Re: [PATCH v4 13/15] livepatch: change to a per-task consistency model

Josh Poimboeuf
2017-02-03T20:39:23Z

On Thu, Feb 02, 2017 at 12:51:16PM +0100, Petr Mladek wrote:
> !!! This is the right version. I am sorry again for the confusion. !!!
>
> > Change livepatch to use a basic per-task consistency model. This is the
> > foundation which will eventually enable us to patch those ~10% of
> > security patches which change function or data semantics. This is the
> > biggest remaining piece needed to make livepatch more generally useful.
> >
> > diff --git a/Documentation/livepatch/livepatch.txt b/Documentation/livepatch/livepatch.txt
> > index 7f04e13..fb00d66 100644
> > --- a/Documentation/livepatch/livepatch.txt
> > +++ b/Documentation/livepatch/livepatch.txt
> > +3.1 Adding consistency model support to new architectures
> > +---------------------------------------------------------
> > +
> > +For adding consistency model support to new architectures, there are a
> > +few options:
> > +
> > +1) Add CONFIG_HAVE_RELIABLE_STACKTRACE. This means porting objtool, and
> > + for non-DWARF unwinders, also making sure there's a way for the stack
> > + tracing code to detect interrupts on the stack.
> > +
> > +2) Alternatively, figure out a way to patch kthreads without stack
> > + checking. If all kthreads sleep in the same place, then we can
> > + designate that place as a patching point. I think Petr M has been
> > + working on that?
>
> Here is version with some more details:
>
> Alternatively, every kthread has to explicitely switch
> current->patch_state on a safe place. Kthreads are typically
> "infinite" loops that do some action repeatedly. The safe
> location is between the loops when there are no locks
> taken and all data structures are in a well defined state.
>
> The location is clear when using the workqueues or the kthread worker
> API. These kthreads process "independent" works in a generic loop.
>
> It is much more complicated with kthreads using a custom loop.
> There the safe place must be carefully localized case by case.

Good clarification.

> > + In that case, arches without
> > + HAVE_RELIABLE_STACKTRACE would still be able to use the
> > + non-stack-checking parts of the consistency model:
> > +
> > + a) patching user tasks when they cross the kernel/user space
> > + boundary; and
> > +
> > + b) patching kthreads and idle tasks at their designated patch points.
> > +
> > + This option isn't as good as option 1 because it requires signaling
> > + most of the tasks to patch them.
>
> Kthreads need to be waken because most of them ignore signals.
>
> And idle tasks might need to be explicitely scheduled if there
> is too big load. Mirek knows more about this.
>
> Well, I am not sure if you want to get into such details.
>
>
> > + But it could still be a good backup
> > + option for those architectures which don't have reliable stack traces
> > + yet.
> > +
> > +In the meantime, patches for such architectures can bypass the
> > +consistency model by setting klp_patch.immediate to true.
>
> I would add that this is perfectly fine for patches that do not
> change semantic of the patched functions. In practice, this is
> usable in about 90% of security and critical fixes.

Another good one.

