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

From: Jonas Bonn
Date: Mon Feb 11 2013 - 07:12:25 EST

On 11 February 2013 12:22, Vineet Gupta <Vineet.Gupta1@xxxxxxxxxxxx> wrote:
> On Monday 11 February 2013 04:23 PM, Jonas Bonn wrote:
>> On 11 February 2013 11:28, James Hogan <james.hogan@xxxxxxxxxx> 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@xxxxxxxxxxxx> 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(???).

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