Re: [PATCH 0/3] livepatch: cleanup kpl_patch kobject release

[Ming Lei <ming.lei@redhat.com>]

On Sun, Oct 31, 2021 at 10:26:18PM -0400, Joe Lawrence wrote:
> On Sat, Oct 30, 2021 at 12:36:28AM +0800, Ming Lei wrote:
> > On Fri, Oct 29, 2021 at 09:51:54AM -0400, Joe Lawrence wrote:
> > > On Thu, Oct 28, 2021 at 08:57:31PM +0800, Ming Lei wrote:
> > > > Hello,
> > > >
> > > > The 1st patch moves module_put() to release handler of klp_patch
> > > > kobject.
> > > >
> > > > The 2nd patch changes to free klp_patch and other kobjects without
> > > > klp_mutex.
> > > >
> > > > The 3rd patch switches to synchronous kobject release for klp_patch.
> > > >
> > > 
> > > Hi Ming,
> > > 
> > > I gave the patchset a spin on top of linus tree @ 1fc596a56b33 and ended
> > > up with a stuck task:
> > > 
> > > Test
> > > ----
> > > Enable the livepatch selftests:
> > >   $ grep CONFIG_TEST_LIVEPATCH .config
> > >   CONFIG_TEST_LIVEPATCH=m
> > > 
> > > Run a continuous kernel build in the background:
> > >   $ while (true); do make clean && make -j$(nproc); done
> > > 
> > > While continuously executing the selftests:
> > >   $ while (true); do make -C tools/testing/selftests/livepatch/ run_tests; done
> > > 
> > > Results
> > > -------
> > 
> > Hello Joe,
> > 
> > Thanks for the test!
> > 
> > Can you replace the 3rd patch with the following one then running the test again?
> > 
> > From 599e96f79aebc388ef3854134312c6039a7884bf Mon Sep 17 00:00:00 2001
> > From: Ming Lei <ming.lei@redhat.com>
> > Date: Thu, 28 Oct 2021 20:11:23 +0800
> > Subject: [PATCH 3/3] livepatch: free klp_patch object synchronously
> > 
> > klp_mutex isn't acquired before calling kobject_put(klp_patch), so it is
> > fine to free klp_patch object synchronously.
> > 
> > One issue is that enabled store() method, in which the klp_patch kobject
> > itself is deleted & released. However, sysfs has provided APIs for dealing
> > with this corner case, so use sysfs_break_active_protection() and
> > sysfs_unbreak_active_protection() for releasing klp_patch kobject from
> > enabled_store().
> > 
> > Signed-off-by: Ming Lei <ming.lei@redhat.com>
> > ---
> >  include/linux/livepatch.h     |  1 -
> >  kernel/livepatch/core.c       | 32 +++++++++++++-------------------
> >  kernel/livepatch/core.h       |  2 +-
> >  kernel/livepatch/transition.c |  2 +-
> >  4 files changed, 15 insertions(+), 22 deletions(-)
> > 
> > diff --git a/include/linux/livepatch.h b/include/linux/livepatch.h
> > index 9712818997c5..4dcebf52fac5 100644
> > --- a/include/linux/livepatch.h
> > +++ b/include/linux/livepatch.h
> > @@ -169,7 +169,6 @@ struct klp_patch {
> >  	struct list_head obj_list;
> >  	bool enabled;
> >  	bool forced;
> > -	struct work_struct free_work;
> >  };
> >  
> >  #define klp_for_each_object_static(patch, obj) \
> > diff --git a/kernel/livepatch/core.c b/kernel/livepatch/core.c
> > index 9ede093d699a..6cfc54f6bdcc 100644
> > --- a/kernel/livepatch/core.c
> > +++ b/kernel/livepatch/core.c
> > @@ -337,6 +337,7 @@ static ssize_t enabled_store(struct kobject *kobj, struct kobj_attribute *attr,
> >  	int ret;
> >  	bool enabled;
> >  	LIST_HEAD(to_free);
> > +	struct kernfs_node *kn = NULL;
> >  
> >  	ret = kstrtobool(buf, &enabled);
> >  	if (ret)
