[PATCH v18 5/9] sched: Mark __schedule() stack frame as non-standard

From: Josh Poimboeuf
Date: Thu Feb 25 2016 - 17:57:50 EST

objtool reports the following warnings for __schedule():

objtool: kernel/sched/core.o: __schedule()+0x3c0: duplicate frame pointer save
objtool: kernel/sched/core.o: __schedule()+0x3fd: sibling call from callable instruction with changed frame pointer
objtool: kernel/sched/core.o: __schedule()+0x40a: call without frame pointer save/setup
objtool: kernel/sched/core.o: __schedule()+0x7fd: frame pointer state mismatch
objtool: kernel/sched/core.o: __schedule()+0x421: frame pointer state mismatch

Basically it's confused by two unusual attributes of the switch_to()

1. It saves prev's frame pointer to the old stack and restores next's
frame pointer from the new stack.

2. For new tasks it jumps directly to ret_from_fork.

Eventually it would probably be a good idea to clean up the
ret_from_fork hack so that new tasks are created with a valid initial
stack, as suggested by Andy:


Then __schedule() could return normally into the new code and objtool
hopefully wouldn't have a problem anymore.

In the meantime, mark its stack frame as non-standard so we can have a
baseline with no objtool warnings. The marker also serves as a reminder
that this code could be improved a bit.

Signed-off-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
kernel/sched/core.c | 2 ++
1 file changed, 2 insertions(+)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 9503d59..641043d 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -74,6 +74,7 @@
#include <linux/binfmts.h>
#include <linux/context_tracking.h>
#include <linux/compiler.h>
+#include <linux/frame.h>

#include <asm/switch_to.h>
#include <asm/tlb.h>
@@ -3288,6 +3289,7 @@ static void __sched notrace __schedule(bool preempt)

+STACK_FRAME_NON_STANDARD(__schedule); /* switch_to() */

static inline void sched_submit_work(struct task_struct *tsk)