// SPDX-License-Identifier: GPL-2.0

#define pr_fmt(fmt) "rethook: " fmt

#include <linux/bug.h>
#include <linux/kallsyms.h>
#include <linux/kprobes.h>
#include <linux/preempt.h>
#include <linux/rethook.h>
#include <linux/slab.h>

/* Return hook list (shadow stack by list) */

/*
 * This function is called from delayed_put_task_struct() when a task is
 * dead and cleaned up to recycle any kretprobe instances associated with
 * this task. These left over instances represent probed functions that
 * have been called but will never return.
 */
void rethook_flush_task(struct task_struct *tk)
{
        struct rethook_node *rhn;
        struct llist_node *node;

        node = __llist_del_all(&tk->rethooks);
        while (node) {
                rhn = container_of(node, struct rethook_node, llist);
                node = node->next;
                preempt_disable();
                rethook_recycle(rhn);
                preempt_enable();
        }
}

static void rethook_free_rcu(struct rcu_head *head)
{
        struct rethook *rh = container_of(head, struct rethook, rcu);
        objpool_fini(&rh->pool);
}

/**
 * rethook_stop() - Stop using a rethook.
 * @rh: the struct rethook to stop.
 *
 * Stop using a rethook to prepare for freeing it. If you want to wait for
 * all running rethook handler before calling rethook_free(), you need to
 * call this first and wait RCU, and call rethook_free().
 */
void rethook_stop(struct rethook *rh)
{
        rcu_assign_pointer(rh->handler, NULL);
}

/**
 * rethook_free() - Free struct rethook.
 * @rh: the struct rethook to be freed.
 *
 * Free the rethook. Before calling this function, user must ensure the
 * @rh::data is cleaned if needed (or, the handler can access it after
 * calling this function.) This function will set the @rh to be freed
 * after all rethook_node are freed (not soon). And the caller must
 * not touch @rh after calling this.
 */
void rethook_free(struct rethook *rh)
{
        rethook_stop(rh);

        call_rcu(&rh->rcu, rethook_free_rcu);
}

static int rethook_init_node(void *nod, void *context)
{
        struct rethook_node *node = nod;

        node->rethook = context;
        return 0;
}

static int rethook_fini_pool(struct objpool_head *head, void *context)
{
        kfree(context);
        return 0;
}

static inline rethook_handler_t rethook_get_handler(struct rethook *rh)
{
        return (rethook_handler_t)rcu_dereference_check(rh->handler,
                                                        rcu_read_lock_any_held());
}

/**
 * rethook_alloc() - Allocate struct rethook.
 * @data: a data to pass the @handler when hooking the return.
 * @handler: the return hook callback function, must NOT be NULL
 * @size: node size: rethook node and additional data
 * @num: number of rethook nodes to be preallocated
 *
 * Allocate and initialize a new rethook with @data and @handler.
 * Return pointer of new rethook, or error codes for failures.
 *
 * Note that @handler == NULL means this rethook is going to be freed.
 */
struct rethook *rethook_alloc(void *data, rethook_handler_t handler,
                              int size, int num)
{
        struct rethook *rh;

        if (!handler || num <= 0 || size < sizeof(struct rethook_node))
                return ERR_PTR(-EINVAL);

        rh = kzalloc_obj(struct rethook);
        if (!rh)
                return ERR_PTR(-ENOMEM);

        rh->data = data;
        rcu_assign_pointer(rh->handler, handler);

        /* initialize the objpool for rethook nodes */
        if (objpool_init(&rh->pool, num, size, GFP_KERNEL, rh,
                         rethook_init_node, rethook_fini_pool)) {
                kfree(rh);
                return ERR_PTR(-ENOMEM);
        }
        return rh;
}

static void free_rethook_node_rcu(struct rcu_head *head)
{
        struct rethook_node *node = container_of(head, struct rethook_node, rcu);
        struct rethook *rh = node->rethook;

        objpool_drop(node, &rh->pool);
}

/**
 * rethook_recycle() - return the node to rethook.
 * @node: The struct rethook_node to be returned.
 *
 * Return back the @node to @node::rethook. If the @node::rethook is already
 * marked as freed, this will free the @node.
 */
void rethook_recycle(struct rethook_node *node)
{
        rethook_handler_t handler;

        handler = rethook_get_handler(node->rethook);
        if (likely(handler))
                objpool_push(node, &node->rethook->pool);
        else
                call_rcu(&node->rcu, free_rethook_node_rcu);
}
NOKPROBE_SYMBOL(rethook_recycle);

/**
 * rethook_try_get() - get an unused rethook node.
 * @rh: The struct rethook which pools the nodes.
 *
 * Get an unused rethook node from @rh. If the node pool is empty, this
 * will return NULL. Caller must disable preemption.
 */
struct rethook_node *rethook_try_get(struct rethook *rh)
{
        rethook_handler_t handler = rethook_get_handler(rh);

        /* Check whether @rh is going to be freed. */
        if (unlikely(!handler))
                return NULL;

