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 21:57:39 +0300
From:	Stas Sergeev <stsp@...t.ru>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Andy Lutomirski <luto@...capital.net>,
	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

13.08.2015 21:35, Linus Torvalds пишет:
> On Thu, Aug 13, 2015 at 10:51 AM, Stas Sergeev <stsp@...t.ru> wrote:
>> Hello Linus, I verified that patch-minimal.diff is enough
>> to fix the problem, BUT! dosemu is in fact using the .fs and
>> .gs fields of sigcontext as a placeholders. Why the minimal
>> patch alone helps is simply because the kernel headers
>> installed in a system do not yet represent the newer kernel
>> developments and have the .fs and .gs fields in.
> Ok. So I'm inclined to do the bigger revert, just to fix the compile
> issue. It would be crazy to force some silly autoconf script for
> random header info.
But OTOH these fields already lost their meaning.
It may make sense to force people to stop using them,
in case you ever want to re-use them again in the future.
 From what Andy says, it seems there are the distant plans
to start restoring FS again. If people still use sigcontext.fs
by that time, you'll get problems. If you force everyone to
stop using them - you'll be safe.

Also, at least in the past, resolving the compile-time problems
was up to the distributions: they always provided the "sanitized up"
version of kernel headers. Not sure what the current policy is...
In fact, here in Fedora-22, I have
/usr/include/asm/sigcontext.h
that is straight from the kernel, but signal.h is instead using
a "sanitized up" version in
/usr/include/bits/sigcontext.h
so the userspace compiles fine.

In fact, I think the "silly autoconf script" you mentioned above,
should indeed be reverted, and instead I should use
sigcontext.reserved1[8] array to store FS/GS? Is this
safer against ever re-using this space? Not sure...
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