From: Davide Libenzi <davidel@xmailserver.org>
To: Heiko Carstens <heiko.carstens@de.ibm.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
David Woodhouse <dwmw2@infradead.org>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
Martin Schwidefsky <schwidefsky@de.ibm.com>
Subject: Re: [patch] add epoll compat code to kernel/compat.c ...
Date: Sun, 11 Feb 2007 15:18:06 -0800 (PST) [thread overview]
Message-ID: <Pine.LNX.4.64.0702111504020.24568@alien.or.mcafeemobile.com> (raw)
In-Reply-To: <20070211214734.GB8631@osiris.ibm.com>
On Sun, 11 Feb 2007, Heiko Carstens wrote:
> On Sun, Feb 11, 2007 at 12:15:24PM -0800, Davide Libenzi wrote:
> >
> > Add epoll compat_ code to kernel/compat.c. IA64 and ARM-OABI are currently
> > using their own version of epoll compat_ code and they could probably wire
> > to the new common code. Patch over 2.6.20.
> > + * epoll (fs/eventpoll.c) compat bits follow ...
> > + */
> > +struct compat_epoll_event {
> > + u32 events;
> > + u32 data[2];
> > +};
> > +
> >[...]
> > +
> > + * We need the compat layer over the epoll_event structure, only if the offset
> > + * of the __u64 data member is not 4 (size of the events member that precedes the
> > + * data one).
> > + */
> > +#define EPOLL_NEED_EVENT_COMPAT() (offsetof(struct epoll_event, data) != 4)
>
> With
>
> struct epoll_event {
> __u32 events;
> __u64 data;
> };
>
> this won't work on s390. offsetof(struct epoll_event, data) is 8 on both
> 31 bit and 64 bit. So it will do the conversion and corrupt all the data.
> Actually we would only need the compat conversion for the sigset_t stuff.
Yup, that's broken not only on s390, but on every arch with alignof(u64) == 8
in 32 bits mode.
The assumption was that for cases like the above, you simply wouldn't wire
the compat_ version. That is true for epoll_wait and epoll_ctl, where the
only need for compat was the "struct epoll_event". But that's not true for
epoll_pwait, since this one needs to be wired because of the sigset_t.
On top of sigset_t, epoll_pwait may need "struct epoll_event" translation.
Now, that *really* sux because two versions of compat_epoll_pwait are
needed, once that does sigset_t translation only, and one that does
sigset_t + "struct epoll_event".
> But then again I thought most 32 bit architectures would add a 4 byte
> pad between events and data, no?
i386 does not, for example ;)
> Maybe we need some arch dependent struct compat_epoll_event and have
> something like
> #define EPOLL_NEED_EVENT_COMPAT() \
> (offsetof(struct epoll_event, data) != offsetof(struct compat_epoll_event, data))
>
> ?
No, it won't work. Unless there is (or we define) a per-arch macro that
tells us how the 32 bits mode align an u64, I'm afraid we can't do any
smart tricks and we need to have the double compat_epoll_pwait.
- Davide
prev parent reply other threads:[~2007-02-11 23:18 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2007-02-11 20:15 [patch] add epoll compat code to kernel/compat.c Davide Libenzi
2007-02-11 21:47 ` Heiko Carstens
2007-02-11 23:18 ` Davide Libenzi [this message]
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