> > @@ -369,7 +370,14 @@ static ssize_t enabled_store(struct kobject *kobj, struct kobj_attribute *attr,
> >  out:
> >  	mutex_unlock(&klp_mutex);
> >  
> > -	klp_free_patches_async(&to_free);
> > +	if (list_empty(&to_free)) {
> > +		kn = sysfs_break_active_protection(kobj, &attr->attr);
> > +		WARN_ON_ONCE(!kn);
> > +		sysfs_remove_file(kobj, &attr->attr);
> > +		klp_free_patches(&to_free);
> > +		if (kn)
> > +			sysfs_unbreak_active_protection(kn);
> > +	}
> >  
> >  	if (ret)
> >  		return ret;
> > @@ -684,32 +692,19 @@ static void klp_free_patch_finish(struct klp_patch *patch)
> >  	kobject_put(&patch->kobj);
> >  }
> >  
> > -/*
> > - * The livepatch might be freed from sysfs interface created by the patch.
> > - * This work allows to wait until the interface is destroyed in a separate
> > - * context.
> > - */
> > -static void klp_free_patch_work_fn(struct work_struct *work)
> > -{
> > -	struct klp_patch *patch =
> > -		container_of(work, struct klp_patch, free_work);
> > -
> > -	klp_free_patch_finish(patch);
> > -}
> > -
> > -static void klp_free_patch_async(struct klp_patch *patch)
> > +static void klp_free_patch(struct klp_patch *patch)
> >  {
> >  	klp_free_patch_start(patch);
> > -	schedule_work(&patch->free_work);
> > +	klp_free_patch_finish(patch);
> >  }
> >  
> > -void klp_free_patches_async(struct list_head *to_free)
> > +void klp_free_patches(struct list_head *to_free)
> >  {
> >  	struct klp_patch *patch, *tmp_patch;
> >  
> >  	list_for_each_entry_safe(patch, tmp_patch, to_free, list) {
> >  		list_del_init(&patch->list);
> > -		klp_free_patch_async(patch);
> > +		klp_free_patch(patch);
> >  	}
> >  }
> >  
> > @@ -873,7 +868,6 @@ static int klp_init_patch_early(struct klp_patch *patch)
> >  	kobject_init(&patch->kobj, &klp_ktype_patch);
> >  	patch->enabled = false;
> >  	patch->forced = false;
> > -	INIT_WORK(&patch->free_work, klp_free_patch_work_fn);
> >  
> >  	klp_for_each_object_static(patch, obj) {
> >  		if (!obj->funcs)
> > diff --git a/kernel/livepatch/core.h b/kernel/livepatch/core.h
> > index 8ff97745ba40..ea593f370049 100644
> > --- a/kernel/livepatch/core.h
> > +++ b/kernel/livepatch/core.h
> > @@ -13,7 +13,7 @@ extern struct list_head klp_patches;
> >  #define klp_for_each_patch(patch)	\
> >  	list_for_each_entry(patch, &klp_patches, list)
> >  
> > -void klp_free_patches_async(struct list_head *to_free);
> > +void klp_free_patches(struct list_head *to_free);
> >  void klp_unpatch_replaced_patches(struct klp_patch *new_patch);
> >  void klp_discard_nops(struct klp_patch *new_patch);
> >  
> > diff --git a/kernel/livepatch/transition.c b/kernel/livepatch/transition.c
> > index a9ebc9c5db02..3eff5fc0deee 100644
> > --- a/kernel/livepatch/transition.c
> > +++ b/kernel/livepatch/transition.c
> > @@ -41,7 +41,7 @@ static void klp_transition_work_fn(struct work_struct *work)
> >  
> >  	mutex_unlock(&klp_mutex);
> >  
> > -	klp_free_patches_async(&to_free);
> > +	klp_free_patches(&to_free);
> >  }
> >  static DECLARE_DELAYED_WORK(klp_transition_work, klp_transition_work_fn);
> >  
> > -- 
> > 2.31.1
> > 
> > 
> 
> Hi Ming,
> 
> The previous test runs without hung tasks, but I noticed that is
> probably because the selftests quickly wedge.  A simple reproducer for
