linux-kernel - Re: [RFC PATCH 2/2] kernel/sched.c: VLA in middle of struct

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Date:	Tue, 12 May 2009 15:03:44 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Rusty Russell <rusty@...tcorp.com.au>
Cc:	Jeff Garzik <jeff@...zik.org>, Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Mike Travis <travis@....com>,
	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>, roland@...hat.com
Subject: Re: [RFC PATCH 2/2] kernel/sched.c: VLA in middle of struct

On Tue, May 12, 2009 at 11:04:51PM +0930, Rusty Russell wrote:
> On Mon, 11 May 2009 12:39:54 am Jeff Garzik wrote:
> > On Sun, May 10, 2009 at 06:19:40PM +0930, Rusty Russell wrote:
> > > Yeah, it's kinda nasty.  Generally, sched_group is dynamically allocated,
> > > so we just allocate sizeof(struct sched_group) + size of nr_cpu_ids bits.
> > >
> > > These ones are static, and it was easier to put this hack in than make
> > > them dynamic.  There's nothing wrong with it, until we really want
> > > NR_CPUS == bignum, or we want to get rid of NR_CPUS altogether for
> > > CONFIG_CPUMASKS_OFFSTACK (which would be very clean, but not clearly
> > > worthwhile).
> >
> > Nothing wrong with it, except
> >
> > - C99 only defines variable-length automatic arrays
> > - VLA in the middle of a struct are difficult to optimize
> > - gcc's VLA handling WILL change, as gcc docs state
> > - other compilers -- and sparse -- puke all over VLAs, making
> >   static analysis impossible for all code with this weirdism
> 
> Jeff, you seem confused.  In my copy of the standard, you'd know this is called 
> a "flexible array member"; it's not a variable length array.  The only GCC 
> specific issue I can find here is that you're not normally allowed to embed 
> structs with them in another struct (according to the gcc docs; I can't 
> actually find this clearly stated in the standard).

6.7.2.1p2.  It's a separate issue from revolting gcc extension that *do*
allow VLA-in-the-middle-of-struct.  And I mean real VLA, not flex array
member :-/

> Anyway, since [] is C99, I thought it preferable to [0] which is a gcc 
> extension.  However, if C99 is really so braindead as to disallow this fairly 
> standard trick, so I'm happy to go with the gcc extension.[1]

No.  There's a standard way to do that in C99; you can put a struct with
that thing into a union.  So correct way to force enought storage for
such an object is

union {
	struct has_flex_array_member foo;
	char [how_much_space_do_I_want];
} bar;

Unions with overlapping members are fine.  Structures are not.
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