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

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Date:	Wed, 24 Jun 2009 05:38:35 +0100
From:	Al Viro <viro@....linux.org.uk>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	Mike Frysinger <vapier@...too.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/4] asm-generic: uaccess: do not expand args multiple
	times

On Sun, Jun 14, 2009 at 10:14:39PM +0200, Arnd Bergmann wrote:
> 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.

Note that sizeof(*(ptr)) does *NOT* evaluate ptr, unless we are dealing
with variably-modified type.  The same goes for typeof.  And chk_user_ptr()
expands to (void)0 during the build.  So I don't believe that existing variant
is incorrect - we do not evaluate the argument twice.
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