#if defined(CONFIG_FTRACE_VALIDATE_RCU_IS_WATCHING) || defined(CONFIG_KPROBE_EVENTS_ON_NOTRACE)
        /*
         * This expects the caller will set up a rethook on a function entry.
         * When the function returns, the rethook will eventually be reclaimed
         * or released in the rethook_recycle() with call_rcu().
         * This means the caller must be run in the RCU-availabe context.
         */
        if (unlikely(!rcu_is_watching()))
                return NULL;
#endif

        return (struct rethook_node *)objpool_pop(&rh->pool);
}
NOKPROBE_SYMBOL(rethook_try_get);

/**
 * rethook_hook() - Hook the current function return.
 * @node: The struct rethook node to hook the function return.
 * @regs: The struct pt_regs for the function entry.
 * @mcount: True if this is called from mcount(ftrace) context.
 *
 * Hook the current running function return. This must be called when the
 * function entry (or at least @regs must be the registers of the function
 * entry.) @mcount is used for identifying the context. If this is called
 * from ftrace (mcount) callback, @mcount must be set true. If this is called
 * from the real function entry (e.g. kprobes) @mcount must be set false.
 * This is because the way to hook the function return depends on the context.
 */
void rethook_hook(struct rethook_node *node, struct pt_regs *regs, bool mcount)
{
        arch_rethook_prepare(node, regs, mcount);
        __llist_add(&node->llist, &current->rethooks);
}
NOKPROBE_SYMBOL(rethook_hook);

/* This assumes the 'tsk' is the current task or is not running. */
static unsigned long __rethook_find_ret_addr(struct task_struct *tsk,
                                             struct llist_node **cur)
{
        struct rethook_node *rh = NULL;
        struct llist_node *node = *cur;

        if (!node)
                node = tsk->rethooks.first;
        else
                node = node->next;

        while (node) {
                rh = container_of(node, struct rethook_node, llist);
                if (rh->ret_addr != (unsigned long)arch_rethook_trampoline) {
                        *cur = node;
                        return rh->ret_addr;
                }
                node = node->next;
        }
        return 0;
}
NOKPROBE_SYMBOL(__rethook_find_ret_addr);

/**
 * rethook_find_ret_addr -- Find correct return address modified by rethook
 * @tsk: Target task
 * @frame: A frame pointer
 * @cur: a storage of the loop cursor llist_node pointer for next call
 *
 * Find the correct return address modified by a rethook on @tsk in unsigned
 * long type.
 * The @tsk must be 'current' or a task which is not running. @frame is a hint
 * to get the currect return address - which is compared with the
 * rethook::frame field. The @cur is a loop cursor for searching the
 * kretprobe return addresses on the @tsk. The '*@cur' should be NULL at the
 * first call, but '@cur' itself must NOT NULL.
 *
 * Returns found address value or zero if not found.
 */
unsigned long rethook_find_ret_addr(struct task_struct *tsk, unsigned long frame,
                                    struct llist_node **cur)
{
        struct rethook_node *rhn = NULL;
        unsigned long ret;

        if (WARN_ON_ONCE(!cur))
                return 0;

        if (tsk != current && task_is_running(tsk))
                return 0;

        do {
                ret = __rethook_find_ret_addr(tsk, cur);
                if (!ret)
                        break;
                rhn = container_of(*cur, struct rethook_node, llist);
        } while (rhn->frame != frame);

        return ret;
}
NOKPROBE_SYMBOL(rethook_find_ret_addr);

void __weak arch_rethook_fixup_return(struct pt_regs *regs,
                                      unsigned long correct_ret_addr)
{
        /*
         * Do nothing by default. If the architecture which uses a
         * frame pointer to record real return address on the stack,
         * it should fill this function to fixup the return address
         * so that stacktrace works from the rethook handler.
         */
}

/* This function will be called from each arch-defined trampoline. */
unsigned long rethook_trampoline_handler(struct pt_regs *regs,
                                         unsigned long frame)
{
        struct llist_node *first, *node = NULL;
        unsigned long correct_ret_addr;
        rethook_handler_t handler;
        struct rethook_node *rhn;

        correct_ret_addr = __rethook_find_ret_addr(current, &node);
        if (!correct_ret_addr) {
                pr_err("rethook: Return address not found! Maybe there is a bug in the kernel\n");
                BUG_ON(1);
        }

        instruction_pointer_set(regs, correct_ret_addr);

        /*
         * These loops must be protected from rethook_free_rcu() because those
         * are accessing 'rhn->rethook'.
         */
        preempt_disable_notrace();

        /*
         * Run the handler on the shadow stack. Do not unlink the list here because
         * stackdump inside the handlers needs to decode it.
         */
        first = current->rethooks.first;
        while (first) {
                rhn = container_of(first, struct rethook_node, llist);
                if (WARN_ON_ONCE(rhn->frame != frame))
                        break;
                handler = rethook_get_handler(rhn->rethook);
                if (handler)
                        handler(rhn, rhn->rethook->data,
                                correct_ret_addr, regs);

                if (first == node)
                        break;
                first = first->next;
        }

        /* Fixup registers for returning to correct address. */
        arch_rethook_fixup_return(regs, correct_ret_addr);

        /* Unlink used shadow stack */
        first = current->rethooks.first;
        current->rethooks.first = node->next;
        node->next = NULL;

        while (first) {
                rhn = container_of(first, struct rethook_node, llist);
                first = first->next;
                rethook_recycle(rhn);
        }
        preempt_enable_notrace();

        return correct_ret_addr;
}
NOKPROBE_SYMBOL(rethook_trampoline_handler);