linux-kernel - Re: pt_regs leak into userspace (was Re: [PATCH v3 20/71] ARC: Signal handling)

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Date:	Mon, 11 Feb 2013 13:12:00 +0100
From:	Jonas Bonn <jonas.bonn@...il.com>
To:	Vineet Gupta <Vineet.Gupta1@...opsys.com>
Cc:	James Hogan <james.hogan@...tec.com>,
	Arnd Bergmann <arnd@...db.de>,
	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Al Viro <viro@...iv.linux.org.uk>
Subject: Re: pt_regs leak into userspace (was Re: [PATCH v3 20/71] ARC: Signal handling)

On 11 February 2013 12:22, Vineet Gupta <Vineet.Gupta1@...opsys.com> wrote:
> On Monday 11 February 2013 04:23 PM, Jonas Bonn wrote:
>> On 11 February 2013 11:28, James Hogan <james.hogan@...tec.com> wrote:
>>> On 11/02/13 10:13, Vineet Gupta wrote:
>>>> On Monday 11 February 2013 03:06 PM, Jonas Bonn wrote:
>>>>> On 11 February 2013 08:26, Vineet Gupta <Vineet.Gupta1@...opsys.com> wrote:
>>>>>
>>>>>> The only downside of this patch is that userspace signal stack grows in size,
>>>>>> since signal frame only cares about scratch regs (pt_regs), but has to accommodate
>>>>>> unused placeholder for callee regs too by virtue of using user_regs_struct.
>>>>> Is this really true?  Don't setcontext and friends require that _all_
>>>>> the registers be part of sigcontext?
>>>>
>>>> But for an ABI - callee saved regs will anyhow be saved/restored even in
>>>> setcontext case ! So collecting it for that purpose seems useless, or am I missing
>>>> something here.
>>>
>>> I think Jonas' point was that signals are asynchronous, i.e. you could
>>> get interrupted by a signal at virtually any time during the program's
>>> execution.
>>
>> No, I agree that the callee-saved regs don't need to be saved across a
>> signal handler invocation.  It's really just the setcontext case that
>> wants to be able to swap out the callee-saved regs.
>
> I don't think that's needed either - and if thats mandated somewhere, it would
> seem a unnecessary mis-optimization IMHO.
>
> See, even a setcontext enabled control flow needs to be ABI compliant so that it
> plays nicely with other normal flows of execution. Thus e.g. it can't fudge a
> callee reg - it needs to save orig callee reg(s) and restore them in the end. And
> if we agree to those semantics - I don't see any value in swapping the callee reg
> context around usage of setcontext as it would be a wasted effort.

Yeah, that makes sense.  I can see where you're coming from... and the
fact that you switch the stack, as James pointed out, means that you
end up restoring whatever callee-saved registers you need in the new
control flow on the way out of your setcontext wrapper.

BUT... a successful call to setcontext() does not return and whatever
code you end up jumping to as a result of the call has its own
expectations about the state of the registers.  Somebody has to set up
the registers to meet these expectations and, as far as I can see,
this means:

i) sigreturn fixes up the internal pt_regs with the new userspace state
ii) the syscall return path restores _all_ the regs, as though there
had been a context switch

What am I missing?  I'm totally open to the idea that I'm the one
who's confused here...

...and perhaps this is all moot since it seems that
getcontext/setcontext are obsolete anyway(???).

/Jonas
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