// SPDX-License-Identifier: GPL-2.0-or-later
/*
 * NET3:        Garbage Collector For AF_UNIX sockets
 *
 * Garbage Collector:
 *      Copyright (C) Barak A. Pearlmutter.
 *
 * Chopped about by Alan Cox 22/3/96 to make it fit the AF_UNIX socket problem.
 * If it doesn't work blame me, it worked when Barak sent it.
 *
 * Assumptions:
 *
 *  - object w/ a bit
 *  - free list
 *
 * Current optimizations:
 *
 *  - explicit stack instead of recursion
 *  - tail recurse on first born instead of immediate push/pop
 *  - we gather the stuff that should not be killed into tree
 *    and stack is just a path from root to the current pointer.
 *
 *  Future optimizations:
 *
 *  - don't just push entire root set; process in place
 *
 *  Fixes:
 *      Alan Cox        07 Sept 1997    Vmalloc internal stack as needed.
 *                                      Cope with changing max_files.
 *      Al Viro         11 Oct 1998
 *              Graph may have cycles. That is, we can send the descriptor
 *              of foo to bar and vice versa. Current code chokes on that.
 *              Fix: move SCM_RIGHTS ones into the separate list and then
 *              skb_free() them all instead of doing explicit fput's.
 *              Another problem: since fput() may block somebody may
 *              create a new unix_socket when we are in the middle of sweep
 *              phase. Fix: revert the logic wrt MARKED. Mark everything
 *              upon the beginning and unmark non-junk ones.
 *
 *              [12 Oct 1998] AAARGH! New code purges all SCM_RIGHTS
 *              sent to connect()'ed but still not accept()'ed sockets.
 *              Fixed. Old code had slightly different problem here:
 *              extra fput() in situation when we passed the descriptor via
 *              such socket and closed it (descriptor). That would happen on
 *              each unix_gc() until the accept(). Since the struct file in
 *              question would go to the free list and might be reused...
 *              That might be the reason of random oopses on filp_close()
 *              in unrelated processes.
 *
 *      AV              28 Feb 1999
 *              Kill the explicit allocation of stack. Now we keep the tree
 *              with root in dummy + pointer (gc_current) to one of the nodes.
 *              Stack is represented as path from gc_current to dummy. Unmark
 *              now means "add to tree". Push == "make it a son of gc_current".
 *              Pop == "move gc_current to parent". We keep only pointers to
 *              parents (->gc_tree).
 *      AV              1 Mar 1999
 *              Damn. Added missing check for ->dead in listen queues scanning.
 *
 *      Miklos Szeredi 25 Jun 2007
 *              Reimplement with a cycle collecting algorithm. This should
 *              solve several problems with the previous code, like being racy
 *              wrt receive and holding up unrelated socket operations.
 */

#include <linux/kernel.h>
#include <linux/string.h>
#include <linux/socket.h>
#include <linux/un.h>
#include <linux/net.h>
#include <linux/fs.h>
#include <linux/skbuff.h>
#include <linux/netdevice.h>
#include <linux/file.h>
#include <linux/proc_fs.h>
#include <linux/mutex.h>
#include <linux/wait.h>

#include <net/sock.h>
#include <net/af_unix.h>
#include <net/scm.h>
#include <net/tcp_states.h>

#include "scm.h"

/* Internal data structures and random procedures: */

static LIST_HEAD(gc_candidates);
static DECLARE_WAIT_QUEUE_HEAD(unix_gc_wait);

static void scan_inflight(struct sock *x, void (*func)(struct unix_sock *),
                          struct sk_buff_head *hitlist)
{
        struct sk_buff *skb;
        struct sk_buff *next;

        spin_lock(&x->sk_receive_queue.lock);
        skb_queue_walk_safe(&x->sk_receive_queue, skb, next) {
                /* Do we have file descriptors ? */
                if (UNIXCB(skb).fp) {
                        bool hit = false;
                        /* Process the descriptors of this socket */
                        int nfd = UNIXCB(skb).fp->count;
                        struct file **fp = UNIXCB(skb).fp->fp;

                        while (nfd--) {
                                /* Get the socket the fd matches if it indeed does so */
                                struct sock *sk = unix_get_socket(*fp++);

                                if (sk) {
                                        struct unix_sock *u = unix_sk(sk);

                                        /* Ignore non-candidates, they could
                                         * have been added to the queues after
                                         * starting the garbage collection
                                         */
                                        if (test_bit(UNIX_GC_CANDIDATE, &u->gc_flags)) {
                                                hit = true;

                                                func(u);
                                        }
                                }
                        }
                        if (hit && hitlist != NULL) {
                                __skb_unlink(skb, &x->sk_receive_queue);
                                __skb_queue_tail(hitlist, skb);
                        }
                }
        }
        spin_unlock(&x->sk_receive_queue.lock);
}

static void scan_children(struct sock *x, void (*func)(struct unix_sock *),
                          struct sk_buff_head *hitlist)
{
        if (x->sk_state != TCP_LISTEN) {
                scan_inflight(x, func, hitlist);
        } else {
                struct sk_buff *skb;
                struct sk_buff *next;
                struct unix_sock *u;
                LIST_HEAD(embryos);

                /* For a listening socket collect the queued embryos
                 * and perform a scan on them as well.
                 */
                spin_lock(&x->sk_receive_queue.lock);
                skb_queue_walk_safe(&x->sk_receive_queue, skb, next) {
                        u = unix_sk(skb->sk);

