linux-kernel - Re: [PATCH] debug: fix BUILD_BUG_ON() for non-constant expressions

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Message-ID: <20080818075459.GH30694@elte.hu>
Date:	Mon, 18 Aug 2008 09:54:59 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Rusty Russell <rusty@...tcorp.com.au>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Alexey Dobriyan <adobriyan@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Sam Ravnborg <sam@...nborg.org>
Subject: Re: [PATCH] debug: fix BUILD_BUG_ON() for non-constant expressions


* Rusty Russell <rusty@...tcorp.com.au> wrote:

> On Monday 18 August 2008 03:33:19 Ingo Molnar wrote:
> > * Linus Torvalds <torvalds@...ux-foundation.org> wrote:
> > > Gag me now.
> > >
> > > Why not just do
> > >
> > >  #define __BBO(c)                sizeof(const char[1 - 2*!!(c)])
> > >  #define __BBONC(c)              __BBO(!__builtin_constant_p(c))
> > >  #define BUILD_BUG_ON_ZERO(c)    (__BBO(c) - __BBONC(c))
> > >  #define BUILD_BUG_ON(c)         (void)BUILD_BUG_ON_ZERO(c)
> > >
> > > and be done with it?
> >
> > yeah, i first tried a few variants of that (compile-time warnings are
> > much better than link time warnings), but none worked when i tested
> > various failure modes.
> 
> Hey, I thought I was the "undisputed ruler of Ugly-land".
> 
> How about this instead:
> 
> #define BUILD_BUG_ON(condition)						\
> do {									\
> 	static struct { char arr[1 - 2*!!(condition)]; } x __maybe_unused;	\
> } while(0)

hm, have you tried it and do we get a severe enough link error about 
that? If the macro gets ignored by the compiler that's really a hard 
error - such things are essential safeguards of kernel sanity:

	/*
	 * Build-time sanity checks on the kernel image and module
	 * area mappings. (these are purely build-time and produce no code)
	 */
	BUILD_BUG_ON(MODULES_VADDR < KERNEL_IMAGE_START);
	BUILD_BUG_ON(MODULES_VADDR-KERNEL_IMAGE_START < KERNEL_IMAGE_SIZE);
	BUILD_BUG_ON(MODULES_LEN + KERNEL_IMAGE_SIZE > 2*PUD_SIZE);
	BUILD_BUG_ON((KERNEL_IMAGE_START & ~PMD_MASK) != 0);
	BUILD_BUG_ON((MODULES_VADDR & ~PMD_MASK) != 0);
	BUILD_BUG_ON(!(MODULES_VADDR > __START_KERNEL));
	BUILD_BUG_ON(!(((MODULES_END - 1) & PGDIR_MASK) ==
				(__START_KERNEL & PGDIR_MASK)));
	BUILD_BUG_ON(__fix_to_virt(__end_of_fixed_addresses) <= MODULES_END);

( and propagating them into runtime failures not only increases bloat,
  it also makes failures harder to debug. These checks 'run' _early_. )

Link time warnings are easy enough to miss.

So unless there's a better way of doing it all at compile time (i'd 
really prefer that!) i'd prefer the link time error about botched 
BUILD_BUG_ON() conditions - as my commits introduce.

	Ingo
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