From mboxrd@z Thu Jan 1 00:00:00 1970 Message-ID: <4B90EB61.4010808@domain.hid> Date: Fri, 05 Mar 2010 12:30:41 +0100 From: Gilles Chanteperdrix MIME-Version: 1.0 References: <4B8E24B4.9000806@domain.hid> <4B8E260D.7070809@domain.hid> <4B8E281F.1040308@domain.hid> <4B8E28D2.6020808@domain.hid> <4B8FFBCD.9020305@domain.hid> <4B8FFDC0.4080903@domain.hid> <4B90173C.9080008@domain.hid> <4B901B53.2050605@domain.hid> <4B90E936.8080502@domain.hid> In-Reply-To: <4B90E936.8080502@domain.hid> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Subject: Re: [Xenomai-core] Potential heap corruption on thread cleanup List-Id: Xenomai life and development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Jan Kiszka Cc: "Mauerer, Wolfgang" , xenomai-core , "Hillier, Gernot" Jan Kiszka wrote: > Gilles Chanteperdrix wrote: >> Jan Kiszka wrote: >>> Gilles Chanteperdrix wrote: >>>> Jan Kiszka wrote: >>>>> Gilles Chanteperdrix wrote: >>>>>> Jan Kiszka wrote: >>>>>>> Gilles Chanteperdrix wrote: >>>>>>>> Jan Kiszka wrote: >>>>>>>>> Hi Gilles, >>>>>>>>> >>>>>>>>> I'm pushing your findings to the list, also as my colleagues showed >>>>>>>>> strong interest - this thing may explain rare corruptions for us as well. >>>>>>>>> >>>>>>>>> I thought a bit about that likely u_mode-related crash in your test case >>>>>>>>> and have the following theory so far: If the xeno_current_mode storage >>>>>>>>> is allocated on the application heap (!HAVE_THREAD, that's also what we >>>>>>>>> are forced to use), it is automatically freed on thread termination in >>>>>>>>> the context of the dying thread. If the thread is already migrated to >>>>>>>>> secondary or if that happens while it is cleaned up (i.e. before calling >>>>>>>>> for exit into the kernel), there is no problem, Xenomai will not touch >>>>>>>>> the mode storage anymore. But if the thread happens to delete the >>>>>>>>> storage "silently", without any migration, the final exit will trigger >>>>>>>>> one further access. And that takes place against an invalid head area at >>>>>>>>> this point. >>>>>>>>> >>>>>>>>> Does this make sense? >>>>>>>> Yes, it is the issue we observed. >>>>>>>> >>>>>>>>> If that is true, all we need to do is to force a migration before >>>>>>>>> releasing the mode storage. Could you check this? >>>>>>>> No, that does not fly. Calling, for instance, __wrap_pthread_mutex_lock >>>>>>>> in another TSD cleanup function is which could be called after the >>>>>>>> current_mode TSD cleanup is allowed and could trigger a switch to >>>>>>>> primary mode and a write to the u_mode. >>>>>>>> >>>>>>> Good point. Mmh. Another, but ABI-breaking, way would be to add a >>>>>>> syscall for deregistering the u_mode pointer... >>>>>> That is the thing we did to verify that we had this bug. But this >>>>>> syscall would be also called too soon, and suffers from the TSD cleanup >>>>>> functions order again. >>>>>> >>>>> Right, the only complete fix without losing functionality is to add an >>>>> option to our ABI for requesting kernel-managed memory if dynamic >>>>> allocation is necessary (i.e. no TLS is available). >>>> No. TLS may as well suffer from the same issue, since it is handled by >>>> the glibc or libgcc, over which we have no control. So yes, it may work >>>> by chance today, but may as well stop working tomorrow. We use >>>> kernel-managed memory all the time, final point. >>> I think we are still in the solution finding process, no need for early >>> conclusions. >>> >>> See, we actually do not need kernel-managed storage for u_mode at all. >>> u_mode is an optimization, mostly for our fast user space mutexes. We >>> can indeed switch off all updates by the kernel and will still be able >>> to provide all required features - just less optimally. Adding a third >>> state, "invalid", we can make all mutex users assume they need the slow >>> syscall path on uncontended acquisition. And assert_nrt will probably be >>> happy about a syscall replacement for u_mode when it became invalid. >>> >>> This invalid state (maybe u_mode == -1 with TLS, and mode_key == NULL >>> without it) is entered during thread clean up with the help of a TSD >>> destructor. The destructor will then deregister our u_mode storage from >>> the kernel so that it doesn't matter if we release the memory >>> immediately and explicitly (w/o TLS) or leave this to glibc (/w TLS). >>> And in this model, it also doesn't matter when precisely the destructor >>> is called. >> We have to add a syscall to propagate this value to kernel-space, and >> clutter the kernel-space code which uses u_mode with tests to see if >> u_mode is valid or not, and we have to clutter the code which uses >> u_mode in user-space to handle that invalid state. And every time we add >> a user of u_mode, we have to think about the invalid state. A lot of >> clutter. >> >> The two last issues may be removed by handling the invalid state only in >> the function which returns the current mode. If the state is invalid, >> then issue the syscall. Admittedly, we get two syscalls for mutex locks, >> but who cares. >> >> However, what for? Allocating u_mode in the process private sem_heap, as >> I suggest since the beggining, looks so much simpler. No test, no >> special case, the address is always valid as long as the tcb is valid. > > Try implementing it. > > I will post a prototype for my approach within a minute. Its major > implementation advantage is that there is no need to touch any skin, > neither on user nor kernel side, and that there is no need for backward > compatible syscalls. > > Another advantage of my approach is that it does not touch the fast > paths of mutex handling (before deregistration) - well, at lest almost > for non-TLS, but absolutely not for TLS. Do not forget the kernel-space part which detects whether we are using the older or newer user-space. -- Gilles.