ipc: convert invalid scenarios to use WARN_ON

Considering Linus' past rants about the (ab)use of BUG in the kernel, I
took a look at how we deal with such calls in ipc.  Given that any errors
or corruption in ipc code are most likely contained within the set of
processes participating in the broken mechanisms, there aren't really many
strong fatal system failure scenarios that would require a BUG call.
Also, if something is seriously wrong, ipc might not be the place for such
a BUG either.

1. For example, recently, a customer hit one of these BUG_ONs in shm
   after failing shm_lock().  A busted ID imho does not merit a BUG_ON,
   and WARN would have been better.

2. MSG_COPY functionality of posix msgrcv(2) for checkpoint/restore.
   I don't see how we can hit this anyway -- at least it should be IS_ERR.
    The 'copy' arg from do_msgrcv is always set by calling prepare_copy()
   first and foremost.  We could also probably drop this check altogether.
    Either way, it does not merit a BUG_ON.

3. No ->fault() callback for the fs getting the corresponding page --
   seems selfish to make the system unusable.

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

ipc/msgutil.c

@@ -123,7 +123,7 @@ struct msg_msg *copy_msg(struct msg_msg *src, struct msg_msg *dst)
 	size_t len = src->m_ts;
 	size_t alen;
 
-	BUG_ON(dst == NULL);
+	WARN_ON(dst == NULL);
 	if (src->m_ts > dst->m_ts)
 		return ERR_PTR(-EINVAL);
 

ipc/shm.c

@@ -159,7 +159,7 @@ static inline struct shmid_kernel *shm_lock(struct ipc_namespace *ns, int id)
 	 * We raced in the idr lookup or with shm_destroy().  Either way, the
 	 * ID is busted.
 	 */
-	BUG_ON(IS_ERR(ipcp));
+	WARN_ON(IS_ERR(ipcp));
 
 	return container_of(ipcp, struct shmid_kernel, shm_perm);
 }
@@ -393,7 +393,7 @@ static int shm_mmap(struct file *file, struct vm_area_struct *vma)
 		return ret;
 	sfd->vm_ops = vma->vm_ops;
 #ifdef CONFIG_MMU
-	BUG_ON(!sfd->vm_ops->fault);
+	WARN_ON(!sfd->vm_ops->fault);
 #endif
 	vma->vm_ops = &shm_vm_ops;
 	shm_open(vma);