> > 4. Livepatch module
> > @@ -134,7 +242,7 @@ Documentation/livepatch/module-elf-format.txt for more details.
> >
> >
> > 4.2. Metadata
> > -------------
> > +-------------
> >
> > The patch is described by several structures that split the information
> > into three levels:
> > @@ -239,9 +347,15 @@ Registered patches might be enabled either by calling klp_enable_patch() or
> > by writing '1' to /sys/kernel/livepatch/<name>/enabled. The system will
> > start using the new implementation of the patched functions at this stage.
> >
> > -In particular, if an original function is patched for the first time, a
> > -function specific struct klp_ops is created and an universal ftrace handler
> > -is registered.
> > +When a patch is enabled, livepatch enters into a transition state where
> > +tasks are converging to the patched state. This is indicated by a value
> > +of '1' in /sys/kernel/livepatch/<name>/transition. Once all tasks have
> > +been patched, the 'transition' value changes to '0'. For more
> > +information about this process, see the "Consistency model" section.
> > +
> > +If an original function is patched for the first time, a function
> > +specific struct klp_ops is created and an universal ftrace handler is
> > +registered.
> >
> > Functions might be patched multiple times. The ftrace handler is registered
> > only once for the given function. Further patches just add an entry to the
> > @@ -261,6 +375,12 @@ by writing '0' to /sys/kernel/livepatch/<name>/enabled. At this stage
> > either the code from the previously enabled patch or even the original
> > code gets used.
> >
> > +When a patch is disabled, livepatch enters into a transition state where
> > +tasks are converging to the unpatched state. This is indicated by a
> > +value of '1' in /sys/kernel/livepatch/<name>/transition. Once all tasks
> > +have been unpatched, the 'transition' value changes to '0'. For more
> > +information about this process, see the "Consistency model" section.
> > +
> > Here all the functions (struct klp_func) associated with the to-be-disabled
> > patch are removed from the corresponding struct klp_ops. The ftrace handler
> > is unregistered and the struct klp_ops is freed when the func_stack list
> > diff --git a/include/linux/init_task.h b/include/linux/init_task.h
> > index 325f649..25f0360 100644
> > --- a/include/linux/init_task.h
> > +++ b/include/linux/init_task.h
> > @@ -14,6 +14,7 @@
> > #include <linux/rbtree.h>
> > #include <net/net_namespace.h>
> > #include <linux/sched/rt.h>
> > +#include <linux/livepatch.h>
> >
> > #include <asm/thread_info.h>
> >
> > @@ -185,6 +186,13 @@ extern struct task_group root_task_group;
> > # define INIT_KASAN(tsk)
> > #endif
> >
> > +#ifdef CONFIG_LIVEPATCH
> > +# define INIT_LIVEPATCH(tsk) \
> > + .patch_state = KLP_UNDEFINED,
> > +#else
> > +# define INIT_LIVEPATCH(tsk)
> > +#endif
> > +
> > #ifdef CONFIG_THREAD_INFO_IN_TASK
> > # define INIT_TASK_TI(tsk) \
> > .thread_info = INIT_THREAD_INFO(tsk), \
> > @@ -271,6 +279,7 @@ extern struct task_group root_task_group;
> > INIT_VTIME(tsk) \
> > INIT_NUMA_BALANCING(tsk) \
> > INIT_KASAN(tsk) \
> > + INIT_LIVEPATCH(tsk) \
> > }
> >
> >
> > diff --git a/include/linux/livepatch.h b/include/linux/livepatch.h
> > index 6602b34..ed90ad1 100644
> > --- a/include/linux/livepatch.h
> > +++ b/include/linux/livepatch.h
> > @@ -28,18 +28,40 @@
> >
> > #include <asm/livepatch.h>
> >
> > +/* task patch states */
> > +#define KLP_UNDEFINED -1
> > +#define KLP_UNPATCHED 0
> > +#define KLP_PATCHED 1
> > +
> > /**
> > * struct klp_func - function structure for live patching
> > * @old_name: name of the function to be patched
> > * @new_func: pointer to the patched function code
> > * @old_sympos: a hint indicating which symbol position the old function
> > * can be found (optional)
> > + * @immediate: patch the func immediately, bypassing safety mechanisms
> > * @old_addr: the address of the function being patched
> > * @kobj: kobject for sysfs resources
> > * @stack_node: list node for klp_ops func_stack list
> > * @old_size: size of the old function
> > * @new_size: size of the new function
> > * @patched: the func has been added to the klp_ops list
> > + * @transition: the func is currently being applied or reverted
> > + *
> > + * The patched and transition variables define the func's patching state. When
> > + * patching, a func is always in one of the following states:
> > + *
> > + * patched=0 transition=0: unpatched
> > + * patched=0 transition=1: unpatched, temporary starting state
> > + * patched=1 transition=1: patched, may be visible to some tasks
> > + * patched=1 transition=0: patched, visible to all tasks
> > + *
> > + * And when unpatching, it goes in the reverse order:
> > + *
> > + * patched=1 transition=0: patched, visible to all tasks
> > + * patched=1 transition=1: patched, may be visible to some tasks
> > + * patched=0 transition=1: unpatched, temporary ending state
> > + * patched=0 transition=0: unpatched
> > */
> > struct klp_func {
> > /* external */
> > @@ -53,6 +75,7 @@ struct klp_func {
> > * in kallsyms for the given object is used.
> > */
> > unsigned long old_sympos;
> > + bool immediate;
> >
> > /* internal */
> > unsigned long old_addr;
> > @@ -60,6 +83,7 @@ struct klp_func {
> > struct list_head stack_node;
> > unsigned long old_size, new_size;
> > bool patched;
> > + bool transition;
> > };
> >
> > /**
> > @@ -68,7 +92,7 @@ struct klp_func {
> > * @funcs: function entries for functions to be patched in the object
> > * @kobj: kobject for sysfs resources
> > * @mod: kernel module associated with the patched object
> > - * (NULL for vmlinux)
> > + * (NULL for vmlinux)
> > * @patched: the object's funcs have been added to the klp_ops list
> > */
> > struct klp_object {
> > @@ -86,6 +110,7 @@ struct klp_object {
> > * struct klp_patch - patch structure for live patching
> > * @mod: reference to the live patch module
> > * @objs: object entries for kernel objects to be patched
> > + * @immediate: patch all funcs immediately, bypassing safety mechanisms
> > * @list: list node for global list of registered patches
> > * @kobj: kobject for sysfs resources
> > * @enabled: the patch is enabled (but operation may be incomplete)
> > @@ -94,6 +119,7 @@ struct klp_patch {
> > /* external */
> > struct module *mod;
> > struct klp_object *objs;
> > + bool immediate;
> >
> > /* internal */
> > struct list_head list;
> > @@ -121,13 +147,27 @@ void arch_klp_init_object_loaded(struct klp_patch *patch,
> > int klp_module_coming(struct module *mod);
> > void klp_module_going(struct module *mod);
> >
> > +void klp_copy_process(struct task_struct *child);
> > void klp_update_patch_state(struct task_struct *task);
> >
> > +static inline bool klp_patch_pending(struct task_struct *task)
> > +{
> > + return test_tsk_thread_flag(task, TIF_PATCH_PENDING);
> > +}
> > +
> > +static inline bool klp_have_reliable_stack(void)
> > +{
> > + return IS_ENABLED(CONFIG_STACKTRACE) &&
> > + IS_ENABLED(CONFIG_HAVE_RELIABLE_STACKTRACE);
> > +}
> > +
> > #else /* !CONFIG_LIVEPATCH */
> >
> > static inline int klp_module_coming(struct module *mod) { return 0; }
> > static inline void klp_module_going(struct module *mod) {}
> > +static inline bool klp_patch_pending(struct task_struct *task) { return false; }
> > static inline void klp_update_patch_state(struct task_struct *task) {}
> > +static inline void klp_copy_process(struct task_struct *child) {}
> >
> > #endif /* CONFIG_LIVEPATCH */
> >
> > diff --git a/include/linux/sched.h b/include/linux/sched.h
> > index 9df1f42..ccbb1fa 100644
> > --- a/include/linux/sched.h
> > +++ b/include/linux/sched.h
> > @@ -1973,6 +1973,9 @@ struct task_struct {
> > /* A live task holds one reference. */
> > atomic_t stack_refcount;
> > #endif
> > +#ifdef CONFIG_LIVEPATCH
> > + int patch_state;
> > +#endif
> > /* CPU-specific state of this task */
> > struct thread_struct thread;
> > /*
> > diff --git a/kernel/fork.c b/kernel/fork.c
> > index 7da33cb..8c8de80 100644
> > --- a/kernel/fork.c
> > +++ b/kernel/fork.c
> > @@ -77,6 +77,7 @@
> > #include <linux/compiler.h>
> > #include <linux/sysctl.h>
> > #include <linux/kcov.h>
> > +#include <linux/livepatch.h>
> >
> > #include <asm/pgtable.h>
> > #include <asm/pgalloc.h>
> > @@ -1765,6 +1766,8 @@ static __latent_entropy struct task_struct *copy_process(
> > p->parent_exec_id = current->self_exec_id;
> > }
> >
> > + klp_copy_process(p);
> > +
> > spin_lock(&current->sighand->siglock);
> >
> > /*
> > diff --git a/kernel/livepatch/Makefile b/kernel/livepatch/Makefile
> > index e136dad..2b8bdb1 100644
> > --- a/kernel/livepatch/Makefile
> > +++ b/kernel/livepatch/Makefile
> > @@ -1,3 +1,3 @@
> > obj-$(CONFIG_LIVEPATCH) += livepatch.o
> >
> > -livepatch-objs := core.o patch.o
> > +livepatch-objs := core.o patch.o transition.o
> > diff --git a/kernel/livepatch/core.c b/kernel/livepatch/core.c
> > index 10ba3a1..4c5a816 100644
> > --- a/kernel/livepatch/core.c
> > +++ b/kernel/livepatch/core.c
> > @@ -31,22 +31,22 @@
> > #include <linux/moduleloader.h>
> > #include <asm/cacheflush.h>
> > #include "patch.h"
> > +#include "transition.h"
> >
> > /*
> > - * The klp_mutex protects the global lists and state transitions of any
> > - * structure reachable from them. References to any structure must be obtained
> > - * under mutex protection (except in klp_ftrace_handler(), which uses RCU to
> > - * ensure it gets consistent data).
> > + * klp_mutex is a coarse lock which serializes access to klp data. All
> > + * accesses to klp-related variables and structures must have mutex protection,
> > + * except within the following functions which carefully avoid the need for it:
> > + *
> > + * - klp_ftrace_handler()
> > + * - klp_update_patch_state()
> > */
> > -static DEFINE_MUTEX(klp_mutex);
> > +DEFINE_MUTEX(klp_mutex);
> >
> > static LIST_HEAD(klp_patches);
> >
> > static struct kobject *klp_root_kobj;
> >
> > -/* TODO: temporary stub */
> > -void klp_update_patch_state(struct task_struct *task) {}
> > -
> > static bool klp_is_module(struct klp_object *obj)
> > {
> > return obj->name;
> > @@ -85,7 +85,6 @@ static void klp_find_object_module(struct klp_object *obj)
> > mutex_unlock(&module_mutex);
> > }
> >
> > -/* klp_mutex must be held by caller */
> > static bool klp_is_patch_registered(struct klp_patch *patch)
> > {
> > struct klp_patch *mypatch;
> > @@ -281,20 +280,25 @@ static int klp_write_object_relocations(struct module *pmod,
> >
> > static int __klp_disable_patch(struct klp_patch *patch)
> > {
> > - struct klp_object *obj;
> > + if (klp_transition_patch)
> > + return -EBUSY;
> >
> > /* enforce stacking: only the last enabled patch can be disabled */
> > if (!list_is_last(&patch->list, &klp_patches) &&
> > list_next_entry(patch, list)->enabled)
> > return -EBUSY;
> >
> > - pr_notice("disabling patch '%s'\n", patch->mod->name);
> > + klp_init_transition(patch, KLP_UNPATCHED);
> >
> > - klp_for_each_object(patch, obj) {
> > - if (obj->patched)
> > - klp_unpatch_object(obj);
> > - }
> > + /*
> > + * Enforce the order of the klp_target_state write in
> > + * klp_init_transition() and the TIF_PATCH_PENDING writes in
> > + * klp_start_transition() to ensure that klp_update_patch_state()
> > + * doesn't set a task->patch_state to KLP_UNDEFINED.
> > + */
> > + smp_wmb();
>
> The description is not clear. The klp_target_state manipulation
> is synchronized by another barrier inside klp_init_transition().

