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 - 18:25:58 EST

14.08.2015 01:11, Andy Lutomirski ÐÐÑÐÑ:
On Thu, Aug 13, 2015 at 3:02 PM, Stas Sergeev <stsp@xxxxxxx> wrote:
14.08.2015 00:46, Linus Torvalds ÐÐÑÐÑ:
On Thu, Aug 13, 2015 at 2:42 PM, Raymond Jennings <shentino@xxxxxxxxx>
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
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. :)
I still don't see how this hypothetical flag would work.

The relevant task state consists of FS and the hidden FS base
register. If you build with -fstack-protector, GCC really wants the
FS base register to point to the right place. So you can't change it
in the middle of a C function (because the prologue and epilogue will
fail to match).

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"?
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at
Please read the FAQ at