On 7/16/19 5:07 PM, Xiaoming Ni wrote: > On 2019/7/16 18:20, Vasily Averin wrote: >> On 7/16/19 5:00 AM, Xiaoming Ni wrote: >>> On 2019/7/15 13:38, Vasily Averin wrote: >>>> On 7/14/19 5:45 AM, Xiaoming Ni wrote: >>>>> On 2019/7/12 22:07, gre...@linuxfoundation.org wrote: >>>>>> On Fri, Jul 12, 2019 at 09:11:57PM +0800, Xiaoming Ni wrote: >>>>>>> On 2019/7/11 21:57, Vasily Averin wrote: >>>>>>>> On 7/11/19 4:55 AM, Nixiaoming wrote: >>>>>>>>> On Wed, July 10, 2019 1:49 PM Vasily Averin wrote: >>>>>>>>>> On 7/10/19 6:09 AM, Xiaoming Ni wrote: >>>>>>>>>>> Registering the same notifier to a hook repeatedly can cause the >>>>>>>>>>> hook >>>>>>>>>>> list to form a ring or lose other members of the list. >>>>>>>>>> >>>>>>>>>> I think is not enough to _prevent_ 2nd register attempt, >>>>>>>>>> it's enough to detect just attempt and generate warning to mark host >>>>>>>>>> in bad state. >>>>>>>>>> >>>>>>>>> >>>>>>>>> Duplicate registration is prevented in my patch, not just "mark host >>>>>>>>> in bad state" >>>>>>>>> >>>>>>>>> Duplicate registration is checked and exited in >>>>>>>>> notifier_chain_cond_register() >>>>>>>>> >>>>>>>>> Duplicate registration was checked in notifier_chain_register() but >>>>>>>>> only >>>>>>>>> the alarm was triggered without exiting. added by commit >>>>>>>>> 831246570d34692e >>>>>>>>> ("kernel/notifier.c: double register detection") >>>>>>>>> >>>>>>>>> My patch is like a combination of 831246570d34692e and >>>>>>>>> notifier_chain_cond_register(), >>>>>>>>> which triggers an alarm and exits when a duplicate registration is >>>>>>>>> detected. >>>>>>>>> >>>>>>>>>> Unexpected 2nd register of the same hook most likely will lead to >>>>>>>>>> 2nd unregister, >>>>>>>>>> and it can lead to host crash in any time: >>>>>>>>>> you can unregister notifier on first attempt it can be too early, it >>>>>>>>>> can be still in use. >>>>>>>>>> on the other hand you can never call 2nd unregister at all. >>>>>>>>> >>>>>>>>> Since the member was not added to the linked list at the time of the >>>>>>>>> second registration, >>>>>>>>> no linked list ring was formed. >>>>>>>>> The member is released on the first unregistration and -ENOENT on the >>>>>>>>> second unregistration. >>>>>>>>> After patching, the fault has been alleviated >>>>>>>> >>>>>>>> You are wrong here. >>>>>>>> 2nd notifier's registration is a pure bug, this should never happen. >>>>>>>> If you know the way to reproduce this situation -- you need to fix it. >>>>>>>> >>>>>>>> 2nd registration can happen in 2 cases: >>>>>>>> 1) missed rollback, when someone forget to call unregister after >>>>>>>> successfull registration, >>>>>>>> and then tried to call register again. It can lead to crash for >>>>>>>> example when according module will be unloaded. >>>>>>>> 2) some subsystem is registered twice, for example from different >>>>>>>> namespaces. >>>>>>>> in this case unregister called during sybsystem cleanup in first >>>>>>>> namespace will incorrectly remove notifier used >>>>>>>> in second namespace, it also can lead to unexpacted behaviour. >>>>>>>> >>>>>>> So in these two cases, is it more reasonable to trigger BUG() directly >>>>>>> when checking for duplicate registration ? >>>>>>> But why does current notifier_chain_register() just trigger WARN() >>>>>>> without exiting ? >>>>>>> notifier_chain_cond_register() direct exit without triggering WARN() ? >>>>>> >>>>>> It should recover from this, if it can be detected. The main point is >>>>>> that not all apis have to be this "robust" when used within the kernel >>>>>> as we do allow for the callers to know what they are doing :) >>>>>> >>>>> In the notifier_chain_register(), the condition ( (*nl) == n) is the same >>>>> registration of the same hook. >>>>> We can intercept this situation and avoid forming a linked list ring to >>>>> make the API more rob >>>> >>>> Once again -- yes, you CAN prevent list corruption, but you CANNOT recover >>>> the host and return it back to safe state. >>>> If double register event was detected -- it means you have bug in kernel. >>>> >>>> Yes, you can add BUG here and crash the host immediately, but I