On Thu, Aug 13, 2015 at 5:00 PM, Stas Sergeev <s...@list.ru> wrote: > 14.08.2015 02:00, Andy Lutomirski пишет: > >> On Thu, Aug 13, 2015 at 3:51 PM, Stas Sergeev <s...@list.ru> wrote: >>> >>> 14.08.2015 01:29, Andy Lutomirski пишет: >>>> >>>> On Thu, Aug 13, 2015 at 3:25 PM, Stas Sergeev <s...@list.ru> wrote: >>>>> >>>>> 14.08.2015 01:11, Andy Lutomirski пишет: >>>>> >>>>>> 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. >>> >>> If someone uses WRFSBASE and wants things to be preserved >>> 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? >> >> DOSEMU, when you set that flag, WRFSBASE gets enabled, and glibc's >> threading library starts using WRFSBASE instead of arch_prctl. > > Hmm, how about the following: > > prctl(ARCH_SET_SIGNAL_FS, my_tls) > If my_tls==NULL - use current fsbase (including one of WRFSBASE). > If my_tls==(void)-1 - don't restore. > > Can this work?
Certainly, but why? ISTM user code should do this itself with a little bit of asm unless there's a good reason it wouldn't work. A good reason it wouldn't work is that high-performance applications need this and an extra syscall is too slow, but IMO that would need evidence. --Andy -- Andy Lutomirski AMA Capital Management, LLC -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/