Yeah. I should also update the barrier comment in klp_init_transition()
to clarify that it also does this.

> A similar barrier is in __klp_enable_patch() and it is correctly
> described there:
>
> It enforces the order of the func->transition writes in
> klp_init_transition() and the ops->func_stack writes in
> klp_patch_object(). The corresponding barrier is in
> klp_ftrace_handler().
>
> But we do not modify ops->func_stack in __klp_disable_patch().
> So we need another explanation.
>
> Huh, I spent few hours thinking about it. I am almost sure
> that it is not needed. But I am not 100% sure. The many times
> rewriten summary looks like:
>
> /*
> * Enforce the order of func->transtion write in
> * klp_init_transition() against TIF_PATCH_PENDING writes
> * in klp_start_transition(). It makes sure that
> * klp_ftrace_hadler() will see func->transition set
> * after the task is migrated by klp_update_patch_state().
> *
> * The barrier probably is not needed because the task
> * must not be migrated when being inside klp_ftrace_handler()
> * and there is another full barrier in
> * klp_update_patch_state().
> * But this is slow path and better be safe than sorry.
> */
> smp_wmb();

This mostly makes sense, but I think the barrier *is* needed for
ordering func->transition and TIF_PATCH_PENDING writes for the rare case
where klp_ftrace_handler() is called right after
klp_update_patch_state(), which could be possible in the idle loop, for
example.

CPU0 CPU1
__klp_disable_patch()
klp_init_transition()
func->transition = true;
(...no barrier...)
klp_start_transition()
set TIF_PATCH_PENDING

klp_update_patch_state()
if (test_and_clear(TIF_PATCH_PENDING))
task->patch_state = KLP_UNPATCHED;
...
klp_ftrace_handler()
smp_rmb();
if (unlikely(func->transition)) <--- false (patched)
...
klp_ftrace_handler()
smp_rmb();
if (unlikely(func->transition)) <--- true (unpatched)

So how about:

/*
* Enforce the order of the func->transition writes in
* klp_init_transition() and the TIF_PATCH_PENDING writes in
* klp_start_transition(). In the rare case where klp_ftrace_handler()
* is called shortly after klp_update_patch_state() switches the task,
* this ensures the handler sees func->transition is set.
*/
smp_wmb();

> > + klp_start_transition();
> > patch->enabled = false;
> >
> > return 0;
> > @@ -337,6 +341,9 @@ static int __klp_enable_patch(struct klp_patch *patch)
> > struct klp_object *obj;
> > int ret;
> >
> > + if (klp_transition_patch)
> > + return -EBUSY;
> > +
> > if (WARN_ON(patch->enabled))
> > return -EINVAL;
> >
> > @@ -347,22 +354,37 @@ static int __klp_enable_patch(struct klp_patch *patch)
> >
> > pr_notice("enabling patch '%s'\n", patch->mod->name);
> >
> > + klp_init_transition(patch, KLP_PATCHED);
> > +
> > + /*
> > + * Enforce the order of the func->transition writes in
> > + * klp_init_transition() and the ops->func_stack writes in
> > + * klp_patch_object(), so that klp_ftrace_handler() will see the
> > + * func->transition updates before the handler is registered and the
> > + * new funcs become visible to the handler.
> > + */
> > + smp_wmb();
> > +
> > klp_for_each_object(patch, obj) {
> > if (!klp_is_object_loaded(obj))
> > continue;
> >
> > ret = klp_patch_object(obj);
> > - if (ret)
> > - goto unregister;
> > + if (ret) {
> > + pr_warn("failed to enable patch '%s'\n",
> > + patch->mod->name);
> > +
> > + klp_unpatch_objects(patch);
>
> We should call here synchronize_rcu() here as we do
> in klp_try_complete_transition(). Some of the affected
> functions might have more versions on the stack and we
> need to make sure that klp_ftrace_handler() will _not_
> see the removed patch on the stack.

Even if the handler sees the new func on the stack, the
task->patch_state is still KLP_UNPATCHED, so it will still choose the
previous version of the function. Or did I miss your point?

