delayacct: align to 8 byte boundary on 64-bit systems

prepare_reply() sets up an skb for the response.  The payload contains:

 +--------------------------------+
 | genlmsghdr - 4 bytes           |
 +--------------------------------+
 | NLA header - 4 bytes           | /* Aggregate header */
 +-+------------------------------+
 | | NLA header - 4 bytes         | /* PID header */
 | +------------------------------+
 | | pid/tgid   - 4 bytes         |
 | +------------------------------+
 | | NLA header - 4 bytes         | /* stats header */
 | + -----------------------------+ <- oops. aligned on 4 byte boundary
 | | struct taskstats - 328 bytes |
 +-+------------------------------+

The start of the taskstats struct must be 8 byte aligned on IA64 (and
other systems with 8 byte alignment rules for 64-bit types) or runtime
alignment warnings will be issued.

This patch pads the pid/tgid field out to sizeof(long), which forces the
alignment of taskstats.  The getdelays userspace code is ok with this
since it assumes 32-bit pid/tgid and then honors that header's length
field.

An array is used to avoid exposing kernel memory contents to userspace in
the response.
Signed-off-by: NJeff Mahoney <jeffm@suse.com>
Cc: Balbir Singh <balbir@in.ibm.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

kernel/taskstats.c

@@ -360,6 +360,12 @@ static struct taskstats *mk_reply(struct sk_buff *skb, int type, u32 pid)
 	struct nlattr *na, *ret;
 	int aggr;
 
+	/* If we don't pad, we end up with alignment on a 4 byte boundary.
+	 * This causes lots of runtime warnings on systems requiring 8 byte
+	 * alignment */
+	u32 pids[2] = { pid, 0 };
+	int pid_size = ALIGN(sizeof(pid), sizeof(long));
+
 	aggr = (type == TASKSTATS_TYPE_PID)
 			? TASKSTATS_TYPE_AGGR_PID
 			: TASKSTATS_TYPE_AGGR_TGID;
@@ -367,7 +373,7 @@ static struct taskstats *mk_reply(struct sk_buff *skb, int type, u32 pid)
 	na = nla_nest_start(skb, aggr);
 	if (!na)
 		goto err;
-	if (nla_put(skb, type, sizeof(pid), &pid) < 0)
+	if (nla_put(skb, type, pid_size, pids) < 0)
 		goto err;
 	ret = nla_reserve(skb, TASKSTATS_TYPE_STATS, sizeof(struct taskstats));
 	if (!ret)