linux-kernel - Re: [regression] x86/signal/64: Fix SS handling for signals delivered to 64-bit programs breaks dosemu

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Message-ID: <CA+55aFyqb29r+1YP2iYStFOij9X0QDWqjgmee=JcTkB-q8iEdA@mail.gmail.com>
Date:	Thu, 13 Aug 2015 14:46:54 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Raymond Jennings <shentino@...il.com>
Cc:	Cyrill Gorcunov <gorcunov@...il.com>,
	Andy Lutomirski <luto@...capital.net>,
	Pavel Emelyanov <xemul@...allels.com>,
	Stas Sergeev <stsp@...t.ru>,
	Linux kernel <linux-kernel@...r.kernel.org>
Subject: Re: [regression] x86/signal/64: Fix SS handling for signals delivered
 to 64-bit programs breaks dosemu

On Thu, Aug 13, 2015 at 2:42 PM, Raymond Jennings <shentino@...il.com> wrote:
>
> I am curious about what's supposed to happen normally on signal delivery.
>
> Is SS a register that's supposed to be preserved like EIP/RIP and CS when a
> signal is delivered?

What exactly does "supposed" mean?

On x86-64, we traditionally haven't touched SS, because it doesn't
really matter in 64-bit long mode. And apparently dosemu depended on
that behavior.

So clearly, we're not "supposed" to save/restore it. Because reality
matters a hell of a lot more than any theoretical arguments.

                Linus
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