> Alternatively, we could move all the code around
> klp_unpatch_objects() from klp_try_complete_transition()
> into klp_complete_transition(), see below.

But klp_target_state is KLP_PATCHED so the synchronize_rcu() at the end
of klp_try_complete_transition() wouldn't get called anyway.

>
> > + klp_complete_transition();
> > +
> > + return ret;
> > + }
> > }
> >
> > + klp_start_transition();
> > patch->enabled = true;
> >
> > return 0;
> > -
> > -unregister:
> > - WARN_ON(__klp_disable_patch(patch));
> > - return ret;
> > }
> >
> > /**
> > @@ -399,6 +421,7 @@ EXPORT_SYMBOL_GPL(klp_enable_patch);
> > * /sys/kernel/livepatch
> > * /sys/kernel/livepatch/<patch>
> > * /sys/kernel/livepatch/<patch>/enabled
> > + * /sys/kernel/livepatch/<patch>/transition
> > * /sys/kernel/livepatch/<patch>/
> > * /sys/kernel/livepatch/<patch>/ /<function,sympos>
> > */
> > @@ -424,7 +447,9 @@ static ssize_t enabled_store(struct kobject *kobj, struct kobj_attribute *attr,
> > goto err;
> > }
> >
> > - if (enabled) {
> > + if (patch == klp_transition_patch) {
> > + klp_reverse_transition();
> > + } else if (enabled) {
> > ret = __klp_enable_patch(patch);
> > if (ret)
> > goto err;
> > @@ -452,9 +477,21 @@ static ssize_t enabled_show(struct kobject *kobj,
> > return snprintf(buf, PAGE_SIZE-1, "%d\n", patch->enabled);
> > }
> >
> > +static ssize_t transition_show(struct kobject *kobj,
> > + struct kobj_attribute *attr, char *buf)
> > +{
> > + struct klp_patch *patch;
> > +
> > + patch = container_of(kobj, struct klp_patch, kobj);
> > + return snprintf(buf, PAGE_SIZE-1, "%d\n",
> > + patch == klp_transition_patch);
> > +}
> > +
> > static struct kobj_attribute enabled_kobj_attr = __ATTR_RW(enabled);
> > +static struct kobj_attribute transition_kobj_attr = __ATTR_RO(transition);
> > static struct attribute *klp_patch_attrs[] = {
> > &enabled_kobj_attr.attr,
> > + &transition_kobj_attr.attr,
> > NULL
> > };
> >
> > @@ -544,6 +581,7 @@ static int klp_init_func(struct klp_object *obj, struct klp_func *func)
> >
> > INIT_LIST_HEAD(&func->stack_node);
> > func->patched = false;
> > + func->transition = false;
> >
> > /* The format for the sysfs directory is <function,sympos> where sympos
> > * is the nth occurrence of this symbol in kallsyms for the patched
> > @@ -740,6 +778,16 @@ int klp_register_patch(struct klp_patch *patch)
> > return -ENODEV;
> >
> > /*
> > + * Architectures without reliable stack traces have to set
> > + * patch->immediate because there's currently no way to patch kthreads
> > + * with the consistency model.
> > + */
> > + if (!klp_have_reliable_stack() && !patch->immediate) {
> > + pr_err("This architecture doesn't have support for the livepatch consistency model.\n");
> > + return -ENOSYS;
> > + }
> > +
> > + /*
> > * A reference is taken on the patch module to prevent it from being
> > * unloaded. Right now, we don't allow patch modules to unload since
> > * there is currently no method to determine if a thread is still
> > @@ -788,7 +836,11 @@ int klp_module_coming(struct module *mod)
> > goto err;
> > }
> >
> > - if (!patch->enabled)
> > + /*
> > + * Only patch the module if the patch is enabled or is
> > + * in transition.
> > + */
> > + if (!patch->enabled && patch != klp_transition_patch)
>
> We need the same change also in klp_module_going().

Ok.

> > break;
> >
> > pr_notice("applying patch '%s' to loading module '%s'\n",
> > diff --git a/kernel/livepatch/patch.c b/kernel/livepatch/patch.c
> > index 5efa262..1a77f05 100644
> > --- a/kernel/livepatch/patch.c
> > +++ b/kernel/livepatch/patch.c
> > @@ -29,6 +29,7 @@
> > #include <linux/bug.h>
> > #include <linux/printk.h>
> > #include "patch.h"
> > +#include "transition.h"
> >
> > static LIST_HEAD(klp_ops);
> >
> > @@ -54,15 +55,58 @@ static void notrace klp_ftrace_handler(unsigned long ip,
> > {
> > struct klp_ops *ops;
> > struct klp_func *func;
> > + int patch_state;
> >
> > ops = container_of(fops, struct klp_ops, fops);
> >
> > rcu_read_lock();
> > +
> > func = list_first_or_null_rcu(&ops->func_stack, struct klp_func,
> > stack_node);
> > +
> > + /*
> > + * func should never be NULL because preemption should be disabled here
> > + * and unregister_ftrace_function() does the equivalent of a
> > + * synchronize_sched() before the func_stack removal.
> > + */
> > + if (WARN_ON_ONCE(!func))
> > + goto unlock;
> > +
> > + /*
> > + * Enforce the order of the ops->func_stack and func->transition reads.
> > + * The corresponding write barrier is in __klp_enable_patch().
> > + */
> > + smp_rmb();
>
> I was curious why the comment did not mention __klp_disable_patch().
> It was related to the hours of thinking. I would like to avoid this
> in the future and add a comment like.
>
> * This barrier probably is not needed when the patch is being
> * disabled. The patch is removed from the stack in
> * klp_try_complete_transition() and there we need to call
> * rcu_synchronize() to prevent seeing the patch on the stack
> * at all.
> *
> * Well, it still might be needed to see func->transition
> * when the patch is removed and the task is migrated. See
> * the write barrier in __klp_disable_patch().

Agreed, though as you mentioned earlier, there's also the implicit
barrier in klp_update_patch_state(), which would execute first in such a
scenario. So I think I'll update the barrier comments in
klp_update_patch_state().

> > + if (unlikely(func->transition)) {
> > +
> > + /*
> > + * Enforce the order of the func->transition and
> > + * current->patch_state reads. Otherwise we could read an
> > + * out-of-date task state and pick the wrong function. The
> > + * corresponding write barriers are in klp_init_transition()
> > + * and __klp_disable_patch().
> > + */
> > + smp_rmb();
>
> IMHO, the corresponding barrier is in klp_init_transition().
>
> The barrier in __klp_disable_patch() is questionable. Anyway,
> it has a different purpose.

Agreed.

