On Wed, Aug 12, 2015 at 1:55 PM, Stas Sergeev <s...@list.ru> wrote: > 12.08.2015 23:47, Andy Lutomirski пишет: > >> On Wed, Aug 12, 2015 at 1:45 PM, Stas Sergeev <s...@list.ru> wrote: >>> >>> 12.08.2015 23:28, Andy Lutomirski пишет: >>> >>>> On Wed, Aug 12, 2015 at 1:14 PM, Stas Sergeev <s...@list.ru> wrote: >>>>> >>>>> 12.08.2015 23:01, Andy Lutomirski пишет: >>>>>> >>>>>> On Wed, Aug 12, 2015 at 12:55 PM, Stas Sergeev <s...@list.ru> wrote: >>>>>>> >>>>>>> 12.08.2015 22:20, Andy Lutomirski пишет: >>>>>>>> >>>>>>>> current kernels, it stays switched. If we change this, it won't >>>>>>>> stay >>>>>>>> switched. Even ignoring old ABI, it's not really clear to me what >>>>>>>> the >>>>>>>> right thing to do is. >>>>>>> >>>>>>> There can be the following cases: >>>>>>> - switch_userspace_thread() switches fs to non-zero selector >>>>>>> - switch_userspace_thread() switches the fs base via syscall >>>>>>> - switch_userspace_thread() switches fs in sigcontext >>>>>>> - switch_userspace_thread() switches fs_base in sigcontext (???) >>>>>>> What exactly case do you have in mind? >>>>>>> I'd say, the way x86_32 is doing things - is good, but the >>>>>>> bases... perhaps, in ideal world, they should be a part of >>>>>>> the sigcontext as well? >>>>>> >>>>>> Any of the above. What do you want the kernel to do, and how does the >>>>>> kernel know you want to do that? The kernel has to pick *some* >>>>>> semantics here. >>>>> >>>>> Assuming the bases are made the part of a sigcontext, >>>>> I'd say there would be no ambiguities remained at all: >>>>> whatever you change in a sigcontext, will be "applied" by >>>>> the sigreturn(). Whatever you put in the registers >>>>> (either segregs or MSRs), is valid until sigreturn(), then >>>>> forgotten forever. >>>>> The mess only comes in when some things are part of >>>>> sigcontext and some are not. But if you have _all_ things >>>>> accessable in sigcontext, then the user has a way of expressing >>>>> his needs very clearly: he'll either touch sigcontext or direct >>>>> values, depending on what he need. >>>>> >>>>> Is this right? >>>> >>>> Maybe, except that doing this might break existing code (Wine and Java >>>> come to mind). I'm not really sure. >>> >>> Yes, but that's why I was talking about some new >>> flag. Maybe a new sigaction() flag? Or something else that >>> will allow the user to request explicitly the new handling >>> where the things are all switched by the kernel. Then >>> the old programs that don't use that flag, will remain >>> unaffected. I realize this may be a lot of work... But please >>> note that there will be no more a chance like this one, >>> when things are already badly broken. :) >> >> I think that, with my patch, we get the best of both worlds. We keep >> the old behavior in cases where it would work, and we switch to the >> new behavior in cases where the old behavior would result in killing >> the task. > > But I mean also fs/TLS. > There is a chance now to fix things for good, all at once. :) > With such an ss patch applied to stable, there will be no more > such a chance ever. What's your opinion on the possibility of > fixing the TLS problem? > Also I am not sure about the sigreturn()'s detection: is it > a subject of the subsequent patch, or you dropped an idea?
I think these things shouldn't be conflated. If we can fix it transparently (i.e. if my patch works), then I think we should do something like my patch. --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/
