On Thu, Aug 13, 2015 at 3:51 PM, Stas Sergeev <stsp@xxxxxxx> wrote:Hmm, how about the following:
14.08.2015 01:29, Andy Lutomirski ÐÐÑÐÑ:DOSEMU, when you set that flag, WRFSBASE gets enabled, and glibc's
On Thu, Aug 13, 2015 at 3:25 PM, Stas Sergeev <stsp@xxxxxxx> wrote:If someone uses WRFSBASE and wants things to be preserved
14.08.2015 01:11, Andy Lutomirski ÐÐÑÐÑ:Yes. sigaltstack says "hey, kernel! here's my SP for signal
Now suppose you set some magic flag and jump (via sigreturn,The same as what happens when you do in userspace:
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?
asm ("mov $0,%%fs\n");
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 aSetting selector and base is what you call an "equivalent of
full-blown FS equivalent of sigaltstack, and that seems like overkill.
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.
in a sighandler, he'll just not set the aforementioned flag. No regression.
Whoever wants to use that flag properly, will not use WRFSBASE,
and will use ARCH_SET_FS or set_thread_area().
What exactly breakage do you have in mind?
threading library starts using WRFSBASE instead of arch_prctl.