> > + patch_state = current->patch_state;
> > +
> > + WARN_ON_ONCE(patch_state == KLP_UNDEFINED);
> > +
> > + if (patch_state == KLP_UNPATCHED) {
> > + /*
> > + * Use the previously patched version of the function.
> > + * If no previous patches exist, continue with the
> > + * original function.
> > + */
> > + func = list_entry_rcu(func->stack_node.next,
> > + struct klp_func, stack_node);
> > +
> > + if (&func->stack_node == &ops->func_stack)
> > + goto unlock;
> > + }
> > + }
> > +
> > klp_arch_set_pc(regs, (unsigned long)func->new_func);
> > unlock:
> > rcu_read_unlock();
> > @@ -211,3 +255,12 @@ int klp_patch_object(struct klp_object *obj)
> >
> > return 0;
> > }
> > +
> > +void klp_unpatch_objects(struct klp_patch *patch)
> > +{
> > + struct klp_object *obj;
> > +
> > + klp_for_each_object(patch, obj)
> > + if (obj->patched)
> > + klp_unpatch_object(obj);
> > +}
> > diff --git a/kernel/livepatch/patch.h b/kernel/livepatch/patch.h
> > index 2d0cce0..0db2271 100644
> > --- a/kernel/livepatch/patch.h
> > +++ b/kernel/livepatch/patch.h
> > @@ -28,5 +28,6 @@ struct klp_ops *klp_find_ops(unsigned long old_addr);
> >
> > int klp_patch_object(struct klp_object *obj);
> > void klp_unpatch_object(struct klp_object *obj);
> > +void klp_unpatch_objects(struct klp_patch *patch);
> >
> > #endif /* _LIVEPATCH_PATCH_H */
> > diff --git a/kernel/livepatch/transition.c b/kernel/livepatch/transition.c
> > new file mode 100644
> > index 0000000..2b87ff9
> > --- /dev/null
> > +++ b/kernel/livepatch/transition.c
> > @@ -0,0 +1,525 @@
> > +/*
> > + * transition.c - Kernel Live Patching transition functions
> > + *
> > + * Copyright (C) 2015-2016 Josh Poimboeuf <jpoimboe@redhat.com>
> > + *
> > + * This program is free software; you can redistribute it and/or
> > + * modify it under the terms of the GNU General Public License
> > + * as published by the Free Software Foundation; either version 2
> > + * of the License, or (at your option) any later version.
> > + *
> > + * This program is distributed in the hope that it will be useful,
> > + * but WITHOUT ANY WARRANTY; without even the implied warranty of
> > + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
> > + * GNU General Public License for more details.
> > + *
> > + * You should have received a copy of the GNU General Public License
> > + * along with this program; if not, see <http://www.gnu.org/licenses/>.
> > + */
> > +
> > +#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
> > +
> > +#include <linux/cpu.h>
> > +#include <linux/stacktrace.h>
> > +#include "patch.h"
> > +#include "transition.h"
> > +#include "../sched/sched.h"
> > +
> > +#define MAX_STACK_ENTRIES 100
> > +#define STACK_ERR_BUF_SIZE 128
> > +
> > +extern struct mutex klp_mutex;
> > +
> > +struct klp_patch *klp_transition_patch;
> > +
> > +static int klp_target_state = KLP_UNDEFINED;
> > +
> > +/*
> > + * This work can be performed periodically to finish patching or unpatching any
> > + * "straggler" tasks which failed to transition in the first attempt.
> > + */
> > +static void klp_try_complete_transition(void);
> > +static void klp_transition_work_fn(struct work_struct *work)
> > +{
> > + mutex_lock(&klp_mutex);
> > +
> > + if (klp_transition_patch)
> > + klp_try_complete_transition();
> > +
> > + mutex_unlock(&klp_mutex);
> > +}
> > +static DECLARE_DELAYED_WORK(klp_transition_work, klp_transition_work_fn);
> > +
> > +/*
> > + * The transition to the target patch state is complete. Clean up the data
> > + * structures.
> > + */
> > +void klp_complete_transition(void)
> > +{
> > + struct klp_object *obj;
> > + struct klp_func *func;
> > + struct task_struct *g, *task;
> > + unsigned int cpu;
> > +
> > + if (klp_transition_patch->immediate)
> > + goto done;
> > +
> > + klp_for_each_object(klp_transition_patch, obj)
> > + klp_for_each_func(obj, func)
> > + func->transition = false;
> > +
> > + /* Prevent klp_ftrace_handler() from seeing KLP_UNDEFINED state */
> > + if (klp_target_state == KLP_PATCHED)
> > + synchronize_rcu();
>
> I forgot why this is done only when the patch is beeing enabled.
> It is because klp_unpatch_objects() and rcu_synchronize() is called
> in klp_try_complete_transition() when klp_target_state ==
> KLP_UNPATCHED.
>
> I would suggest to move the code below the "success" label from
> klp_try_complete_transtion() to klp_complete_transition().
> It will get the two related things close to each other.
> Also it would fix the missing rcu_synchronize() in
> the error path in __klp_enable_patch(), see above.

As I mentioned, I don't see how that will fix the __klp_enable_patch()
error path, nor can I see why it needs fixing in the first place.

Regardless, it does seem like a good idea to move the end of
klp_try_complete_transition() into klp_complete_transition().

> > + read_lock(&tasklist_lock);
> > + for_each_process_thread(g, task) {
> > + WARN_ON_ONCE(test_tsk_thread_flag(task, TIF_PATCH_PENDING));
> > + task->patch_state = KLP_UNDEFINED;
> > + }
> > + read_unlock(&tasklist_lock);
> > +
> > + for_each_possible_cpu(cpu) {
> > + task = idle_task(cpu);
> > + WARN_ON_ONCE(test_tsk_thread_flag(task, TIF_PATCH_PENDING));
> > + task->patch_state = KLP_UNDEFINED;
> > + }
> > +
> > +done:
> > + klp_target_state = KLP_UNDEFINED;
> > + klp_transition_patch = NULL;
> > +}
> > +
> > +/*
> > + * Switch the patched state of the task to the set of functions in the target
> > + * patch state.
> > + *
> > + * NOTE: If task is not 'current', the caller must ensure the task is inactive.
> > + * Otherwise klp_ftrace_handler() might read the wrong 'patch_state' value.
> > + */
> > +void klp_update_patch_state(struct task_struct *task)
> > +{
> > + rcu_read_lock();
> > +
> > + /*
> > + * This test_and_clear_tsk_thread_flag() call also serves as a read
> > + * barrier to enforce the order of the TIF_PATCH_PENDING and
> > + * klp_target_state reads. The corresponding write barrier is in
> > + * __klp_disable_patch().
>
> The corresponding barrier is in klp_init_transition(), see below.
>
> In each case, we need the corresponding write barrier in both enable
> and disable paths. They both set klp_target_state and
> TIF_PATCH_PENDING. The write barrier in klp_init_transition() perfectly
> fits the purpose.

