Messages in this thread | | | Date | Fri, 1 Apr 2022 17:12:59 +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 11:08:02AM +0200, Jakob Koschel wrote: > > > > On 1. Apr 2022, at 11:05, Li Fei1 <fei1.li@intel.com> wrote: > > > > 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. > > Now I'm very confused. > Didn't you say the dev_msg() can be called with has_pending == false && retry == 0? > Then the 'client' used in the dev_msg() cannot be valid. I think I would not reply you before you understand how ACRN ioreq works.
> > >> > >> 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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