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
> 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. > thanks, > > greg k-h > > . > Thanks Xiaoming Ni