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

From: Andy Lutomirski
Date: Thu Aug 13 2015 - 18:30:14 EST

On Thu, Aug 13, 2015 at 3:25 PM, Stas Sergeev <stsp@xxxxxxx> wrote:
> 14.08.2015 01:11, Andy Lutomirski ÐÐÑÐÑ:
>> Now suppose you set some magic flag and jump (via sigreturn,
>> trampoline, whatever) into DOS code. The DOS code loads 0x7 into FS
>> and then gets #GP. You land in a signal handler. As far as the
>> kernel's concerned, the FS base register is whatever the base of LDT
>> entry 0 is. What else is the kernel supposed to shove in there?
> The same as what happens when you do in userspace:
> ---
> asm ("mov $0,%%fs\n");
> prctl(ARCH_SET_FS, my_tls_base);
> ---
> This was the trick I did before gcc started to use FS in prolog,
> now I have to do this in asm.
> But how simpler for the kernel is to do the same?
>> I think that making this work fully in the kernel would require a
>> full-blown FS equivalent of sigaltstack, and that seems like overkill.
> Setting selector and base is what you call an "equivalent of sigaltstack"?

Yes. sigaltstack says "hey, kernel! here's my SP for signal
handling." I think we'd need something similar to tell the kernel
what my_tls_base is. Using the most recent thing passed to
ARCH_SET_FS is no good because WRFSBASE systems might not use
ARCH_SET_FS, and we can't break DOSEMU on Ivy Bridge and newer as soon
as we enable WRFSBASE.

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