                        /* An embryo cannot be in-flight, so it's safe
                         * to use the list link.
                         */
                        BUG_ON(!list_empty(&u->link));
                        list_add_tail(&u->link, &embryos);
                }
                spin_unlock(&x->sk_receive_queue.lock);

                while (!list_empty(&embryos)) {
                        u = list_entry(embryos.next, struct unix_sock, link);
                        scan_inflight(&u->sk, func, hitlist);
                        list_del_init(&u->link);
                }
        }
}

static void dec_inflight(struct unix_sock *usk)
{
        atomic_long_dec(&usk->inflight);
}

static void inc_inflight(struct unix_sock *usk)
{
        atomic_long_inc(&usk->inflight);
}

static void inc_inflight_move_tail(struct unix_sock *u)
{
        atomic_long_inc(&u->inflight);
        /* If this still might be part of a cycle, move it to the end
         * of the list, so that it's checked even if it was already
         * passed over
         */
        if (test_bit(UNIX_GC_MAYBE_CYCLE, &u->gc_flags))
                list_move_tail(&u->link, &gc_candidates);
}

static bool gc_in_progress;
#define UNIX_INFLIGHT_TRIGGER_GC 16000

void wait_for_unix_gc(void)
{
        /* If number of inflight sockets is insane,
         * force a garbage collect right now.
         */
        if (unix_tot_inflight > UNIX_INFLIGHT_TRIGGER_GC && !gc_in_progress)
                unix_gc();
        wait_event(unix_gc_wait, gc_in_progress == false);
}

/* The external entry point: unix_gc() */
void unix_gc(void)
{
        struct unix_sock *u;
        struct unix_sock *next;
        struct sk_buff_head hitlist;
        struct list_head cursor;
        LIST_HEAD(not_cycle_list);

        spin_lock(&unix_gc_lock);

        /* Avoid a recursive GC. */
        if (gc_in_progress)
                goto out;

        gc_in_progress = true;
        /* First, select candidates for garbage collection.  Only
         * in-flight sockets are considered, and from those only ones
         * which don't have any external reference.
         *
         * Holding unix_gc_lock will protect these candidates from
         * being detached, and hence from gaining an external
         * reference.  Since there are no possible receivers, all
         * buffers currently on the candidates' queues stay there
         * during the garbage collection.
         *
         * We also know that no new candidate can be added onto the
         * receive queues.  Other, non candidate sockets _can_ be
         * added to queue, so we must make sure only to touch
         * candidates.
         */
        list_for_each_entry_safe(u, next, &gc_inflight_list, link) {
                long total_refs;
                long inflight_refs;

                total_refs = file_count(u->sk.sk_socket->file);
                inflight_refs = atomic_long_read(&u->inflight);

                BUG_ON(inflight_refs < 1);
                BUG_ON(total_refs < inflight_refs);
                if (total_refs == inflight_refs) {
                        list_move_tail(&u->link, &gc_candidates);
                        __set_bit(UNIX_GC_CANDIDATE, &u->gc_flags);
                        __set_bit(UNIX_GC_MAYBE_CYCLE, &u->gc_flags);
                }
        }

        /* Now remove all internal in-flight reference to children of
         * the candidates.
         */
        list_for_each_entry(u, &gc_candidates, link)
                scan_children(&u->sk, dec_inflight, NULL);

        /* Restore the references for children of all candidates,
         * which have remaining references.  Do this recursively, so
         * only those remain, which form cyclic references.
         *
         * Use a "cursor" link, to make the list traversal safe, even
         * though elements might be moved about.
         */
        list_add(&cursor, &gc_candidates);
        while (cursor.next != &gc_candidates) {
                u = list_entry(cursor.next, struct unix_sock, link);

                /* Move cursor to after the current position. */
                list_move(&cursor, &u->link);

                if (atomic_long_read(&u->inflight) > 0) {
                        list_move_tail(&u->link, &not_cycle_list);
                        __clear_bit(UNIX_GC_MAYBE_CYCLE, &u->gc_flags);
                        scan_children(&u->sk, inc_inflight_move_tail, NULL);
                }
        }
        list_del(&cursor);

        /* Now gc_candidates contains only garbage.  Restore original
         * inflight counters for these as well, and remove the skbuffs
         * which are creating the cycle(s).
         */
        skb_queue_head_init(&hitlist);
        list_for_each_entry(u, &gc_candidates, link)
                scan_children(&u->sk, inc_inflight, &hitlist);

        /* not_cycle_list contains those sockets which do not make up a
         * cycle.  Restore these to the inflight list.
         */
        while (!list_empty(&not_cycle_list)) {
                u = list_entry(not_cycle_list.next, struct unix_sock, link);
                __clear_bit(UNIX_GC_CANDIDATE, &u->gc_flags);
                list_move_tail(&u->link, &gc_inflight_list);
        }

        spin_unlock(&unix_gc_lock);

        /* Here we are. Hitlist is filled. Die. */
        __skb_queue_purge(&hitlist);

        spin_lock(&unix_gc_lock);

        /* All candidates should have been detached by now. */
        BUG_ON(!list_empty(&gc_candidates));
        gc_in_progress = false;
        wake_up(&unix_gc_wait);

 out:
        spin_unlock(&unix_gc_lock);
}