linux-kernel - [PATCH] Fix BUILD_BUG_ON in fs/compat_ioctl.c to build with gcc 4.5 snapshot

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Message-ID: <20091220165315.GA13208@basil.fritz.box>
Date:	Sun, 20 Dec 2009 17:53:15 +0100
From:	Andi Kleen <andi@...stfloor.org>
To:	torvalds@...ux-foundation.org, linux-kernel@...r.kernel.org,
	jbeulich@...ell.com, rusty@...tcorp.com.au, rguenther@...e.de
Cc:	arnd@...db.de
Subject: [PATCH] Fix BUILD_BUG_ON in fs/compat_ioctl.c to build with gcc
	4.5 snapshot

Fix BUILD_BUG_ON in fs/compat_ioctl.c to build with gcc 4.5 snapshot

The BUILD_BUG_ON in compat_ioctl_check_table() fails
with a recent gcc mainline snapshot (gcc version 4.5.0 20091219)
(GCC)), even though it works with older compilers.

 const int max = ARRAY_SIZE(ioctl_pointer) - 1;
 BUILD_BUG_ON(max >= (1 << 16));

I replaced the BUILD_BUG_ON with a old style extern reference and 
that works.

That shows that the actual expression is evaluated ok and something must be 
wrong with the BUILD_BUG_ON macro itself. Maybe Rusty's rework
will fix that (I haven't tried but it's probably worth investigating) 

I filed a bug on the compiler too 
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=42439

According to Joseph Myers the problem is that even when declared
with const "max" is not a constant expression in the C standard sense,
so the code is indeed incorrect.

Anyways with this workaround a relatively standard defconfig like 
configuration and a 64bit allyes configuration builds again with gcc 4.5

Cc: jbeulich@...ell.com
Cc: rusty@...tcorp.com.au
Cc: rguenther@...e.de

Signed-off-by: Andi Kleen <ak@...ux.intel.com>

---
 fs/compat_ioctl.c |    4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

Index: linux-2.6.33-rc1-ak/fs/compat_ioctl.c
===================================================================
--- linux-2.6.33-rc1-ak.orig/fs/compat_ioctl.c
+++ linux-2.6.33-rc1-ak/fs/compat_ioctl.c
@@ -1649,8 +1649,10 @@ static int compat_ioctl_check_table(unsi
 {
 	int i;
 	const int max = ARRAY_SIZE(ioctl_pointer) - 1;
+	extern void __ioctl_pointer_too_large(void);
 
-	BUILD_BUG_ON(max >= (1 << 16));
+	if (max >= (1 << 16))
+		__ioctl_pointer_too_large();
 
 	/* guess initial offset into table, assuming a
 	   normalized distribution */
--
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