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. --Andy -- 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/
