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

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Message-ID: <tip-8e05e96ac949c80704d0a38420bf60dcf18c938f@git.kernel.org>
Date:	Mon, 29 Feb 2016 02:59:56 -0800
From:	tip-bot for Josh Poimboeuf <tipbot@...or.com>
To:	linux-tip-commits@...r.kernel.org
Cc:	bp@...en8.de, luto@...nel.org, linux-kernel@...r.kernel.org,
	palves@...hat.com, tglx@...utronix.de,
	torvalds@...ux-foundation.org, jslaby@...e.cz,
	bernd@...rovitsch.priv.at, peterz@...radead.org, mingo@...nel.org,
	chris.j.arges@...onical.com, hpa@...or.com, namhyung@...il.com,
	mmarek@...e.cz, jpoimboe@...hat.com, akpm@...ux-foundation.org,
	acme@...nel.org
Subject: [tip:core/objtool] sched: Mark __schedule() stack frame as
 non-standard

Commit-ID:  8e05e96ac949c80704d0a38420bf60dcf18c938f
Gitweb:     http://git.kernel.org/tip/8e05e96ac949c80704d0a38420bf60dcf18c938f
Author:     Josh Poimboeuf <jpoimboe@...hat.com>
AuthorDate: Sun, 28 Feb 2016 22:22:38 -0600
Committer:  Ingo Molnar <mingo@...nel.org>
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()
macro:

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:

  https://lkml.kernel.org/r/CALCETrWsqCw4L1qKO9j9L5F+4ED4viuLQTFc=n1pKBZfFPQUFg@mail.gmail.com

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@...hat.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: Andy Lutomirski <luto@...nel.org>
Cc: Arnaldo Carvalho de Melo <acme@...nel.org>
Cc: Bernd Petrovitsch <bernd@...rovitsch.priv.at>
Cc: Borislav Petkov <bp@...en8.de>
Cc: Chris J Arges <chris.j.arges@...onical.com>
Cc: Jiri Slaby <jslaby@...e.cz>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Michal Marek <mmarek@...e.cz>
Cc: Namhyung Kim <namhyung@...il.com>
Cc: Pedro Alves <palves@...hat.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: live-patching@...r.kernel.org
Link: http://lkml.kernel.org/r/91190e324ebd7fcd01748d508d0dfd4693e84d91.1456719558.git.jpoimboe@redhat.com
Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
 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)
 
 	balance_callback(rq);
 }
+STACK_FRAME_NON_STANDARD(__schedule); /* switch_to() */
 
 static inline void sched_submit_work(struct task_struct *tsk)
 {