[tip:core/objtool] sched: Mark __schedule() stack frame as non-standard

From: tip-bot for Josh Poimboeuf
Date: Mon Feb 29 2016 - 06:01:25 EST

Commit-ID: 8e05e96ac949c80704d0a38420bf60dcf18c938f
Gitweb: http://git.kernel.org/tip/8e05e96ac949c80704d0a38420bf60dcf18c938f
Author: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
AuthorDate: Sun, 28 Feb 2016 22:22:38 -0600
Committer: Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Mon, 29 Feb 2016 08:35:11 +0100

sched: Mark __schedule() stack frame as non-standard

objtool reports the following warnings for __schedule():

kernel/sched/core.o: warning: objtool:__schedule()+0x3c0: duplicate frame pointer save
kernel/sched/core.o: warning: objtool:__schedule()+0x3fd: sibling call from callable instruction with changed frame pointer
kernel/sched/core.o: warning: objtool:__schedule()+0x40a: call without frame pointer save/setup
kernel/sched/core.o: warning: objtool:__schedule()+0x7fd: frame pointer state mismatch
kernel/sched/core.o: warning: objtool:__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>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxx>
Cc: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
Cc: Bernd Petrovitsch <bernd@xxxxxxxxxxxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxxxx>
Cc: Chris J Arges <chris.j.arges@xxxxxxxxxxxxx>
Cc: Jiri Slaby <jslaby@xxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Michal Marek <mmarek@xxxxxxx>
Cc: Namhyung Kim <namhyung@xxxxxxxxx>
Cc: Pedro Alves <palves@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: live-patching@xxxxxxxxxxxxxxx
Link: http://lkml.kernel.org/r/91190e324ebd7fcd01748d508d0dfd4693e84d91.1456719558.git.jpoimboe@xxxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@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)