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 15:07:38 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Cyrill Gorcunov <gorcunov@...il.com>
Cc:	Denys Vlasenko <vda.linux@...glemail.com>,
	Andy Lutomirski <luto@...capital.net>,
	Pavel Emelyanov <xemul@...allels.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

I do not have a strong opinion, I leave this to you and Andy...

But,

On 03/20, Cyrill Gorcunov wrote:
> 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

If we add this special case now, I am not sure we can remove it later.

> > > just don't forget
> > > to document it in a good comment

and perhaps with WARN_ON_ONCE().

>  | 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.

Another downside (to me) is that this special case can help, but only
"by accident".

For example, criu has ->ss = 0, but it was not initialized explicitly.

But as I said, I won't argue.

Oleg.

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