linux-kernel - Re: [PATCH v3 1/2] x86_64,signal: Fix SS handling for signals delivered to 64-bit programs

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Date:	Fri, 20 Mar 2015 15:04:19 +0300
From:	Cyrill Gorcunov <gorcunov@...il.com>
To:	Denys Vlasenko <vda.linux@...glemail.com>
Cc:	Andy Lutomirski <luto@...capital.net>,
	Pavel Emelyanov <xemul@...allels.com>,
	Oleg Nesterov <oleg@...hat.com>,
	Andrey Wagin <avagin@...il.com>,
	Andy Lutomirski <luto@...nel.org>,
	Ingo Molnar <mingo@...nel.org>,
	Andi Kleen <andi@...stfloor.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Al Viro <viro@...iv.linux.org.uk>, X86 ML <x86@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Borislav Petkov <bp@...en8.de>,
	Pavel Emelyanov <xemul@...nvz.org>
Subject: Re: [PATCH v3 1/2] x86_64,signal: Fix SS handling for signals
 delivered to 64-bit programs

On Fri, Mar 20, 2015 at 02:56:22PM +0300, Cyrill Gorcunov wrote:
> > >
> > > fwiw currently we're passing zero in this __pad0 (replying to your
> > > previous email, so we can workaround in the kernel assuming zero
> > > as a special case, not that good but better than nothing).
> > 
> > Special-casing zero sounds not that bad to me.
> > It can be removed after a few years - just don't forget
> > to document it in a good comment: why we have special
> > case? What software required it?
> > In which version of that software the need to have this hack
> > was eliminated?
> 
> To be fair, such special case would be ideal for us, so that
> if noone object against such hack, i would cook a patch.

Denys, note though that Andy pointed a downside for such approach as well:

 | The only real down side I can see to special casing zero is that it
 | really is possible to end up with zero in there.  For example, the
 | SIGSEGV you get do to the failed sigreturn probably has sigcontext->ss
 | == 0 :)

which I don't know how to resolve.
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