prefer to >>>> use warning in such situations. >>>> >>>>>> If this does not cause any additional problems or slow downs, it's >>>>>> probably fine to add. >>>>>> >>>>> Notifier_chain_register() is not a system hotspot function. >>>>> At the same time, there is already a WARN_ONCE judgment. There is no new >>>>> judgment in the new patch. >>>>> It only changes the processing under the condition of (*nl) == n, which >>>>> will not cause performance problems. >>>>> At the same time, avoiding the formation of a link ring can make the >>>>> system more robust. >>>> >>>> I disagree, >>>> yes, node will have correct list, but anyway node will work wrong and can >>>> crash the host in any time. >>> >>> Sorry, my description is not accurate. >>> >>> My patch feature does not prevent users from repeatedly registering hooks. >>> But avoiding the chain ring caused by the user repeatedly registering the >>> hook >>> >>> There are no modules for duplicate registration hooks in the current system. >>> But considering that not all modules are in the kernel source tree, >>> In order to improve the robustness of the kernel API, we should avoid the >>> linked list ring caused by repeated registration. >>> Or in order to improve the efficiency of problem location, when the >>> duplicate registration is checked, the system crashes directly. >> >> Detect of duplicate registration means an unrecoverable error, >> from this point of view it makes sense to replace WARN_ONCE by BUG_ON. >> >>> On the other hand, the difference between notifier_chain_register() and >>> notifier_chain_cond_register() for duplicate registrations is confusing: >>> Blocking the formation of the linked list ring in >>> notifier_chain_cond_register() >>> There is no interception of the linked list ring in >>> notifier_chain_register(), just an alarm. >>> Give me the illusion: Isn't notifier_chain_register() allowed to create a >>> linked list ring? >> >> I'm not sure that I understood your question correctly but will try to >> answer. >> As far as I see all callers of notifier_chain_cond_register checks return >> value, expect possible failure and handle it somehow. >> On the other hand callers of notifier_chain_register() in many cases do not >> check return value and always expect success. >> The goal of original WARN_ONCE -- to detect possible misuse of notifiers and >> it seems for me it correctly handles this task. >> > Notifier_chain_cond_register() has only one return value: 0
It looks wrong for me. > At the same time, it is only called by > blocking_notifier_chain_cond_register(). > In the function comment of blocking_notifier_chain_cond_register there is " > Currently always returns zero." > Therefore, the user cannot check whether the hook has duplicate registration > or other errors by checking the return value. I think notifier_chain_cond_register can be changed to return error. It is safe now, all its in-tree callers checks return value and can properly react on such error. On the other hand, in all cases notifier_chain_cond_register are __init functions, they are called once only and double registration seems is impossible here: even if some old notifier was lost and was not properly unregistered, new one will have another address. And even if these addresses was equal -- it is critical error and I prefer to generate warning instead of silent failure of module load. > If the interceptor list ring is added to notifier_chain_register(), > notifier_chain_register() > And notifier_chain_cond_register() becomes redundant code, we can delete one > of them Yes, I'm agree, at present there are no difference between notifier_chain_cond_register() and notifier_chain_register() Question is -- how to improve it. You propose to remove notifier_chain_cond_register() by some way. Another option is return an error, for some abstract callers who expect possible double registration. Frankly speaking I prefer second one, however because of kernel do not have any such callers right now seems you are right, and we can delete notifier_chain_cond_register(). So let me finally accept your patch-set. Thank you, Vasily Averin