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From: Arnd Bergmann <arnd@arndb.de>
To: Kyle McMartin <kyle@infradead.org>
Subject: Re: [PATCH v3 0/3] preadv & pwritev syscalls.
Date: Tue, 16 Dec 2008 22:03:05 +0100
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On Tuesday 16 December 2008, Kyle McMartin wrote:
> > I'd prefer to have the ordering coded explicitly instead, like this:
> > 
> > asmlinkage int compat_sys_pwritev(unsigned long fd,
> >        const struct compat_iovec __user *vec, unsigned long vlen,
> >        unsigned pos_low, unsigned pos_high)
> > {
> >       loff_t pos = pos_low | (loff_t)pos_high << 32;
> >         [ ... ]
> > 
> 
> Sadly this isn't possible without a wrapper unless you can guarantee
> it was passed low high instead of high low. Unless you munge it in
> userspace, you can't.
> 

Exactly, we have to munge it in userspace anyway because of the
argument ordering requirement we get if we pass it as a single u64.
I vote for the explicit split as well, this will give us a straightforward
wrapper in user space, unlike the swapped arguments that are just asking
for trouble, and will make it easy to have a common compat_sys_pwritev
function.

	Arnd <><