Agreed.

> > + */
> > + if (test_and_clear_tsk_thread_flag(task, TIF_PATCH_PENDING))
> > + task->patch_state = READ_ONCE(klp_target_state);
> > +
> > + rcu_read_unlock();
> > +}
> > +
> > +/*
> > + * Determine whether the given stack trace includes any references to a
> > + * to-be-patched or to-be-unpatched function.
> > + */
> > +static int klp_check_stack_func(struct klp_func *func,
> > + struct stack_trace *trace)
> > +{
> > + unsigned long func_addr, func_size, address;
> > + struct klp_ops *ops;
> > + int i;
> > +
> > + if (func->immediate)
> > + return 0;
> > +
> > + for (i = 0; i < trace->nr_entries; i++) {
> > + address = trace->entries[i];
> > +
> > + if (klp_target_state == KLP_UNPATCHED) {
> > + /*
> > + * Check for the to-be-unpatched function
> > + * (the func itself).
> > + */
> > + func_addr = (unsigned long)func->new_func;
> > + func_size = func->new_size;
> > + } else {
> > + /*
> > + * Check for the to-be-patched function
> > + * (the previous func).
> > + */
> > + ops = klp_find_ops(func->old_addr);
> > +
> > + if (list_is_singular(&ops->func_stack)) {
> > + /* original function */
> > + func_addr = func->old_addr;
> > + func_size = func->old_size;
> > + } else {
> > + /* previously patched function */
> > + struct klp_func *prev;
> > +
> > + prev = list_next_entry(func, stack_node);
> > + func_addr = (unsigned long)prev->new_func;
> > + func_size = prev->new_size;
> > + }
> > + }
> > +
> > + if (address >= func_addr && address < func_addr + func_size)
> > + return -EAGAIN;
> > + }
> > +
> > + return 0;
> > +}
> > +
> > +/*
> > + * Determine whether it's safe to transition the task to the target patch state
> > + * by looking for any to-be-patched or to-be-unpatched functions on its stack.
> > + */
> > +static int klp_check_stack(struct task_struct *task, char *err_buf)
> > +{
> > + static unsigned long entries[MAX_STACK_ENTRIES];
> > + struct stack_trace trace;
> > + struct klp_object *obj;
> > + struct klp_func *func;
> > + int ret;
> > +
> > + trace.skip = 0;
> > + trace.nr_entries = 0;
> > + trace.max_entries = MAX_STACK_ENTRIES;
> > + trace.entries = entries;
> > + ret = save_stack_trace_tsk_reliable(task, &trace);
> > + WARN_ON_ONCE(ret == -ENOSYS);
> > + if (ret) {
> > + snprintf(err_buf, STACK_ERR_BUF_SIZE,
> > + "%s: %s:%d has an unreliable stack\n",
> > + __func__, task->comm, task->pid);
> > + return ret;
> > + }
> > +
> > + klp_for_each_object(klp_transition_patch, obj) {
> > + if (!obj->patched)
> > + continue;
> > + klp_for_each_func(obj, func) {
> > + ret = klp_check_stack_func(func, &trace);
> > + if (ret) {
> > + snprintf(err_buf, STACK_ERR_BUF_SIZE,
> > + "%s: %s:%d is sleeping on function %s\n",
> > + __func__, task->comm, task->pid,
> > + func->old_name);
> > + return ret;
> > + }
> > + }
> > + }
> > +
> > + return 0;
> > +}
> > +
> > +/*
> > + * Try to safely switch a task to the target patch state. If it's currently
> > + * running, or it's sleeping on a to-be-patched or to-be-unpatched function, or
> > + * if the stack is unreliable, return false.
> > + */
> > +static bool klp_try_switch_task(struct task_struct *task)
> > +{
> > + struct rq *rq;
> > + struct rq_flags flags;
> > + int ret;
> > + bool success = false;
> > + char err_buf[STACK_ERR_BUF_SIZE];
> > +
> > + err_buf[0] = '\0';
> > +
> > + /* check if this task has already switched over */
> > + if (task->patch_state == klp_target_state)
> > + return true;
> > +
> > + /*
> > + * For arches which don't have reliable stack traces, we have to rely
> > + * on other methods (e.g., switching tasks at kernel exit).
> > + */
> > + if (!klp_have_reliable_stack())
> > + return false;
> > +
> > + /*
> > + * Now try to check the stack for any to-be-patched or to-be-unpatched
> > + * functions. If all goes well, switch the task to the target patch
> > + * state.
> > + */
> > + rq = task_rq_lock(task, &flags);
> > +
> > + if (task_running(rq, task) && task != current) {
> > + snprintf(err_buf, STACK_ERR_BUF_SIZE,
> > + "%s: %s:%d is running\n", __func__, task->comm,
> > + task->pid);
> > + goto done;
> > + }
> > +
> > + ret = klp_check_stack(task, err_buf);
> > + if (ret)
> > + goto done;
> > +
> > + success = true;
> > +
> > + clear_tsk_thread_flag(task, TIF_PATCH_PENDING);
> > + task->patch_state = klp_target_state;
> > +
> > +done:
> > + task_rq_unlock(rq, task, &flags);
> > +
> > + /*
> > + * Due to console deadlock issues, pr_debug() can't be used while
> > + * holding the task rq lock. Instead we have to use a temporary buffer
> > + * and print the debug message after releasing the lock.
> > + */
> > + if (err_buf[0] != '\0')
> > + pr_debug("%s", err_buf);
> > +
> > + return success;
> > +
> > +}
> > +
> > +/*
> > + * Try to switch all remaining tasks to the target patch state by walking the
> > + * stacks of sleeping tasks and looking for any to-be-patched or
> > + * to-be-unpatched functions. If such functions are found, the task can't be
> > + * switched yet.
> > + *
> > + * If any tasks are still stuck in the initial patch state, schedule a retry.
> > + */
> > +static void klp_try_complete_transition(void)
> > +{
> > + unsigned int cpu;
> > + struct task_struct *g, *task;
> > + bool complete = true;
> > +
> > + WARN_ON_ONCE(klp_target_state == KLP_UNDEFINED);
> > +
> > + /*
> > + * If the patch can be applied or reverted immediately, skip the
> > + * per-task transitions.
> > + */
> > + if (klp_transition_patch->immediate)
> > + goto success;
> > +
> > + /*
> > + * Try to switch the tasks to the target patch state by walking their
> > + * stacks and looking for any to-be-patched or to-be-unpatched
> > + * functions. If such functions are found on a stack, or if the stack
> > + * is deemed unreliable, the task can't be switched yet.
> > + *
> > + * Usually this will transition most (or all) of the tasks on a system
> > + * unless the patch includes changes to a very common function.
> > + */
> > + read_lock(&tasklist_lock);
> > + for_each_process_thread(g, task)
> > + if (!klp_try_switch_task(task))
> > + complete = false;
> > + read_unlock(&tasklist_lock);
> > +
> > + /*
> > + * Ditto for the idle "swapper" tasks.
> > + */
> > + get_online_cpus();
> > + for_each_possible_cpu(cpu) {
> > + task = idle_task(cpu);
> > + if (cpu_online(cpu)) {
> > + if (!klp_try_switch_task(task))
> > + complete = false;
> > + } else if (task->patch_state != klp_target_state) {
> > + /* offline idle tasks can be switched immediately */
> > + clear_tsk_thread_flag(task, TIF_PATCH_PENDING);
> > + task->patch_state = klp_target_state;
> > + }
> > + }
> > + put_online_cpus();
> > +
> > + /*
> > + * Some tasks weren't able to be switched over. Try again later and/or
> > + * wait for other methods like kernel exit switching.
> > + */
> > + if (!complete) {
> > + schedule_delayed_work(&klp_transition_work,
> > + round_jiffies_relative(HZ));
> > + return;
> > + }
> > +
> > +success:
> > +
> > + if (klp_target_state == KLP_UNPATCHED) {
> > + /*
> > + * All tasks have transitioned to KLP_UNPATCHED so we can now
> > + * remove the new functions from the func_stack.
> > + */
> > + klp_unpatch_objects(klp_transition_patch);
> > +
> > + /*
> > + * Make sure klp_ftrace_handler() sees that the functions have
> > + * been removed from ops->func_stack before we clear
> > + * func->transition. Otherwise it may pick the wrong
> > + * transition.
> > + */
>
> I was a bit confused by the comment. It might be my problem. I just
> wonder if might be more clear with somethink like:
>
> /*
> * Make sure klp_ftrace_handler() can not longer see functions
> * from this patch on the stack. Otherwise it may use
> * the being-removed function after func->transition is cleared.
> */

