/*
 * 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 "core.h"
#include "patch.h"
#include "transition.h"
#include "../sched/sched.h"

#define MAX_STACK_ENTRIES  100
#define STACK_ERR_BUF_SIZE 128

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_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);

/*
 * This function is just a stub to implement a hard force
 * of synchronize_sched(). This requires synchronizing
 * tasks even in userspace and idle.
 */
static void klp_sync(struct work_struct *work)
{
}

/*
 * We allow to patch also functions where RCU is not watching,
 * e.g. before user_exit(). We can not rely on the RCU infrastructure
 * to do the synchronization. Instead hard force the sched synchronization.
 *
 * This approach allows to use RCU functions for manipulating func_stack
 * safely.
 */
static void klp_synchronize_transition(void)
{
        schedule_on_each_cpu(klp_sync);
}

/*
 * The transition to the target patch state is complete.  Clean up the data
 * structures.
 */
static void klp_complete_transition(void)
{
        struct klp_object *obj;
        struct klp_func *func;
        struct task_struct *g, *task;
        unsigned int cpu;
        bool immediate_func = false;

        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() can no longer see functions
                 * from this patch on the ops->func_stack.  Otherwise, after
                 * func->transition gets cleared, the handler may choose a
                 * removed function.
                 */
                klp_synchronize_transition();
        }

        if (klp_transition_patch->immediate)
                goto done;

        klp_for_each_object(klp_transition_patch, obj) {
                klp_for_each_func(obj, func) {
                        func->transition = false;
                        if (func->immediate)
                                immediate_func = true;
                }
        }

        if (klp_target_state == KLP_UNPATCHED && !immediate_func)
                module_put(klp_transition_patch->mod);

        /* Prevent klp_ftrace_handler() from seeing KLP_UNDEFINED state */
        if (klp_target_state == KLP_PATCHED)
                klp_synchronize_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;
}

/*
 * This is called in the error path, to cancel a transition before it has
 * started, i.e. klp_init_transition() has been called but
 * klp_start_transition() hasn't.  If the transition *has* been started,
 * klp_reverse_transition() should be used instead.
 */
void klp_cancel_transition(void)
{
        if (WARN_ON_ONCE(klp_target_state != KLP_PATCHED))
                return;

        klp_target_state = KLP_UNPATCHED;
        klp_complete_transition();
}

/*
 * 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)
{
        /*
         * A variant of synchronize_sched() is used to allow patching functions
         * where RCU is not watching, see klp_synchronize_transition().
         */
        preempt_disable_notrace();

        /*
         * This test_and_clear_tsk_thread_flag() call also serves as a read
         * barrier (smp_rmb) for two cases:
         *
         * 1) Enforce the order of the TIF_PATCH_PENDING read and the
         *    klp_target_state read.  The corresponding write barrier is in
         *    klp_init_transition().
         *
         * 2) Enforce the order of the TIF_PATCH_PENDING read and a future read
         *    of func->transition, if klp_ftrace_handler() is called later on
         *    the same CPU.  See __klp_disable_patch().
         */
        if (test_and_clear_tsk_thread_flag(task, TIF_PATCH_PENDING))
                task->patch_state = READ_ONCE(klp_target_state);

        preempt_enable_notrace();
}

/*
 * 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.
 */
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();

        if (!complete) {
                /*
                 * Some tasks weren't able to be switched over.  Try again
                 * later and/or wait for other methods like kernel exit
                 * switching.
                 */
                schedule_delayed_work(&klp_transition_work,
                                      round_jiffies_relative(HZ));
                return;
        }

success:
        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)
                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);
        }
}

/*
 * 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 klp_ftrace_handler() doesn't
         * see a func in transition with a task->patch_state of KLP_UNDEFINED.
         *
         * Also enforce the order of the klp_target_state write and future
         * TIF_PATCH_PENDING writes to ensure klp_update_patch_state() doesn't
         * set a task->patch_state to KLP_UNDEFINED.
         */
        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 */
        klp_synchronize_transition();

        klp_start_transition();
}

/* 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() */
}