > >- if (sma->sem_perm.deleted) > > > >+ if (!ipc_valid_object(&sma->sem_perm)) > > > > goto out_unlock_free; > > > simple semtimedop() operation do not acquire sem_perm.lock, they > > > only acquire the per-semaphore lock and check that sem_perm.lock is > > > not held. This is sufficient to prevent races with RMID. > > > > > > Could you update the comment? > > > > The comment for ipc_valid_object() is not entirely wrong, as holding the spinlock > > is clearly necessary for all cases except for this one you pointed above. > > When I dropped the assert as Davilohr suggested, I then could have this one exception > > case (where the check can, eventually, be done lockless) converted too, but I did not include > > an exception comment at that particular checkpoint. Perhaps, that's what I should have done, or > > perhaps the best thing is to just let all that as is sits right now. > > Yeah, Manfred is entirely correct - I didn't mention that sem_lock() > tries to be fine grained about its locking, so semaphores can in fact > not take the larger ipc lock (kern perm), but just the sem->lock > instead. This means that ipc_valid_object() must be called either way > with some lock held, but that assertion is indeed incorrect, not just > redundant like I suggested before. So, I think that if you update the > comment mentioning this corner case, then it should be ok. >
Folks,
Before I re-submit the v3 with the commentary changes requested, I'm pasting here what I'm planning to amend to v2 patch: --- diff --git a/ipc/sem.c b/ipc/sem.c index ed0057a..23379b6 100644 --- a/ipc/sem.c +++ b/ipc/sem.c @@ -1846,6 +1846,14 @@ SYSCALL_DEFINE4(semtimedop, int, semid, struct sembuf __u error = -EIDRM; locknum = sem_lock(sma, sops, nsops); + /* + * We eventually might perform the following check in a lockless + * fashion here, considering ipc_valid_object() locking constraints. + * If nsops == 1 and there's no contention for sem_perm.lock, then + * only a per-semaphore lock is held and it's OK to go on the check + * below. More details on the fine grained locking scheme entangled + * here, and why it's RMID race safe on comments at sem_lock() + */ if (!ipc_valid_object(&sma->sem_perm)) goto out_unlock_free; /* diff --git a/ipc/util.h b/ipc/util.h index 071ed58..d05b708 100644 --- a/ipc/util.h +++ b/ipc/util.h @@ -190,7 +190,8 @@ static inline void ipc_unlock(struct kern_ipc_perm *perm) * where the respective ipc_ids.rwsem is not being held down. * Checks whether the ipc object is still around or if it's gone already, as * ipc_rmid() may have already freed the ID while the ipc lock was spinning. - * Needs to be called with kern_ipc_perm.lock held. + * Needs to be called with kern_ipc_perm.lock held -- exception made for one + * checkpoint case at sys_semtimedop() as noted in code commentary. */ static inline bool ipc_valid_object(struct kern_ipc_perm *perm) { --- Do we need to change somthing else? Looking forward your thoughts! -- Rafael
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