Ok.

> > + synchronize_rcu();
> > + }
> > +
> > + pr_notice("'%s': %s complete\n", klp_transition_patch->mod->name,
> > + klp_target_state == KLP_PATCHED ? "patching" : "unpatching");
> > +
> > + /* we're done, now cleanup the data structures */
> > + klp_complete_transition();
> > +}
> > +
> > +/*
> > + * Start the transition to the specified target patch state so tasks can begin
> > + * switching to it.
> > + */
> > +void klp_start_transition(void)
> > +{
> > + struct task_struct *g, *task;
> > + unsigned int cpu;
> > +
> > + WARN_ON_ONCE(klp_target_state == KLP_UNDEFINED);
> > +
> > + pr_notice("'%s': %s...\n", klp_transition_patch->mod->name,
> > + klp_target_state == KLP_PATCHED ? "patching" : "unpatching");
> > +
> > + /*
> > + * If the patch can be applied or reverted immediately, skip the
> > + * per-task transitions.
> > + */
> > + if (klp_transition_patch->immediate)
>
> We should call klp_try_complete_transition() here. Otherwise, it will
> never be called and the transition will never get completed.
>
> Alternative solution would be to move klp_try_complete_transition()
> from klp_start_transition() and explicitely call it from
> __klp_disable_patch() and klp_enable_patch(). It would actually
> solve one issue with klp_revert_patch(), see below.
>
> I kind of like the alternative solution. I hope that it was not
> me who suggested to move klp_try_complete_transition() into
> klp_start_transtion().

I'll go with your alternative solution. (BTW, moving the
klp_try_complete_transition() call was my idea. It was an "improvement"
I made as part of moving the klp_transition_work to transition.c.)

> > + return;
> > +
> > + /*
> > + * Mark all normal tasks as needing a patch state update. They'll
> > + * switch either in klp_try_complete_transition() or as they exit the
> > + * kernel.
> > + */
> > + read_lock(&tasklist_lock);
> > + for_each_process_thread(g, task)
> > + if (task->patch_state != klp_target_state)
> > + set_tsk_thread_flag(task, TIF_PATCH_PENDING);
> > + read_unlock(&tasklist_lock);
> > +
> > + /*
> > + * Mark all idle tasks as needing a patch state update. They'll switch
> > + * either in klp_try_complete_transition() or at the idle loop switch
> > + * point.
> > + */
> > + for_each_possible_cpu(cpu) {
> > + task = idle_task(cpu);
> > + if (task->patch_state != klp_target_state)
> > + set_tsk_thread_flag(task, TIF_PATCH_PENDING);
> > + }
> > +
> > + klp_try_complete_transition();
> > +}
> > +
> > +/*
> > + * Initialize the global target patch state and all tasks to the initial patch
> > + * state, and initialize all function transition states to true in preparation
> > + * for patching or unpatching.
> > + */
> > +void klp_init_transition(struct klp_patch *patch, int state)
> > +{
> > + struct task_struct *g, *task;
> > + unsigned int cpu;
> > + struct klp_object *obj;
> > + struct klp_func *func;
> > + int initial_state = !state;
> > +
> > + WARN_ON_ONCE(klp_target_state != KLP_UNDEFINED);
> > +
> > + klp_transition_patch = patch;
> > +
> > + /*
> > + * Set the global target patch state which tasks will switch to. This
> > + * has no effect until the TIF_PATCH_PENDING flags get set later.
> > + */
> > + klp_target_state = state;
> > +
> > + /*
> > + * If the patch can be applied or reverted immediately, skip the
> > + * per-task transitions.
> > + */
> > + if (patch->immediate)
> > + return;
> > +
> > + /*
> > + * Initialize all tasks to the initial patch state to prepare them for
> > + * switching to the target state.
> > + */
> > + read_lock(&tasklist_lock);
> > + for_each_process_thread(g, task) {
> > + WARN_ON_ONCE(task->patch_state != KLP_UNDEFINED);
> > + task->patch_state = initial_state;
> > + }
> > + read_unlock(&tasklist_lock);
> > +
> > + /*
> > + * Ditto for the idle "swapper" tasks.
> > + */
> > + for_each_possible_cpu(cpu) {
> > + task = idle_task(cpu);
> > + WARN_ON_ONCE(task->patch_state != KLP_UNDEFINED);
> > + task->patch_state = initial_state;
> > + }
> > +
> > + /*
> > + * Enforce the order of the task->patch_state initializations and the
> > + * func->transition updates to ensure that, in the enable path,
> > + * klp_ftrace_handler() doesn't see a func in transition with a
> > + * task->patch_state of KLP_UNDEFINED.
>
> It has one more purpose:
>
> *
> * Also enforce the order between klp_target_state and
> * TIF_PATCH_PENDING. The corresponding read barrier is in
> * klp_update_patch_state().

