Messages in this thread | | | Date | Fri, 1 Apr 2022 17:05:45 +0800 | From | Li Fei1 <> | Subject | Re: [PATCH] virt: acrn: fix invalid check past list iterator |
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On Fri, Apr 01, 2022 at 10:50:51AM +0200, Jakob Koschel wrote: > > > > On 1. Apr 2022, at 09:57, Li Fei1 <fei1.li@intel.com> wrote: > > > > On Fri, Apr 01, 2022 at 09:16:48AM +0200, Jakob Koschel wrote: > >> > >> > >>> On 1. Apr 2022, at 05:57, Li Fei1 <fei1.li@intel.com> wrote: > >>> > >>> On Fri, Apr 01, 2022 at 05:22:36AM +0200, Jakob Koschel wrote: > >>>> > >>>>> On 1. Apr 2022, at 03:15, Li Fei1 <fei1.li@intel.com> wrote: > >>>>> > >>>>> On Thu, Mar 31, 2022 at 01:20:50PM +0200, Jakob Koschel wrote: > >>>>>> > >>>>>>> On 30. Mar 2022, at 09:57, Li Fei1 <fei1.li@intel.com> wrote: > >>>>>>> > >>>>>>> On Sat, Mar 19, 2022 at 09:38:19PM +0100, Jakob Koschel wrote: > >>>>>>>> The condition retry == 0 is theoretically possible even if 'client' > >>>>>>>> does not point to a valid element because no break was hit. > >>>>>>>> > >>>>>>>> To only execute the dev_warn if actually a break within the loop was > >>>>>>>> hit, a separate variable is used that is only set if it is ensured to > >>>>>>>> point to a valid client struct. > >>>>>>>> > >>>>>>> Hi Koschel > >>>>>>> > >>>>>>> Thanks for you to help us to try to improve the code. Maybe you don't get the point. > >>>>>>> The dev_warn should only been called when has_pending = true && retry == 0 > >>>>>> > >>>>>> Maybe I don't understand but looking isolated at this function I could see a way to call > >>>>>> the dev_warn() with has_pending = false && retry == 0. > >>>>> Yes, even has_pending = true && retry == 0 at the beginning, we could not make sure > >>>>> has_pending is true after schedule_timeout_interruptible and we even didn't check > >>>>> there're other pending client on the ioreq_clients (because we can't make sure > >>>>> when we dev_warn this clent is still pending). So we just use dev_warn not higher log level. > >>>> > >>>> I'm sorry, I don't quite understand what you mean by that. > >>>> Do you agree that has_pending = false && retry == 0 is possible when calling the dev_warn() > >>>> or not? > >>>> > >>> Yes, so what ? It just a hint there may have pending request. > >> > >> if has_pending == false && retry == 0 when calling dev_warn() there are very clear > >> dependencies met: > >> > >> * has_pending == false means that the list_for_each_entry() macro it *not* exit early. > >> * since list_for_each_entry() did *not* exit early, client will not hold a valid list entry > >> * using client->name is not safe and will not point to a valid string (perhaps not even an address) > > So you'd better to check when the client in ioreq_clients would been destroyed, right ? > > I'm afraid I don't know exactly what you mean here. > > More specifically: > to check what? and I'm also not sure what you mean with "when client in ioreq_clients would been destroyed". > > Actually thinking about it more the check should be > > if (client && retry == 0) > > to be correct. The client in ioreq_clients would always been "valid" (here valid means the client struct would not been destroyed) when this function been called. That's guaranteed by the code logic. > > if has_pending == false I read the code as if no client was found that has a pending request > so I don't follow how: > > "%s cannot flush pending request!