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SubjectRe: [PATCH v4 3/3] livepatch: add atomic replace
On Thu 2017-10-12 17:12:29, Jason Baron wrote:
> Sometimes we would like to revert a particular fix. This is currently
> This is not easy because we want to keep all other fixes active and we
> could revert only the last applied patch.
>
> One solution would be to apply new patch that implemented all
> the reverted functions like in the original code. It would work
> as expected but there will be unnecessary redirections. In addition,
> it would also require knowing which functions need to be reverted at
> build time.
>
> A better solution would be to say that a new patch replaces
> an older one. This might be complicated if we want to replace
> a particular patch. But it is rather easy when a new cummulative
> patch replaces all others.
>
> diff --git a/kernel/livepatch/core.c b/kernel/livepatch/core.c
> index f53eed5..d1c7a06 100644
> --- a/kernel/livepatch/core.c
> +++ b/kernel/livepatch/core.c
> @@ -283,8 +301,21 @@ static int klp_write_object_relocations(struct module *pmod,
> return ret;
> }
>
> +atomic_t klp_nop_release_count;
> +static DECLARE_WAIT_QUEUE_HEAD(klp_nop_release_wait);
> +
> static void klp_kobj_release_object(struct kobject *kobj)
> {
> + struct klp_object *obj;
> +
> + obj = container_of(kobj, struct klp_object, kobj);
> + /* Free dynamically allocated object */
> + if (!obj->funcs) {
> + kfree(obj->name);
> + kfree(obj);
> + atomic_dec(&klp_nop_release_count);
> + wake_up(&klp_nop_release_wait);

I would slightly optimize this by

if (atomic_dec_and_test((&klp_nop_release_count))
wake_up(&klp_nop_release_wait);

> + }
> }
>
> static struct kobj_type klp_ktype_object = {
> @@ -294,6 +325,16 @@ static struct kobj_type klp_ktype_object = {
>
> static void klp_kobj_release_func(struct kobject *kobj)
> {
> + struct klp_func *func;
> +
> + func = container_of(kobj, struct klp_func, kobj);
> + /* Free dynamically allocated functions */
> + if (!func->new_func) {
> + kfree(func->old_name);
> + kfree(func);
> + atomic_dec(&klp_nop_release_count);
> + wake_up(&klp_nop_release_wait);

Same here

if (atomic_dec_and_test((&klp_nop_release_count))
wake_up(&klp_nop_release_wait);


> + }
> }
>
> static struct kobj_type klp_ktype_func = {
> @@ -436,8 +480,14 @@ static int klp_init_object(struct klp_patch *patch, struct klp_object *obj)
> if (ret)
> return ret;
>
> - klp_for_each_func(obj, func) {
> - ret = klp_init_func(obj, func);
> + list_add(&obj->obj_entry, &patch->obj_list);
> + INIT_LIST_HEAD(&obj->func_list);
> +
> + if (nop)
> + return 0;

Ah, this is something that I wanted to avoid. It makes the code
very hard to read and maintain. It forces us to duplicate
some code in klp_alloc_nop_func(). I think that I complained
about this in v2 already.

I understand that you actually kept it because of me.
It is related to the possibility to re-enable released
patches :-(

The klp_init_*() stuff is called from __klp_enable_patch()
for the "nop" functions now. And it has already been called
for the statically defined structures in klp_register_patch().
Therefore we need to avoid calling it twice for the static
structures.

One solution would be to do these operations for the statically
defined structures in __klp_enable_patch() as well. But this
would mean a big redesign of the code.

Another solution would be to give up on the idea that the replaced
patches might be re-enabled without re-loading. I am afraid
that this the only reasonable approach. It will help to avoid
also the extra klp_replaced_patches list. All this will help to
make the code much easier.

I am really sorry that I asked you to do this exercise and
support the patch re-enablement. It looked like a good idea.
I did not expect that it would be that complicated.

I stop reviewing this patch because it will look quite
different again. I will only keep some random comments
around that I added before finding this main design flaw.

Thanks a lot for the hard work. v4 looks much better than
v2 in many ways. I think that we are going on the right way.
> +
> + klp_for_each_func_static(obj, func) {
> + ret = klp_init_func(obj, func, false);
> if (ret)
> goto free;
> }
> @@ -456,6 +506,226 @@ static int klp_init_object(struct klp_patch *patch, struct klp_object *obj)
> return ret;
> }

[...]

> +/* Add 'nop' functions which simply return to the caller to run
> + * the original function. The 'nop' functions are added to a
> + * patch to facilitate a 'replace' mode
> + */
> +static int klp_add_nops(struct klp_patch *patch)
> +{
> + struct klp_patch *old_patch;
> + struct klp_object *old_obj;
> + int err = 0;
> +
> + if (!patch->replace)
> + return 0;

It would be more sane if this returns -EINVAL and
we call the following in __klp_enable_patch().

if (patch->replace) {
ret = klp_add_nops(patch);
...
}
Otherwise, people might wonder why klp_add_nops() is always called.
They would need to look how it is implemented just to realize that
it is a NOP when patch->replace is not set.

> +
> + list_for_each_entry(old_patch, &klp_patches, list) {
> + if (old_patch == patch)
> + break;
> +
> + klp_for_each_object(old_patch, old_obj) {
> + err = klp_add_nop_object(patch, old_obj);
> + if (err) {
> + klp_remove_nops(patch);
> + return err;
> + }
> + }
> + }
> +
> + return 0;
> +}
> +
> static int __klp_disable_patch(struct klp_patch *patch)
> {
> if (klp_transition_patch)
> @@ -527,10 +798,22 @@ static int __klp_enable_patch(struct klp_patch *patch)
> if (WARN_ON(patch->enabled))
> return -EINVAL;
>
> + if (klp_is_patch_replaced(patch)) {
> + list_move_tail(&patch->list, &klp_patches);
> + replaced = true;
> + }
> +
> /* enforce stacking: only the first disabled patch can be enabled */
> if (patch->list.prev != &klp_patches &&
> - !list_prev_entry(patch, list)->enabled)
> - return -EBUSY;
> + !list_prev_entry(patch, list)->enabled) {
> + ret = -EBUSY;
> + goto cleanup_list;
> + }
> +
> + /* setup nops */

Please, remove the comment. It does not add much information.

> + ret = klp_add_nops(patch);
> + if (ret)
> + goto cleanup_list;

It will be more obvious when we do:

if (patch->replace) {
ret = klp_add_nops(patch);
if (ret)
goto cleanup_list;
}
>
> /*
> * A reference is taken on the patch module to prevent it from being

Best Regards,
Petr

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