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

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Message-ID: <20130211140704.GI4503@ZenIV.linux.org.uk>
Date:	Mon, 11 Feb 2013 14:07:05 +0000
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Jonas Bonn <jonas.bonn@...il.com>
Cc:	Vineet Gupta <Vineet.Gupta1@...opsys.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>
Subject: Re: pt_regs leak into userspace (was Re: [PATCH v3 20/71] ARC:
 Signal handling)

On Mon, Feb 11, 2013 at 11:30:50AM +0100, Jonas Bonn wrote:

> >> 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?)

I'd suggest asking itanic folks; they do *not* put callee-saved stuff into
sigcontext.  AFAICS, they don't have setcontext() implemented as a syscall
at all - it's done as sigprocmask() + doing to callee-saved registers what
longjmp() does.
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