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

From: Stas Sergeev
Date: Thu Aug 13 2015 - 18:02:37 EST

14.08.2015 00:46, Linus Torvalds ÐÐÑÐÑ:
On Thu, Aug 13, 2015 at 2:42 PM, Raymond Jennings <shentino@xxxxxxxxx> 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.
Unless you introduce some clever flag to explicitly request its restoring.
There is another problem as well which is that gcc assumes
FS base to point to TLS at function prolog. Since FS is not
restored too, the only suggestion I get is to write a sighandlers
in asm... I wonder if someone really should write a sighandler in
asm to restore FS base manually with a syscall.
So I think the reality is asking for a new flag. :)
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