Yes.

> > + */
> > + smp_wmb();
> > +
> > + /*
> > + * Set the func transition states so klp_ftrace_handler() will know to
> > + * switch to the transition logic.
> > + *
> > + * When patching, the funcs aren't yet in the func_stack and will be
> > + * made visible to the ftrace handler shortly by the calls to
> > + * klp_patch_object().
> > + *
> > + * When unpatching, the funcs are already in the func_stack and so are
> > + * already visible to the ftrace handler.
> > + */
> > + klp_for_each_object(patch, obj)
> > + klp_for_each_func(obj, func)
> > + func->transition = true;
> > +}
> > +
> > +/*
> > + * This function can be called in the middle of an existing transition to
> > + * reverse the direction of the target patch state. This can be done to
> > + * effectively cancel an existing enable or disable operation if there are any
> > + * tasks which are stuck in the initial patch state.
> > + */
> > +void klp_reverse_transition(void)
> > +{
> > + unsigned int cpu;
> > + struct task_struct *g, *task;
> > +
> > + klp_transition_patch->enabled = !klp_transition_patch->enabled;
> > +
> > + klp_target_state = !klp_target_state;
> > +
> > + /*
> > + * Clear all TIF_PATCH_PENDING flags to prevent races caused by
> > + * klp_update_patch_state() running in parallel with
> > + * klp_start_transition().
> > + */
> > + read_lock(&tasklist_lock);
> > + for_each_process_thread(g, task)
> > + clear_tsk_thread_flag(task, TIF_PATCH_PENDING);
> > + read_unlock(&tasklist_lock);
> > +
> > + for_each_possible_cpu(cpu)
> > + clear_tsk_thread_flag(idle_task(cpu), TIF_PATCH_PENDING);
> > +
> > + /* Let any remaining calls to klp_update_patch_state() complete */
> > + synchronize_rcu();
> > +
> > + klp_start_transition();
>
> Hmm, we should not call klp_try_complete_transition() when
> klp_start_transition() is called from here. I can't find a safe
> way to cancel klp_transition_work() when we own klp_mutex.
> It smells with a possible deadlock.
>
> I suggest to move move klp_try_complete_transition() outside
> klp_start_transition() and explicitely call it from
> __klp_disable_patch() and __klp_enabled_patch().
> This would fix also the problem with immediate patches, see
> klp_start_transition().

Agreed. I'll fix it as you suggest and I'll put the mod_delayed_work()
call in klp_reverse_transition() again.

> > +}
> > +
> > +/* Called from copy_process() during fork */
> > +void klp_copy_process(struct task_struct *child)
> > +{
> > + child->patch_state = current->patch_state;
> > +
> > + /* TIF_PATCH_PENDING gets copied in setup_thread_stack() */
> > +}
> > diff --git a/kernel/livepatch/transition.h b/kernel/livepatch/transition.h
> > new file mode 100644
> > index 0000000..3ee625f
> > --- /dev/null
> > +++ b/kernel/livepatch/transition.h
> > @@ -0,0 +1,13 @@
> > +#ifndef _LIVEPATCH_TRANSITION_H
> > +#define _LIVEPATCH_TRANSITION_H
> > +
> > +#include <linux/livepatch.h>
> > +
> > +extern struct klp_patch *klp_transition_patch;
> > +
> > +void klp_init_transition(struct klp_patch *patch, int state);
> > +void klp_start_transition(void);
> > +void klp_reverse_transition(void);
> > +void klp_complete_transition(void);
> > +
> > +#endif /* _LIVEPATCH_TRANSITION_H */
> > diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c
> > index 6a4bae0..a8b3f1a 100644
> > --- a/kernel/sched/idle.c
> > +++ b/kernel/sched/idle.c
> > @@ -9,6 +9,7 @@
> > #include <linux/mm.h>
> > #include <linux/stackprotector.h>
> > #include <linux/suspend.h>
> > +#include <linux/livepatch.h>
> >
> > #include <asm/tlb.h>
> >
> > @@ -264,6 +265,9 @@ static void do_idle(void)
> >
> > sched_ttwu_pending();
> > schedule_preempt_disabled();
> > +
> > + if (unlikely(klp_patch_pending(current)))
> > + klp_update_patch_state(current);
> > }
> >
> > bool cpu_in_idle(unsigned long pc)
> > diff --git a/samples/livepatch/livepatch-sample.c b/samples/livepatch/livepatch-sample.c
> > index e34f871..629e0dc 100644
> > --- a/samples/livepatch/livepatch-sample.c
> > +++ b/samples/livepatch/livepatch-sample.c
> > @@ -17,6 +17,8 @@
> > * along with this program; if not, see <http://www.gnu.org/licenses/>.
> > */
> >
> > +#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
> > +
> > #include <linux/module.h>
> > #include <linux/kernel.h>
> > #include <linux/livepatch.h>
> > @@ -69,6 +71,21 @@ static int livepatch_init(void)
> > {
> > int ret;
> >
> > + if (!klp_have_reliable_stack() && !patch.immediate) {
> > + /*
> > + * WARNING: Be very careful when using 'patch.immediate' in
> > + * your patches. It's ok to use it for simple patches like
> > + * this, but for more complex patches which change function
> > + * semantics, locking semantics, or data structures, it may not
> > + * be safe. Use of this option will also prevent removal of
> > + * the patch.
> > + *
> > + * See Documentation/livepatch/livepatch.txt for more details.
> > + */
> > + patch.immediate = true;
> > + pr_notice("The consistency model isn't supported for your architecture. Bypassing safety mechanisms and applying the patch immediately.\n");
> > + }
> > +
> > ret = klp_register_patch(&patch);
> > if (ret)
> > return ret;
>
> I am sorry that the review took me so long. I was interrupted several
> times by other tasks. The logic is very complex. I wanted to
> make sure that my comments are consistent and make sense.
>
> Anyway, I think that we are getting close.

Yeah, it does take a while to wrap your head around the code, especially
with respect to synchronization. If there's any way to simplify things,
I'm all for it. Thanks again for your excellent reviews.

--
Josh