On Thu, Aug 13, 2015 at 10:13 AM, Stas Sergeev <s...@list.ru> wrote: > 13.08.2015 19:59, Andy Lutomirski пишет: > >> On Thu, Aug 13, 2015 at 9:48 AM, Stas Sergeev <s...@list.ru> wrote: >>> >>> 13.08.2015 19:42, Andy Lutomirski пишет: >>> >>>> On Thu, Aug 13, 2015 at 9:38 AM, Stas Sergeev <s...@list.ru> wrote: >>>>> >>>>> 13.08.2015 19:24, Andy Lutomirski пишет: >>>>> >>>>>> On Thu, Aug 13, 2015 at 9:20 AM, Stas Sergeev <s...@list.ru> wrote: >>>>>>> >>>>>>> 13.08.2015 19:09, Andy Lutomirski пишет: >>>>>>> >>>>>>>> On Thu, Aug 13, 2015 at 9:03 AM, Stas Sergeev <s...@list.ru> wrote: >>>>>>>>> >>>>>>>>> 13.08.2015 18:38, Andy Lutomirski пишет: >>>>>>>>>> >>>>>>>>>> >>>>>>>>>> So... what do we do about it? We could revert the whole mess. >>>>>>>>>> We >>>>>>>>>> could tell everyone to fix their DOSEMU, which violates policy and >>>>>>>>>> is >>>>>>>>>> especially annoying given how much effort we've put into keeping >>>>>>>>>> 16-bit mode fully functional lately. We could add yet more >>>>>>>>>> heuristics >>>>>>>>>> and teach sigreturn to ignore the saved SS value in sigcontext if >>>>>>>>>> the >>>>>>>>>> saved CS is 64-bit and the saved SS is unusable. >>>>>>>>> >>>>>>>>> Andy, why do you constantly ignore the proposal to make >>>>>>>>> new behaviour explicitly controlable? You don't have to agree >>>>>>>>> with it, but you could at least comment on that possibility >>>>>>>>> and/or mention it with the ones you listed above. >>>>>>>> >>>>>>>> I'm not sure what the proposal is exactly. >>>>>>>> >>>>>>>> We could add a new uc_flags flag. If set, it means that >>>>>>>> sigcontext->ss is valid and should be used by sigreturn. If clear, >>>>>>>> then we ignore sigcontext->ss and just restore __USER_DS. >>>>>>>> >>>>>>>> The problem is that, by itself, this won't fix old DOSEMU. We >>>>>>>> somehow >>>>>>>> need to either detect that something funny is going on or just leave >>>>>>>> the flag clear by default. >>>>>>>> >>>>>>>> We could do this: always save SS to sigcontext->ss, but only restore >>>>>>>> sigcontext->ss if userspace explicitly sets the flag before >>>>>>>> sigreturn. >>>>>>>> If we do that, we'd need to also add my patch to preserve the actual >>>>>>>> HW SS selector if possible so that old DOSEMU knows what SS to >>>>>>>> program >>>>>>>> into its trampoline. >>>>>>>> >>>>>>>> This at least lets *new* DOSEMU set the flag and get the improved >>>>>>>> behavior. I still don't know what effect it'll have on Wine and >>>>>>>> CRIU. >>>>>>>> >>>>>>>> Stas, is that what you were thinking, or were you thinking of >>>>>>>> something >>>>>>>> else? >>>>>>> >>>>>>> Not quite. >>>>>>> I mean the flag that will control not only sigreturn, but >>>>>>> the signal delivery as well. This may probably be a sigaction() >>>>>>> flag or some other. If not set - ss is ignored by both signal >>>>>>> delivery and sigreturn(). If set - ss is saved/restored (and in >>>>>>> the future - also fs/gs). >>>>>>> Is such a flag possible? >>>>>> >>>>>> Maybe. I think I'm more nervous about adding new flags in sigaction >>>>>> than I am in uc_flags. >>>>> >>>>> Isn't uc_flags read-only for the user? >>>>> I look into setup_rt_frame >>>>> <http://lxr.free-electrons.com/ident?v=2.4.37;i=setup_rt_frame>() and >>>>> see >>>>> --- >>>>> /* Create the ucontext. */ >>>>> err |= __put_user(0, &frame->uc.uc_flags); >>>>> --- >>>>> so it doesn't look like the flag that user can use to _request_ >>>>> something from the kernel. And I am talking about exactly >>>>> the flag to request the new behaviour, as only that can remove >>>>> the regression completely without patching dosemu. >>>> >>>> User code could rewrite it in the signal handler to request something. >>> >>> But that's too late to affect the signal _delivery_ anyhow, no? >>> Any idea about the flag that can control both delivery and return? >> >> I think my LAR patch should cover the signal delivery part. > > Ah, I see your point now. > But that's not what I mean, as it doesn't cover fs/gs, which > is what Linus is looking to revert now too (I am building the > testing kernels now). > So you obviously don't want the flag that will control all 3 > things together without any lar heuristics, but I don't understand why... > Yes, your heuristic+uc_flag may work, but IMHO far from > perfection and TLS problem is not covered. I can test such > a patch but I don't understand why you don't want the flag > that will just control all things together.
The fs/gs patch doesn't change anything, so there's nothing to control. It just renamed fields that did nothing. (It turns out they did something back before arch_prctl existed, but there's only a narrow range of kernels like that, and I'm not at all convinced that those kernels are ABI-compatible with modern kernels at all. This is all pre-git.) Sure, it might make sense to change TLS behavior in signals at some point, but I don't think we're there yet. We need to deal with fsgsbase first, and that's a *huge* can of worms. --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/
