Messages in this thread | | | Date | Wed, 5 Dec 2012 09:44:51 +1100 | Subject | Re: PTRACE_SYSCALL && vsyscall (Was: arch_check_bp_in_kernelspace: fix the range check) | From | u3557@miso ... |
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Dear Oleg,
> Yes, I understand, so DR_RW_EXECUTE should probably work. And I even > sent the patch (untested/uncompiled). But given that even the simple > bugfix which started this thread was ignored by maintainers, I am > not sure how we can convince them this change makes sense ;)
Just to confirm, DR_RW_EXECUTE won't only "probably" work - it DOES work, I have tested it.
A fully super-duper automatic and transparent emulation of PTRACE_SYSCALL, including the faking of user-registers, would undoubtedly be great, but it's complex and will require a large patch, while here is a trivial 1-2 line fix which doesn't harm anyone and allows ptracers to trap a vsyscall in no time.
> But If we want to allow to trace vsyscall's, hw bp doesn't look very > nice imo. HBP_NUM = 4 and you need to setup 3 bp's to trace them all.
The next solution in line, in terms of patch-size, if we don't want to waste debug registers, is to have the execute permission of the vsyscall page changed on context switches when a process/ptracer requests so, in a manner similar to prctl(PR_SET_TSC).
Best Regards, Amnon.
>> >> > However. Of course it would be nice to avoid the new option. IMO it >> > would be better to do nothing ;) vsyscall is deprecated, and EMULATE >> > is x86-specific. >> >> The problem is that the current static glibc invokes the vsyscall page, > > Yes I know. > > Still I'd like to avoid to change the ptrace API, even if the change is > simple. This emulate_vsyscall() is too "exotic" imho. > >> > You forgot again that EMULATE does not execute the code in the >> > vsyscall page. >> >> The beauty of using the x86 debug-registers, is that they do not >> trap the instruction, but rather the fact that the program-counter >> has a given value. > > Yes, I understand, so DR_RW_EXECUTE should probably work. And I even > sent the patch (untested/uncompiled). But given that even the simple > bugfix which started this thread was ignored by maintainers, I am > not sure how we can convince them this change makes sense ;) > > However. This looks like a hack to me, because this code is never > executed. But this is sudjective and I am not saying this can't work. > And yes, this doesn't add new ptrace hacks. > > > > But If we want to allow to trace vsyscall's, hw bp doesn't look very > nice imo. HBP_NUM = 4 and you need to setup 3 bp's to trace them all. > > And what about strace? It won't be easy to change it to use hwbp. > > > That is why I think PTRACE_SYSCALL should "simply work" somehow. And > so far I think that "just report syscall_exit with orig_ax = -1" is > the best (and simple) solution. > > OK. We can do more. We can report both syscall_enter/exit and we can > change orig_ax/ax temporary to "fool" the tracer, so that everything > will look as a "normal" syscall. Like vsyscall_seccomp() does. > > But this needs much more changes. > > Oleg. > >
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