linux-kernel - Re: [patch] (2nd try) add epoll compat code to kernel/compat.c ...

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Date:	Mon, 12 Feb 2007 23:26:42 -0800 (PST)
From:	Davide Libenzi <davidel@...ilserver.org>
To:	Stephen Rothwell <sfr@...b.auug.org.au>
cc:	Andrew Morton <akpm@...ux-foundation.org>,
	David Woodhouse <dwmw2@...radead.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-arch@...r.kernel.org
Subject: Re: [patch] (2nd try)  add epoll compat code to kernel/compat.c ...

On Tue, 13 Feb 2007, Stephen Rothwell wrote:

> A better way here might be to have each 64 bit architecture define
> compat_epoll_event in its asm/compat.h and then you can just use:
> 
> 	if (copy_from_user(&user, event, sizeof(user)))
> 		return -EFAULT;
> 	kernel = compat_alloc_user_space(sizeof(struct epoll_event));
> 	err |= __put_user(user.events, &kernel->events);
> 	err |= __put_user(user.data, &kernel->data);
> 
> And you shouldn't need the compat routine if
> offsetof(struct compat_epoll_event, data) == offsetof(struct epoll_event, data).

That is *definitely* better, because at that point you can make them also
define a NEED_COMPAT_EPOLL_{CTL,WAIT}, and that code can be excluded 
altogether if not needed. I simply wanted to reduce work for arch 
maintainers, but I'm all for something like the above.



> OK, I have thought about this some more and I *think* the only
> architecture that needs compat_sys_epoll_ctl or compat_sys_epoll_wait is
> ia64 where the 64 bit version of struct epoll_event is different from the
> 32 bit version.  On x86_64, the struct is explictly packed (so it is the
> same as the 32 bit version) and on all the other 64 bit architectures the
> alignment of the u64 is the same as the equivalent 32 bit version.
> 
> Since ia64 already has its own version of these two, we only have to
> worry about epoll_pwait and then the struct epoll_event is only a problem
> for ia64.
> 
> Am I right?  (I have cc'd linux-arch for guidance.)

ARM-OABI also defines them, dunno why. Rmk?



- Davide


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