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/*

 * NET3:        Garbage Collector For AF_UNIX sockets

 *

 * Garbage Collector:

 *      Copyright (C) Barak A. Pearlmutter.

 *      Released under the GPL version 2 or later.

 *

 * 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

 *

 *      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.

 *

 *  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/slab.h>

#include <linux/skbuff.h>

#include <linux/netdevice.h>

#include <linux/file.h>

#include <linux/proc_fs.h>

#include <linux/mutex.h>

#include <net/sock.h>

#include <net/af_unix.h>

#include <net/scm.h>

#include <net/tcp_states.h>

/* Internal data structures and random procedures: */

static LIST_HEAD(gc_inflight_list);

static LIST_HEAD(gc_candidates);

static DEFINE_SPINLOCK(unix_gc_lock);

unsigned int unix_tot_inflight;

static struct sock *unix_get_socket(struct file *filp)

{

        struct sock *u_sock = NULL;

        struct inode *inode = filp->f_path.dentry->d_inode;

        /*

         *      Socket ?

         */

        if (S_ISSOCK(inode->i_mode)) {

                struct socket * sock = SOCKET_I(inode);

                struct sock * s = sock->sk;

                /*

                 *      PF_UNIX ?

                 */

                if (s && sock->ops && sock->ops->family == PF_UNIX)

                        u_sock = s;

        }

        return u_sock;

}

/*

 *      Keep the number of times in flight count for the file

 *      descriptor if it is for an AF_UNIX socket.

 */

void unix_inflight(struct file *fp)

{

        struct sock *s = unix_get_socket(fp);

        if(s) {

                struct unix_sock *u = unix_sk(s);

                spin_lock(&unix_gc_lock);

                if (atomic_inc_return(&u->inflight) == 1) {

                        BUG_ON(!list_empty(&u->link));

                        list_add_tail(&u->link, &gc_inflight_list);

                } else {

                        BUG_ON(list_empty(&u->link));

                }

                unix_tot_inflight++;

                spin_unlock(&unix_gc_lock);

        }

}

void unix_notinflight(struct file *fp)

{

        struct sock *s = unix_get_socket(fp);

        if(s) {

                struct unix_sock *u = unix_sk(s);

                spin_lock(&unix_gc_lock);

                BUG_ON(list_empty(&u->link));

                if (atomic_dec_and_test(&u->inflight))

                        list_del_init(&u->link);

                unix_tot_inflight--;

                spin_unlock(&unix_gc_lock);

        }

}

static inline struct sk_buff *sock_queue_head(struct sock *sk)

{

        return (struct sk_buff *) &sk->sk_receive_queue;

}

#define receive_queue_for_each_skb(sk, next, skb) \

        for (skb = sock_queue_head(sk)->next, next = skb->next; \

             skb != sock_queue_head(sk); skb = next, next = skb->next)

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);

        receive_queue_for_each_skb(x, next, skb) {

                /*

                 *      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) {

                                        hit = true;

                                        func(unix_sk(sk));

                                }

                        }

                        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);

                receive_queue_for_each_skb(x, next, skb) {

                        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_dec(&usk->inflight);

}

static void inc_inflight(struct unix_sock *usk)

{

        atomic_inc(&usk->inflight);

}

static void inc_inflight_move_tail(struct unix_sock *u)

{

        atomic_inc(&u->inflight);

        /*

         * If this is still a candidate, move it to the end of the

         * list, so that it's checked even if it was already passed

         * over

         */

        if (u->gc_candidate)

                list_move_tail(&u->link, &gc_candidates);

}

/* The external entry point: unix_gc() */

void unix_gc(void)

{

        static bool gc_in_progress = false;

        struct unix_sock *u;

        struct unix_sock *next;

        struct sk_buff_head hitlist;

        struct list_head cursor;

        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.  This also means, that since there are no

         * possible receivers, the receive queues of these sockets are

         * static during the GC, even though the dequeue is done

         * before the detach without atomicity guarantees.

         */

        list_for_each_entry_safe(u, next, &gc_inflight_list, link) {

                int total_refs;

                int inflight_refs;

                total_refs = file_count(u->sk.sk_socket->file);

                inflight_refs = atomic_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);

                        u->gc_candidate = 1;

                }

        }

        /*

         * 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_read(&u->inflight) > 0) {

                        list_move_tail(&u->link, &gc_inflight_list);

                        u->gc_candidate = 0;

                        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);

        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;

 out:

        spin_unlock(&unix_gc_lock);

}