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

Jeff Mahoney
2010-02-12T16:48:33Z

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@suse.com>
---
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