Messages in this thread | | | Date | Wed, 18 Dec 2013 13:11:29 +0100 | From | Manfred Spraul <> | Subject | Re: [PATCH v2] ipc: introduce ipc_valid_object() helper to sort out IPC_RMID races |
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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? [...] > @@ -1116,7 +1116,7 @@ long do_shmat(int shmid, char __user *shmaddr, int shmflg, ulong *raddr, > ipc_lock_object(&shp->shm_perm); > > /* check if shm_destroy() is tearing down shp */ > - if (shp->shm_file == NULL) { > + if (!ipc_valid_object(&shp->shm_perm)) { > ipc_unlock_object(&shp->shm_perm); > err = -EIDRM; > goto out_unlock; Please mention the change from "shm_file == NULL" to perm.deleted in the changelog. With regards to the impact of this change: No idea, I've never worked on the shm code.
-- Manfred
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