linux-kernel - Re: [PATCH 1/4] asm-generic: uaccess: do not expand args multiple times

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Message-Id: <200906142214.40532.arnd@arndb.de>
Date:	Sun, 14 Jun 2009 22:14:39 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	Mike Frysinger <vapier@...too.org>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/4] asm-generic: uaccess: do not expand args multiple times

On Sunday 14 June 2009, Mike Frysinger wrote:
> While it's debatable whether {get,put}_user() should be called with
> arguments that have side effects, macro's should be written safely in the
> first place.  In this case, a slightly off version of put_user() ended up
> causing random userspace corruption and these things aren't trivial to
> track down.
> 
> While some of these conversions aren't strictly necessary, I think it's
> better to do all of them so as to be proactive in people accidently
> screwing it up in the future.

I've tried this and failed. This change adds an endless number of sparse
warnings in put_user and even gcc warnings in get_user. The problem
is that typeof() carries over the 'const' and '__user' modifiers, both
of which prevent you from assigning data to the new pointer that you
constructed.

I'd love to see a way to do this correctly, but this patch won't cut it.

	Arnd <><
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