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

-- Joe

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