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Date: Sun, 11 Feb 2007 22:47:34 +0100
From: Heiko Carstens <heiko.carstens@de.ibm.com>
To: Davide Libenzi <davidel@xmailserver.org>
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 ...
Message-ID: <20070211214734.GB8631@osiris.ibm.com>
References: <Pine.LNX.4.64.0702111155180.24568@alien.or.mcafeemobile.com>
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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.

But then again I thought most 32 bit architectures would add a 4 byte
pad between events and data, no?

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))

?