[PATCH] x86/fpu, bug.h: Move CHECK_MEMBER_AT_END_OF() to a generic header and use it in generic code

[Ingo Molnar <mingo@kernel.org>]

* Ingo Molnar <mingo@kernel.org> wrote:

> 
> * Peter Zijlstra <peterz@infradead.org> wrote:
> 
> > On Thu, Jul 16, 2015 at 12:14:37PM -0700, Dave Hansen wrote:
> > > +++ b/arch/x86/kernel/fpu/init.c	2015-07-16 12:02:15.284280976 -0700
> > > @@ -136,6 +136,45 @@ static void __init fpu__init_system_gene
> > >  unsigned int xstate_size;
> > >  EXPORT_SYMBOL_GPL(xstate_size);
> > >  
> > > +#define CHECK_MEMBER_AT_END_OF(TYPE, MEMBER)	\
> > > +	BUILD_BUG_ON((sizeof(TYPE) -			\
> > > +			offsetof(TYPE, MEMBER) -	\
> > > +			sizeof(((TYPE *)0)->MEMBER)) > 	\
> > > +			0)				\
> > > +
> > > +/*
> > > + * We append the 'struct fpu' to the task_struct.
> > > + */
> > > +int __weak arch_task_struct_size(void)
> > > +{
> > > +	int task_size = sizeof(struct task_struct);
> > > +
> > > +	/*
> > > +	 * Subtract off the static size of the register state.
> > > +	 * It potentially has a bunch of padding.
> > > +	 */
> > > +	task_size -= sizeof(((struct task_struct *)0)->thread.fpu.state);
> > > +
> > > +	/*
> > > +	 * Add back the dynamically-calculated register state
> > > +	 * size.
> > > +	 */
> > > +	task_size += xstate_size;
> > > +
> > > +	/*
> > > +	 * We dynamically size 'struct fpu', so we require that
> > > +	 * it be at the end of 'thread_struct' and that
> > > +	 * 'thread_struct' be at the end of 'task_struct'.  If
> > > +	 * you hit a compile error here, check the structure to
> > > +	 * see if something got added to the end.
> > > +	 */
> > > +	CHECK_MEMBER_AT_END_OF(struct fpu, state);
> > > +	CHECK_MEMBER_AT_END_OF(struct thread_struct, fpu);
> > > +	CHECK_MEMBER_AT_END_OF(struct task_struct, thread);
> > > +
> > > +	return task_size;
> > > +}
> > 
> > Since you want these invariants true at all times, maybe put the
> > BUILD_BUG_ON() in generic code instead of x86 specific? That way people
> > poking at other archs are less likely to accidentally break your stuff.
> 
> Yeah.

The patch below implements this. Only build tested.

Thanks,

	Ingo

===============>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 arch/x86/kernel/fpu/init.c |   17 ++---------------
 include/linux/bug.h        |    4 ++++
 kernel/fork.c              |   18 ++++++++++++++++++
 3 files changed, 24 insertions(+), 15 deletions(-)

Index: tip/arch/x86/kernel/fpu/init.c
===================================================================
--- tip.orig/arch/x86/kernel/fpu/init.c
+++ tip/arch/x86/kernel/fpu/init.c
@@ -136,16 +136,10 @@ static void __init fpu__init_system_gene
 unsigned int xstate_size;
 EXPORT_SYMBOL_GPL(xstate_size);
 
-#define CHECK_MEMBER_AT_END_OF(TYPE, MEMBER)	\
-	BUILD_BUG_ON((sizeof(TYPE) -			\
-			offsetof(TYPE, MEMBER) -	\
-			sizeof(((TYPE *)0)->MEMBER)) > 	\
-			0)				\
-
 /*
  * We append the 'struct fpu' to the task_struct.
  */
-int __weak arch_task_struct_size(void)
+int arch_task_struct_size(void)
 {
 	int task_size = sizeof(struct task_struct);
 
@@ -161,16 +155,9 @@ int __weak arch_task_struct_size(void)
 	 */
 	task_size += xstate_size;
 
-	/*
-	 * We dynamically size 'struct fpu', so we require that
-	 * it be at the end of 'thread_struct' and that
-	 * 'thread_struct' be at the end of 'task_struct'.  If
-	 * you hit a compile error here, check the structure to
-	 * see if something got added to the end.
-	 */
+	/* Build time FPU structure layout debug checks: */
 	CHECK_MEMBER_AT_END_OF(struct fpu, state);
 	CHECK_MEMBER_AT_END_OF(struct thread_struct, fpu);
-	CHECK_MEMBER_AT_END_OF(struct task_struct, thread);
 
 	return task_size;
 }
Index: tip/include/linux/bug.h
===================================================================
--- tip.orig/include/linux/bug.h
+++ tip/include/linux/bug.h
@@ -85,6 +85,10 @@ struct pt_regs;
 
 #endif	/* __CHECKER__ */
 
+/* Enforce that 'MEMBER' is the last field of 'TYPE': */
+#define CHECK_MEMBER_AT_END_OF(TYPE, MEMBER) \
+	BUILD_BUG_ON(sizeof(TYPE) != offsetofend(TYPE, MEMBER))
+
 #ifdef CONFIG_GENERIC_BUG
 #include <asm-generic/bug.h>
 
Index: tip/kernel/fork.c
===================================================================
--- tip.orig/kernel/fork.c
+++ tip/kernel/fork.c
@@ -287,8 +287,26 @@ static void set_max_threads(unsigned int
 	max_threads = clamp_t(u64, threads, MIN_THREADS, MAX_THREADS);
 }
 
+#define CHECK_MEMBER_AT_END_OF(TYPE, MEMBER) \
+	BUILD_BUG_ON(sizeof(TYPE) != offsetofend(TYPE, MEMBER))
+
+/*
+ * This function can be overridden by the architecture to support dynamic sizing
+ * of the task_struct:
+ */
 int __weak arch_task_struct_size(void)
 {
+	/*
+	 * Build-time checks for structure layout constraints:
+	 *
+	 * On some architectures we dynamically size 'struct thread_struct',
+	 * so we require that it be at the end of 'task_struct'.
+	 *
+	 * If you hit a compile error here, check the structure to
+	 * see if something got added to the end.
+	 */
+	CHECK_MEMBER_AT_END_OF(struct task_struct, thread);
+
 	return sizeof(struct task_struct);
 }