[PATCH v2] sched: fix build with GCC_PLUGIN_RANDSTRUCT

[Mike Rapoport]

From: Mike Rapoport <rppt@xxxxxxxxxxxxx>

Since the commit a148866489fb ("sched: Replace rq::wake_list")
task_struct and CSD_TYPE_TTWU objects can be on the same queue and this
requires that have layout similar enough.

This assumption is broken when CONFIG_GCC_PLUGIN_RANDSTRUCT is enabled:

  CHK     include/generated/compile.h
  CC      kernel/smp.o
In file included from arch/x86/include/asm/atomic.h:5,
                 from include/linux/atomic.h:7,
                 from include/linux/llist.h:51,
                 from include/linux/irq_work.h:5,
                 from kernel/smp.c:10:
kernel/smp.c: In function âsmp_initâ:
include/linux/compiler.h:392:38: error: call to â__compiletime_assert_157â declared with attribute error: BUILD_BUG_ON failed: offsetof(struct task_struct, wake_entry_type) - offsetof(struct task_struct, wake_entry) != offsetof(struct __call_single_data, flags) - offsetof(struct __call_single_data, llist)
  392 |  _compiletime_assert(condition, msg, __compiletime_assert_, __COUNTER__)
      |                                      ^
include/linux/compiler.h:373:4: note: in definition of macro â__compiletime_assertâ
  373 |    prefix ## suffix();    \
      |    ^~~~~~
include/linux/compiler.h:392:2: note: in expansion of macro â_compiletime_assertâ
  392 |  _compiletime_assert(condition, msg, __compiletime_assert_, __COUNTER__)
      |  ^~~~~~~~~~~~~~~~~~~
include/linux/build_bug.h:39:37: note: in expansion of macro âcompiletime_assertâ
   39 | #define BUILD_BUG_ON_MSG(cond, msg) compiletime_assert(!(cond), msg)
      |                                     ^~~~~~~~~~~~~~~~~~
include/linux/build_bug.h:50:2: note: in expansion of macro âBUILD_BUG_ON_MSGâ
   50 |  BUILD_BUG_ON_MSG(condition, "BUILD_BUG_ON failed: " #condition)
      |  ^~~~~~~~~~~~~~~~
kernel/smp.c:687:2: note: in expansion of macro âBUILD_BUG_ONâ
  687 |  BUILD_BUG_ON(offsetof(struct task_struct, wake_entry_type) - offsetof(struct task_struct, wake_entry) !=
      |  ^~~~~~~~~~~~

Wrap 'wake_entry' and 'wake_entry_type' fiels of task_struct in an
anonymous struct to keep their relative layout intact during
randomization.

Suggested-by: Steven Rostedt <rostedt@xxxxxxxxxxx>
Signed-off-by: Mike Rapoport <rppt@xxxxxxxxxxxxx>
---

v2: use anonymous struct as Steven suggested.

 include/linux/sched.h | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index b62e6aaf28f0..7e30a09df616 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -654,8 +654,15 @@ struct task_struct {
 	unsigned int			ptrace;
 
 #ifdef CONFIG_SMP
-	struct llist_node		wake_entry;
-	unsigned int			wake_entry_type;
+       /*
+        * The layout of these fields must match the layout of CSD_TYPE_TTWU
+        * so they can be on the same @call_single_queue
+        */
+	struct {
+		struct llist_node		wake_entry;
+		unsigned int			wake_entry_type;
+	};
+
 	int				on_cpu;
 #ifdef CONFIG_THREAD_INFO_IN_TASK
 	/* Current CPU: */
-- 
2.25.4