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

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Message-Id: <f3e44131b7aa6b3f3ead20792b993346a3777f54.1456440439.git.jpoimboe@redhat.com>
Date:	Thu, 25 Feb 2016 16:57:12 -0600
From:	Josh Poimboeuf <jpoimboe@...hat.com>
To:	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org
Cc:	linux-kernel@...r.kernel.org, live-patching@...r.kernel.org,
	Michal Marek <mmarek@...e.cz>,
	Peter Zijlstra <peterz@...radead.org>,
	Andy Lutomirski <luto@...nel.org>,
	Borislav Petkov <bp@...en8.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andi Kleen <andi@...stfloor.org>,
	Pedro Alves <palves@...hat.com>,
	Namhyung Kim <namhyung@...il.com>,
	Bernd Petrovitsch <bernd@...rovitsch.priv.at>,
	Chris J Arges <chris.j.arges@...onical.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jiri Slaby <jslaby@...e.cz>,
	Arnaldo Carvalho de Melo <acme@...nel.org>
Subject: [PATCH v18 5/9] sched: Mark __schedule() stack frame as non-standard

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()
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>
---
 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)
 {
-- 
2.4.3