linux-kernel - Re: [PATCH v4 10/10] x86: Support compiling out userspace IO (iopl and ioperm)

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Message-ID: <20141103152748.327032f8@alan.etchedpixels.co.uk>
Date:	Mon, 3 Nov 2014 15:27:48 +0000
From:	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>
To:	Josh Triplett <josh@...htriplett.org>
Cc:	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
	Kees Cook <keescook@...omium.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-kernel@...r.kernel.org,
	virtualization@...ts.linux-foundation.org, x86@...nel.org
Subject: Re: [PATCH v4 10/10] x86: Support compiling out userspace IO (iopl
 and ioperm)

> > This isn't unreasonable but there are drivers with userspace helpers that
> > use iopl/ioperm type functionality where you should be doing a SELECT of
> > X86_IOPORT. The one that comes to mind is the uvesa driver. From a quick
> > scan it may these days be the only mainstream one that needs the select
> > adding.
> 
> Should kernel drivers really express dependencies that only their
> (current instances of) corresponding userspace components need?
> Something seems wrong about that.

uvesafb will always need X86_IOPORT. It's kind of implicit in the design.
I'm not suggesting that fbdev should select X86_IOPORT but in the uvesafb
case at least it's completely useless to have one and not the other.

> IO_BITMAP_LONGS already gets defined to (0/sizeof(long)).  And as far as
> I can tell, that would only work for init_tss_io, not anything else.
> Even then, that would only work with a zero-size array left around in
> tss_struct, which doesn't seem appropriate.  The remaining ifdefs wrap
> code that GCC could not constant-fold away, and making that code
> constant-foldable seems significantly more invasive than the ifdefs.

OK
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