ipc,sysv: make return -EIDRM when racing with RMID consistent

The ipc_lock helper is used by all forms of sysv ipc to acquire the ipc
object's spinlock.  Upon error (bogus identifier), we always return
-EINVAL, whether the problem be in the idr path or because we raced with a
task performing RMID.  For the later, however, all ipc related manpages,
state the that for:

       EIDRM  <ID> points to a removed identifier.

And return:

       EINVAL Invalid <ID> value, or unaligned, etc.

Which (EINVAL) should only return once the ipc resource is deleted.  For
all types of ipc this is done immediately upon a RMID command.  However,
shared memory behaves slightly different as it can merely mark a segment
for deletion, and delay the actual freeing until there are no more active
consumers.  Per shmctl(IPC_RMID) manpage:

""
Mark  the  segment to be destroyed.  The segment will only actually
be destroyed after the last process detaches it (i.e., when the
shm_nattch member of the associated structure shmid_ds is zero).
""

Unlike ipc_lock, paths that behave "correctly", at least per the manpage,
involve controlling the ipc resource via *ctl(), doing the exact same
validity check as ipc_lock after right acquiring the spinlock:

	if (!ipc_valid_object()) {
		err = -EIDRM;
		goto out_unlock;
	}

Thus make ipc_lock consistent with the rest of ipc code and return -EIDRM
in ipc_lock when !ipc_valid_object().

Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
Cc: Manfred Spraul <manfred@colorfullife.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 8 insertions, 5 deletions

diff --git a/ipc/util.c b/ipc/util.c
index 3fdfabfdd9c3..b99038699fee 100644
--- a/ipc/util.c
+++ b/ipc/util.c
@@ -583,19 +583,22 @@ struct kern_ipc_perm *ipc_lock(struct ipc_ids *ids, int id)
         rcu_read_lock();
         out = ipc_obtain_object_idr(ids, id);
         if (IS_ERR(out))
-                goto err1;
+                goto err;
 
         spin_lock(&out->lock);
 
-        /* ipc_rmid() may have already freed the ID while ipc_lock
-         * was spinning: here verify that the structure is still valid
+        /*
+         * ipc_rmid() may have already freed the ID while ipc_lock()
+         * was spinning: here verify that the structure is still valid.
+         * Upon races with RMID, return -EIDRM, thus indicating that
+         * the ID points to a removed identifier.
          */
         if (ipc_valid_object(out))
                 return out;
 
         spin_unlock(&out->lock);
-        out = ERR_PTR(-EINVAL);
-err1:
+        out = ERR_PTR(-EIDRM);
+err:
         rcu_read_unlock();
         return out;
 }