[PATCH] Fix BUILD_BUG_ON in fs/compat_ioctl.c to build with gcc 4.5 snapshot
Andi Kleen
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: jbeu...@novell.com
Cc: ru...@rustcorp.com.au
Cc: rgue...@suse.de
Signed-off-by: Andi Kleen <a...@linux.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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Rusty Russell
> 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.
We have a proper fix for BUILD_BUG_ON which I sent Linus a pull req for,
but I think it was too late:
git://git.kernel.org/pub/scm/linux/kernel/git/rusty/linux-2.6-for-linus.git BUILD_BUG_ON
It's not really high priority, but it'd be nice to fix this properly.
Thanks,
Rusty.
Andi Kleen
> On Mon, 21 Dec 2009 03:23:15 am Andi Kleen wrote:
> > 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.
>
> We have a proper fix for BUILD_BUG_ON which I sent Linus a pull req for,
> but I think it was too late:
>
> git://git.kernel.org/pub/scm/linux/kernel/git/rusty/linux-2.6-for-linus.git BUILD_BUG_ON
>
> It's not really high priority, but it'd be nice to fix this properly.
AFAIK the code I fixed was not correct in the first place because
"const int xxx" is not a constant expression in the standards sense.
So it's orthogonal to whatever changes you come up.
Building on gcc 4.5 is at least medium priority I would say.
-Andi
--
a...@linux.intel.com -- Speaking for myself only.
Arnd Bergmann
> ===================================================================
> --- 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 */
Acked-by: Arnd Bergmann <ar...@arndb.de>
Rusty Russell
> On Mon, Dec 21, 2009 at 09:02:39PM +1030, Rusty Russell wrote:
> > On Mon, 21 Dec 2009 03:23:15 am Andi Kleen wrote:
> > > 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.
> >
> > We have a proper fix for BUILD_BUG_ON which I sent Linus a pull req for,
> > but I think it was too late:
> >
> > git://git.kernel.org/pub/scm/linux/kernel/git/rusty/linux-2.6-for-linus.git BUILD_BUG_ON
> >
> > It's not really high priority, but it'd be nice to fix this properly.
>
> AFAIK the code I fixed was not correct in the first place because
> "const int xxx" is not a constant expression in the standards sense.
>
> So it's orthogonal to whatever changes you come up.
No, it's solving the same problem, generically. There are numerous places
where we want to use BUILD_BUG_ON() but we don't have a constant expression.
Look for MAYBE_BUILD_BUG_ON().
Subject: BUILD_BUG_ON: make it handle more cases
BUILD_BUG_ON used to use the optimizer to do code elimination or fail
at link time; it was changed to first the size of a negative array (a
nicer compile time error), then (in
8c87df457cb58fe75b9b893007917cf8095660a0) to a bitfield.
bitfields: needs a literal constant at parse time, and can't be put under
"if (__builtin_constant_p(x))" for example.
negative array: can handle anything, but if the compiler can't tell it's
a constant, silently has no effect.
link time: breaks link if the compiler can't determine the value, but the
linker output is not usually as informative as a compiler error.
If we use the negative-array-size method *and* the link time trick,
we get the ability to use BUILD_BUG_ON() under __builtin_constant_p()
branches, and maximal ability for the compiler to detect errors at
build time.
Signed-off-by: Rusty Russell <ru...@rustcorp.com.au>
Acked-by: Hollis Blanchard <hol...@us.ibm.com>
---
include/linux/kernel.h | 33 +++++++++++++++++++++++++++------
1 file changed, 27 insertions(+), 6 deletions(-)
diff --git a/include/linux/kernel.h b/include/linux/kernel.h
--- a/include/linux/kernel.h
+++ b/include/linux/kernel.h
@@ -683,12 +683,6 @@ struct sysinfo {
char _f[20-2*sizeof(long)-sizeof(int)]; /* Padding: libc5 uses this.. */
};
-/* Force a compilation error if condition is true */
-#define BUILD_BUG_ON(condition) ((void)BUILD_BUG_ON_ZERO(condition))
-
-/* Force a compilation error if condition is constant and true */
-#define MAYBE_BUILD_BUG_ON(cond) ((void)sizeof(char[1 - 2 * !!(cond)]))
-
/* Force a compilation error if condition is true, but also produce a
result (of value 0 and type size_t), so the expression can be used
e.g. in a structure initializer (or where-ever else comma expressions
@@ -696,6 +690,33 @@ struct sysinfo {
#define BUILD_BUG_ON_ZERO(e) (sizeof(struct { int:-!!(e); }))
#define BUILD_BUG_ON_NULL(e) ((void *)sizeof(struct { int:-!!(e); }))
+/**
+ * BUILD_BUG_ON - break compile if a condition is true.
+ * @cond: the condition which the compiler should know is false.
+ *
+ * If you have some code which relies on certain constants being equal, or
+ * other compile-time-evaluated condition, you should use BUILD_BUG_ON to
+ * detect if someone changes it.
+ *
+ * The implementation uses gcc's reluctance to create a negative array, but
+ * gcc (as of 4.4) only emits that error for obvious cases (eg. not arguments
+ * to inline functions). So as a fallback we use the optimizer; if it can't
+ * prove the condition is false, it will cause a link error on the undefined
+ * "__build_bug_on_failed". This error message can be harder to track down
+ * though, hence the two different methods.
+ */
+#ifndef __OPTIMIZE__
+#define BUILD_BUG_ON(condition) ((void)sizeof(char[1 - 2*!!(condition)]))
+#else
+extern int __build_bug_on_failed;
+#define BUILD_BUG_ON(condition) \
+ do { \
+ ((void)sizeof(char[1 - 2*!!(condition)])); \
+ if (condition) __build_bug_on_failed = 1; \
+ } while(0)
+#endif
+#define MAYBE_BUILD_BUG_ON(condition) BUILD_BUG_ON(condition)
+
/* Trap pasters of __FUNCTION__ at compile-time */
#define __FUNCTION__ (__func__)
Andi Kleen
> where we want to use BUILD_BUG_ON() but we don't have a constant expression.
> Look for MAYBE_BUILD_BUG_ON().
Ok so you're saying that should be just a MAYBE_BUILD_BUG_ON()?
>
> Subject: BUILD_BUG_ON: make it handle more cases
>
> BUILD_BUG_ON used to use the optimizer to do code elimination or fail
> at link time; it was changed to first the size of a negative array (a
> nicer compile time error), then (in
> 8c87df457cb58fe75b9b893007917cf8095660a0) to a bitfield.
>
> bitfields: needs a literal constant at parse time, and can't be put under
> "if (__builtin_constant_p(x))" for example.
> negative array: can handle anything, but if the compiler can't tell it's
> a constant, silently has no effect.
> link time: breaks link if the compiler can't determine the value, but the
> linker output is not usually as informative as a compiler error.
>
> If we use the negative-array-size method *and* the link time trick,
> we get the ability to use BUILD_BUG_ON() under __builtin_constant_p()
> branches, and maximal ability for the compiler to detect errors at
> build time.
Ok maybe that works, but we need a fix for 2.6.33 too.
-Andi