[PATCH] workqueue: Fix an object aliasing bug with `work_data_bits'

[Maciej W. Rozycki]

Fix an aliasing issue causing a MIPS port build error:

In file included from include/linux/srcu.h:34:0,
                 from include/linux/notifier.h:15,
                 from ./arch/mips/include/asm/uprobes.h:9,
                 from include/linux/uprobes.h:61,
                 from include/linux/mm_types.h:13,
                 from ./arch/mips/include/asm/vdso.h:14,
                 from arch/mips/vdso/vdso.h:27,
                 from arch/mips/vdso/gettimeofday.c:11:
include/linux/workqueue.h: In function 'work_static':
include/linux/workqueue.h:186:2: error: dereferencing type-punned pointer will break strict-aliasing rules [-Werror=strict-aliasing]
  return *work_data_bits(work) & WORK_STRUCT_STATIC;
  ^
cc1: all warnings being treated as errors
make[2]: *** [arch/mips/vdso/gettimeofday.o] Error 1

with a CONFIG_DEBUG_OBJECTS_WORK configuration and GCC 5.2.0.  Use a
union to switch between pointers as per ISO C language rules.

Signed-off-by: Maciej W. Rozycki <macro@xxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx # v2.6.20+
---
 This has been probably missed with ports which do not use `-Werror' and 
consequently let warnings scroll by unnoticed.  Yet undefined behaviour 
results as the compiler is free to optimise (e.g. remove) code under the 
assumption that the two objects do not alias.

 Please apply then.  This reaches back to 2.6.20 I believe.

  Maciej

linux-work-data-bits.diff
Index: linux-sfr-shell/include/linux/workqueue.h
===================================================================
--- linux-sfr-shell.orig/include/linux/workqueue.h	2016-04-22 17:31:08.000000000 +0100
+++ linux-sfr-shell/include/linux/workqueue.h	2016-05-17 05:26:18.772193000 +0100
@@ -23,7 +23,16 @@ void delayed_work_timer_fn(unsigned long
  * The first word is the work queue pointer and the flags rolled into
  * one
  */
-#define work_data_bits(work) ((unsigned long *)(&(work)->data))
+#define work_data_bits(work)						\
+({									\
+	union {								\
+		atomic_long_t *a;					\
+		unsigned long *l;					\
+	} u;								\
+									\
+	u.a = &(work)->data;						\
+	u.l;								\
+})
 
 enum {
 	WORK_STRUCT_PENDING_BIT	= 0,	/* work item is pending execution */