[PULL] BUILD_BUG_ON improvements

From: Rusty Russell
Date: Thu Dec 17 2009 - 21:05:27 EST

The following changes since commit b8a7f3cd7e8212e5c572178ff3b5a514861036a5:
Linus Torvalds (1):
Merge branch 'master' of git://git.kernel.org/.../viro/vfs-2.6

are available in the git repository at:

ssh://master.kernel.org/pub/scm/linux/kernel/git/rusty/linux-2.6-for-linus.git BUILD_BUG_ON

Rusty Russell (3):
BUILD_BUG_ON: make it handle more cases
Remove MAYBE_BUILD_BUG_ON
kernel.h: move BUILD_BUG_ON et al inside __KERNEL__

include/linux/gfp.h | 2 +-
include/linux/kernel.h | 80 +++++++++++++++++++++++++----------------
include/linux/kmemcheck.h | 2 +-
include/linux/virtio_config.h | 5 ++-
4 files changed, 55 insertions(+), 34 deletions(-)

commit a7a9f439b8380b930a47a0b17b217f87458844f6
Author: Rusty Russell <rusty@xxxxxxxxxxxxxxx>
Date: Fri Dec 18 12:32:55 2009 -0600

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 <rusty@xxxxxxxxxxxxxxx>
Acked-by: Hollis Blanchard <hollisb@xxxxxxxxxx>

include/linux/kernel.h | 33 +++++++++++++++++++++++++++------
1 files changed, 27 insertions(+), 6 deletions(-)

commit 7f6d9e683ab862e7b102d65766c131bc6136e6ca
Author: Rusty Russell <rusty@xxxxxxxxxxxxxxx>
Date: Fri Dec 18 12:32:57 2009 -0600

Remove MAYBE_BUILD_BUG_ON

Now BUILD_BUG_ON() can handle optimizable constants, we don't need
MAYBE_BUILD_BUG_ON any more.

Signed-off-by: Rusty Russell <rusty@xxxxxxxxxxxxxxx>

include/linux/gfp.h | 2 +-
include/linux/kernel.h | 1 -
include/linux/kmemcheck.h | 2 +-
include/linux/virtio_config.h | 5 ++++-
4 files changed, 6 insertions(+), 4 deletions(-)

commit 10b483dd7905f59062f7aa2986a1d4c7f5129dc3
Author: Rusty Russell <rusty@xxxxxxxxxxxxxxx>
Date: Fri Dec 18 12:32:58 2009 -0600

kernel.h: move BUILD_BUG_ON et al inside __KERNEL__

Recent warning caused by change in BUILD_BUG_ON:

usr/include/linux/kernel.h:53: userspace cannot call function or variable defined in the kernel

Macros no longer accessible to userspace:

BUILD_BUG_ON_ZERO, BUILD_BUG_ON_NULL, BUILD_BUG_ON,
__FUNCTION__, NUMA_BUILD, REBUILD_DUE_TO_FTRACE_MCOUNT_RECORD.

(The last two were in #ifdef CONFIG anyway, so already useless).

Also, avoid silly re-test of __KERNEL__ immediately below.

Signed-off-by: Rusty Russell <rusty@xxxxxxxxxxxxxxx>

include/linux/kernel.h | 50 +++++++++++++++++++++++------------------------
1 files changed, 24 insertions(+), 26 deletions(-)


diff --git a/include/linux/gfp.h b/include/linux/gfp.h
index 557bdad..f53e9b8 100644
--- a/include/linux/gfp.h
+++ b/include/linux/gfp.h
@@ -220,7 +220,7 @@ static inline enum zone_type gfp_zone(gfp_t flags)
((1 << ZONES_SHIFT) - 1);

if (__builtin_constant_p(bit))
- MAYBE_BUILD_BUG_ON((GFP_ZONE_BAD >> bit) & 1);
+ BUILD_BUG_ON((GFP_ZONE_BAD >> bit) & 1);
else {
#ifdef CONFIG_DEBUG_VM
BUG_ON((GFP_ZONE_BAD >> bit) & 1);
diff --git a/include/linux/kernel.h b/include/linux/kernel.h
index 3fc9f5a..57ffaa0 100644
--- a/include/linux/kernel.h
+++ b/include/linux/kernel.h
@@ -702,13 +702,59 @@ static inline void ftrace_dump(void) { }
struct sysinfo;
extern int do_sysinfo(struct sysinfo *info);

