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

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Message-ID: <CACM3HyF2GHz5PKuurYG2Hg+UEKeto+zme5gcv2fxQDC=tpZYRw@mail.gmail.com>
Date:	Mon, 11 Feb 2013 11:30:50 +0100
From:	Jonas Bonn <jonas.bonn@...il.com>
To:	Vineet Gupta <Vineet.Gupta1@...opsys.com>
Cc:	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 11:13, Vineet Gupta <Vineet.Gupta1@...opsys.com> 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.

Yes, I agree with you on the ABI bit... but set/get/swapcontext are
special in that they use rt_sigreturn to "trick" the kernel into
swapping out _all_ registers; it's a light-weight (userspace) context
switch.

(I really don't know this area very well, though, so I'm hoping
somebody more knowledgable jumps in here... Al?)

/Jonas
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