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

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Date:	Thu, 13 Aug 2015 17:40:59 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Stas Sergeev <stsp@...t.ru>, Raymond Jennings <shentino@...il.com>,
	Cyrill Gorcunov <gorcunov@...il.com>,
	Pavel Emelyanov <xemul@...allels.com>,
	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 5:24 PM, Andy Lutomirski <luto@...capital.net> wrote:
>
> So yes, it mostly works.  It also sucks, and it makes it extremely
> unpleasant for any other program to do this.

Well, I'd argue that

 (a) we don't really _want_ any other programs to do that

 (b) but yeah, we might want to make it easier to do cleanly and
explicitly for dosemu (and any other possible programs that do want to
do it)

so if this is for just things like dosemu and for test applications
etc, let's just introduce something like SA_RESTORE_SS for those to
explicitly opt in to "I want this signal handler to save and restore
SS".

That's how we have handled these kinds of things in the past (ie
SA_SIGINFO extends the stack frame with siginfo, sa_restorer says that
user space will use its own sigrestore function, etc etc).

Yeah, the SA_xyzzy flags aren't exactly _common_, but it's how we've
handled these kinds of compat issues in the past (SA_ONESHOT, SA_MASK,
and SA_NOCLDWAIT are obviously about the traditional BSD/SysV
differences, and SA_RESTORER is a Linux internal compatibility thing
etc)

                      Linus
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