> that:
> 
> # (background load)
> % while(true); do make clean && make -j$(nproc); done
> vs.
> # (selftests)
> % while(true); do ./tools/testing/selftests/livepatch/test-livepatch.sh || break; done
> TEST: basic function patching ... ok
> TEST: multiple livepatches ... ok
> TEST: atomic replace livepatch ... ok
> TEST: basic function patching ... ok
> TEST: multiple livepatches ... ok
> TEST: atomic replace livepatch ... ok
> TEST: basic function patching ... ok
> TEST: multiple livepatches ... ERROR: failed to disable livepatch test_klp_livepatch
> 
> % lsmod | grep test_klp
> test_klp_livepatch     16384  1
> 
> % cat /sys/kernel/livepatch/test_klp_livepatch/enabled 
> 0
> 
> % rmmod test_klp_livepatch
> rmmod: ERROR: Module test_klp_livepatch is in use
> 
> [  249.870587] ===== TEST: multiple livepatches =====
> [  249.885520] % modprobe test_klp_livepatch
> [  249.916987] livepatch: enabling patch 'test_klp_livepatch'
> [  249.923131] livepatch: 'test_klp_livepatch': initializing patching transition
> [  249.925575] livepatch: 'test_klp_livepatch': starting patching transition
> [  249.934215] livepatch: 'test_klp_livepatch': completing patching transition
> [  249.934337] livepatch: 'test_klp_livepatch': patching complete
> [  249.946294] test_klp_livepatch: this has been live patched
> [  249.963337] % modprobe test_klp_atomic_replace replace=0
> [  249.996371] livepatch: enabling patch 'test_klp_atomic_replace'
> [  250.002998] livepatch: 'test_klp_atomic_replace': initializing patching transition
> [  250.005333] livepatch: 'test_klp_atomic_replace': starting patching transition
> [  250.014364] livepatch: 'test_klp_atomic_replace': completing patching transition
> [  250.014471] livepatch: 'test_klp_atomic_replace': patching complete
> [  250.027259] test_klp_livepatch: this has been live patched
> [  250.036050] test_klp_atomic_replace: this has been live patched
> [  250.046347] % echo 0 > /sys/kernel/livepatch/test_klp_atomic_replace/enabled
> [  250.054403] livepatch: 'test_klp_atomic_replace': initializing unpatching transition
> [  250.054574] livepatch: 'test_klp_atomic_replace': starting unpatching transition
> [  251.764849] livepatch: 'test_klp_atomic_replace': completing unpatching transition
> [  251.799266] livepatch: 'test_klp_atomic_replace': unpatching complete
> [  251.911706] % rmmod test_klp_atomic_replace
> [  251.970438] test_klp_livepatch: this has been live patched
> [  251.980347] % echo 0 > /sys/kernel/livepatch/test_klp_livepatch/enabled
> [  251.987936] livepatch: 'test_klp_livepatch': initializing unpatching transition
> [  251.988115] livepatch: 'test_klp_livepatch': starting unpatching transition
> [  251.997033] livepatch: 'test_klp_livepatch': completing unpatching transition
> [  252.027090] livepatch: 'test_klp_livepatch': unpatching complete
> [  313.289932] ERROR: failed to disable livepatch test_klp_livepatch
> 
> In this case, the "failed to disable" msg occurs because the sysfs
> interface / module remain present.

Hi Joe,

Thanks for your test!

Hammm, the check of list_empty() in enabled_store() is inverted, please try the
V3:

https://lore.kernel.org/lkml/20211101112548.3364086-4-ming.lei@redhat.com/T/#u


Thanks,
Ming