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SubjectRe: [PATCH] delayacct: align to 8 byte boundary on 64-bit systems
* Andrew Morton <akpm@linux-foundation.org> [2010-02-12 10:19:57]:

> On Fri, 12 Feb 2010 11:48:27 -0500
> Jeff Mahoney <jeffm@suse.com> wrote:
>
> > 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 |
>
> So we put another four zero bytes in here and add four to the "PID header".
>

Do you know if these breaks existing applications?

> > | +------------------------------+
> > | | 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.
> >

Could you define OK above? Does the application work without
recompiling? Have you checked for endianness issues?

> > 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 };

Shouldn't this be endianness dependent?

> > + 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)
>
> So any code which assumes that the pid/tgid field is four bytes long
> will break. Code which takes that length from the netlink message
> header will work OK.
>

I think we assume that PID/TGID is 32 bits long, we use the length to
extract the rest of the message. We cast NLA_DATA to int* in
getdelays.c.

> 32-bit architectures are unaltered.
>
> Seems safe enough. We'd be safer still if we didn't do this on 64-bit
> architectures which don't need it. ie: x86_64. But if we do that we
> add a risk that people will develop shoddy code which works on x86_64
> and doesn't work on ia64.
>

May be, this deserves an ABI and version bump in taskstats. I'll test
this patch soon.

--
Three Cheers,
Balbir


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