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

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Date:	Fri, 17 Jul 2015 23:03:14 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Andy Lutomirski <luto@...capital.net>
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 12:58:12PM -0700, Andy Lutomirski wrote:
> 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? 

Yep, but context_switch() gets inlined and __switch_to() is a macro, so
it all ends up being __schedule().

> 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?

I've never really looked at the x86 details, but yes it would be good to
get rid of that fork special case.
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