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

[Jeff Mahoney]

 prepare_reply sets up an skb for the response. If I understand it correctly,
 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: Jeff Mahoney <jeffm@xxxxxxxx>
---
 kernel/taskstats.c |    8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

--- a/kernel/taskstats.c
+++ b/kernel/taskstats.c
@@ -362,6 +362,12 @@ static struct taskstats *mk_reply(struct
 	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;
@@ -369,7 +375,7 @@ static struct taskstats *mk_reply(struct
 	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)
-- 
Jeff Mahoney
SUSE Labs
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