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

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Message-ID: <5118D478.3010205@synopsys.com>
Date:	Mon, 11 Feb 2013 16:52:32 +0530
From:	Vineet Gupta <Vineet.Gupta1@...opsys.com>
To:	Jonas Bonn <jonas.bonn@...il.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 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.

-Vineet
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