-#endif /* __KERNEL__ */
+/* 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
+ aren't permitted). */
+#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
+
+/* Trap pasters of __FUNCTION__ at compile-time */
+#define __FUNCTION__ (__func__)
+
+/* This helps us to avoid #ifdef CONFIG_NUMA */
+#ifdef CONFIG_NUMA
+#define NUMA_BUILD 1
+#else
+#define NUMA_BUILD 0
+#endif
+
+/* Rebuild everything on CONFIG_FTRACE_MCOUNT_RECORD */
+#ifdef CONFIG_FTRACE_MCOUNT_RECORD
+# define REBUILD_DUE_TO_FTRACE_MCOUNT_RECORD
+#endif

+#else /* __KERNEL__ */
#ifndef __EXPORTED_HEADERS__
-#ifndef __KERNEL__
#warning Attempt to use kernel headers from user space, see http://kernelnewbies.org/KernelHeaders
-#endif /* __KERNEL__ */
#endif /* __EXPORTED_HEADERS__ */
+#endif /* !__KERNEL__ */

#define SI_LOAD_SHIFT 16
struct sysinfo {
@@ -728,32 +774,4 @@ 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
- aren't permitted). */
-#define BUILD_BUG_ON_ZERO(e) (sizeof(struct { int:-!!(e); }))
-#define BUILD_BUG_ON_NULL(e) ((void *)sizeof(struct { int:-!!(e); }))
-
-/* Trap pasters of __FUNCTION__ at compile-time */
-#define __FUNCTION__ (__func__)
-
-/* This helps us to avoid #ifdef CONFIG_NUMA */
-#ifdef CONFIG_NUMA
-#define NUMA_BUILD 1
-#else
-#define NUMA_BUILD 0
-#endif
-
-/* Rebuild everything on CONFIG_FTRACE_MCOUNT_RECORD */
-#ifdef CONFIG_FTRACE_MCOUNT_RECORD
-# define REBUILD_DUE_TO_FTRACE_MCOUNT_RECORD
-#endif
-
#endif
diff --git a/include/linux/kmemcheck.h b/include/linux/kmemcheck.h
index e880d4c..136cdcd 100644
--- a/include/linux/kmemcheck.h
+++ b/include/linux/kmemcheck.h
@@ -152,7 +152,7 @@ static inline bool kmemcheck_is_obj_initialized(unsigned long addr, size_t size)
\
_n = (long) &((ptr)->name##_end) \
- (long) &((ptr)->name##_begin); \
- MAYBE_BUILD_BUG_ON(_n < 0); \
+ BUILD_BUG_ON(_n < 0); \
\
kmemcheck_mark_initialized(&((ptr)->name##_begin), _n); \
} while (0)
diff --git a/include/linux/virtio_config.h b/include/linux/virtio_config.h
index 0093dd7..800617b 100644
--- a/include/linux/virtio_config.h
+++ b/include/linux/virtio_config.h
@@ -109,7 +109,10 @@ static inline bool virtio_has_feature(const struct virtio_device *vdev,
unsigned int fbit)
{
/* Did you forget to fix assumptions on max features? */
- MAYBE_BUILD_BUG_ON(fbit >= 32);
+ if (__builtin_constant_p(fbit))
+ BUILD_BUG_ON(fbit >= 32);
+ else
+ BUG_ON(fbit >= 32);

if (fbit < VIRTIO_TRANSPORT_F_START)
virtio_check_driver_offered_feature(vdev, fbit);
--
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