\n", client->name); > > can be valid since no client has a pending request. > > > > >> > >> I'm *only* talking about the case where has_pending == false, in case that's not clear. > >> > >> > >>> Even retry == 0 && has_pending = true, > >>> When we call dev_warn, the clent may not is pending. > >>> > >>> > >>>>>> > >>>>>>> list_for_each_entry(client, &vm->ioreq_clients, list) { > >>>>>>> has_pending = has_pending_request(client); > >>>>>>> if (has_pending) > >>>>>>> } > >>>>>>> spin_unlock_bh(&vm->ioreq_clients_lock); > >>>>>> > >>>>>> imagine has_pending == false && retry == 1 here, then client will not hold a valid list entry. > >>>>> What do you mean "client will not hold a valid list entry" ? > >>>> > >>>> Imagine a very simple example: > >>>> > >>>> struct acrn_ioreq_client *client; > >>>> list_for_each_entry(client, &vm->ioreq_clients, list) { > >>>> continue; > >>>> } > >>>> > >>>> dev_warn(acrn_dev.this_device, > >>>> "%s cannot flush pending request!\n", client->name); /* NOT GOOD */ > >>>> > >>> If there're pending request, we would call schedule to schedule out then schedule back > >>> to check the list from the beginning. If there's no pending client, we point to the last > >>> client and break out the while loop. > >>> > >>> The code doesn't want to find the pending client and break out the while loop and call > >>> dev_warn. Please see the function comment. > >>> > >>> > >>>> > >>>> Since there is no break for the list_for_each_entry() iterator to return early, > >>>> client *cannot* be a valid entry of the list. In fact if you look at the list_for_each_entry() > >>>> macro, it will be an 'bogus' pointer, pointing somewhere into 'vm'. > >>>> Essentially before the terminating condition of the list traversal the following code is called: > >>>> > >>>> list_entry(&vm->ioreq_clients, struct acrn_ioreq_client *, list); > >>>> > >>>> resulting in a: > >>>> > >>>> container_of(&vm->ioreq_clients, struct acrn_ioreq_client *, list); > >>>> > >>>> &vm->ioreq_clients however is not contained in a struct acrn_ioreq_client, making > >>>> this call compute an invalid pointer. > >>>> Therefore using 'client' as in the example above (e.g. client->name) after the loop is > >>>> not safe. Since the loop can never return early in the simple example above it will > >>>> always break. On other cases (the one we are discussing here) there might be a chance that > >>>> there is one code path (in theory) where the loop did not exit early and 'client' > >>>> holds that 'invalid entry'. > >>>> > >>>> This would be the case with has_pending = false && retry == 0. > >>>> > >>>> I hope this makes sense. > >>>> > >>>>> > >>>>>> > >>>>>>> > >>>>>>> if (has_pending) > >>>>>>> schedule_timeout_interruptible(HZ / 100); > >>>>>>> } while (has_pending && --retry > 0); > >>>>>> > >>>>>> since has_pending && --retry > 0 is no longer true the loop stops. > >>>>>> > >>>>>>> if (retry == 0) > >>>>>>> dev_warn(acrn_dev.this_device, > >>>>>>> "%s cannot flush pending request!\n", client->name); > >>>>>> client->name is accessed since retry == 0 now, but client is not a valid struct ending up > >>>>>> in a type confusion. > >>>>>> > >>>>>>> > >>>>>>> If retry > 0 and has_pending is true, we would call schedule_timeout_interruptible > >>>>>>> to schedule out to wait all the pending I/O requests would been completed. > >>>>>>> > >>>>>>> Thanks. > >>>>>> > >>>>>> Again, I'm not sure if this is realistically possible. I'm trying to remove > >>>>>> any use of the list iterator after the loop to make such potentially issues detectable > >>>>> You may think we still in the loop (could we ?), even that we didn't write the list iterator then. > >>>> > >>>> I'm not exactly sure which loop you are referring to but no, I don't think we are still in > >>>> the do while loop. > >>>> > >>>> The only thing we know after the do while loop is that: !has_pending || retry == 0 > >>>> And iff has_pending && retry == 0, then we shouldn't call the dev_warn(). > >>>> > >>>>>> at compile time instead of relying on certain (difficult to maintain) conditions to be met > >>>>>> to avoid the type confusion. > >>>>>> > >>>>>> Thanks, > >>>>>> Jakob > >>>> > >>>> Jakob > >> > >> Jakob >
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