linux-kernel - Re: [RFC PATCH 12/21] sched: Add __schedule() to stackvalidate whitelist

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Date:	Fri, 17 Jul 2015 12:58:12 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Josh Poimboeuf <jpoimboe@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, Michal Marek <mmarek@...e.cz>,
	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>, X86 ML <x86@...nel.org>,
	live-patching@...r.kernel.org,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH 12/21] sched: Add __schedule() to stackvalidate whitelist

On Fri, Jul 17, 2015 at 12:46 PM, Peter Zijlstra <peterz@...radead.org> wrote:
> On Fri, Jul 17, 2015 at 11:47:28AM -0500, Josh Poimboeuf wrote:
>> stackvalidate reports the following warnings for __schedule():
>>
>>   stackvalidate: kernel/sched/core.o: __schedule()+0x3e7: duplicate frame pointer save
>>   stackvalidate: kernel/sched/core.o: __schedule()+0x424: sibling call from callable instruction with changed frame pointer
>>   stackvalidate: kernel/sched/core.o: __schedule()+0x431: call without frame pointer save/setup
>>   stackvalidate: kernel/sched/core.o: __schedule()+0x8b8: frame pointer state mismatch
>>   stackvalidate: kernel/sched/core.o: __schedule()+0x447: frame pointer state mismatch
>>
>> __schedule() is obviously a special case which is allowed to do unusual
>> things with the frame pointer.
>
> Yes, but is the code actually correct? We can't dismiss the warnings
> just on that basis alone.

It's really only __switch_to that does weird things, right?  I kinda
want to rework that thing anyway to have a well-defined saved state
format anyway, which would have the nice benefit of letting us get rid
of all the ret_from_fork crap.

That is, we'd context switch like:

static inline void __switch_to(...) {
  switch extra stuff;
  switch everything except gpregs;
  asm volatile ("call __switch_stack_and_ip" : [__sp thing goes here]
: "S" (&prev->bottom_of_stack), "D" (&next->bottom_of_stack) :
"basically all regs and flags");
}

Then the low level bit would be:

__switch_stack_and_ip:
  pushq %rbp
  mov %rsp, (%rsi)
  mov (%rdi), %rsp
  popq %rbp
  ret

Now, when we create a new task, we can set up its stack directly
rather than setting some TI flag, because we actually know the layout.

Hmm?

--Andy
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