linux/net/unix/garbage.c
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   1/*
   2 * NET3:        Garbage Collector For AF_UNIX sockets
   3 *
   4 * Garbage Collector:
   5 *      Copyright (C) Barak A. Pearlmutter.
   6 *      Released under the GPL version 2 or later.
   7 *
   8 * Chopped about by Alan Cox 22/3/96 to make it fit the AF_UNIX socket problem.
   9 * If it doesn't work blame me, it worked when Barak sent it.
  10 *
  11 * Assumptions:
  12 *
  13 *  - object w/ a bit
  14 *  - free list
  15 *
  16 * Current optimizations:
  17 *
  18 *  - explicit stack instead of recursion
  19 *  - tail recurse on first born instead of immediate push/pop
  20 *  - we gather the stuff that should not be killed into tree
  21 *    and stack is just a path from root to the current pointer.
  22 *
  23 *  Future optimizations:
  24 *
  25 *  - don't just push entire root set; process in place
  26 *
  27 *      This program is free software; you can redistribute it and/or
  28 *      modify it under the terms of the GNU General Public License
  29 *      as published by the Free Software Foundation; either version
  30 *      2 of the License, or (at your option) any later version.
  31 *
  32 *  Fixes:
  33 *      Alan Cox        07 Sept 1997    Vmalloc internal stack as needed.
  34 *                                      Cope with changing max_files.
  35 *      Al Viro         11 Oct 1998
  36 *              Graph may have cycles. That is, we can send the descriptor
  37 *              of foo to bar and vice versa. Current code chokes on that.
  38 *              Fix: move SCM_RIGHTS ones into the separate list and then
  39 *              skb_free() them all instead of doing explicit fput's.
  40 *              Another problem: since fput() may block somebody may
  41 *              create a new unix_socket when we are in the middle of sweep
  42 *              phase. Fix: revert the logic wrt MARKED. Mark everything
  43 *              upon the beginning and unmark non-junk ones.
  44 *
  45 *              [12 Oct 1998] AAARGH! New code purges all SCM_RIGHTS
  46 *              sent to connect()'ed but still not accept()'ed sockets.
  47 *              Fixed. Old code had slightly different problem here:
  48 *              extra fput() in situation when we passed the descriptor via
  49 *              such socket and closed it (descriptor). That would happen on
  50 *              each unix_gc() until the accept(). Since the struct file in
  51 *              question would go to the free list and might be reused...
  52 *              That might be the reason of random oopses on filp_close()
  53 *              in unrelated processes.
  54 *
  55 *      AV              28 Feb 1999
  56 *              Kill the explicit allocation of stack. Now we keep the tree
  57 *              with root in dummy + pointer (gc_current) to one of the nodes.
  58 *              Stack is represented as path from gc_current to dummy. Unmark
  59 *              now means "add to tree". Push == "make it a son of gc_current".
  60 *              Pop == "move gc_current to parent". We keep only pointers to
  61 *              parents (->gc_tree).
  62 *      AV              1 Mar 1999
  63 *              Damn. Added missing check for ->dead in listen queues scanning.
  64 *
  65 *      Miklos Szeredi 25 Jun 2007
  66 *              Reimplement with a cycle collecting algorithm. This should
  67 *              solve several problems with the previous code, like being racy
  68 *              wrt receive and holding up unrelated socket operations.
  69 */
  70
  71#include <linux/kernel.h>
  72#include <linux/string.h>
  73#include <linux/socket.h>
  74#include <linux/un.h>
  75#include <linux/net.h>
  76#include <linux/fs.h>
  77#include <linux/slab.h>
  78#include <linux/skbuff.h>
  79#include <linux/netdevice.h>
  80#include <linux/file.h>
  81#include <linux/proc_fs.h>
  82#include <linux/mutex.h>
  83
  84#include <net/sock.h>
  85#include <net/af_unix.h>
  86#include <net/scm.h>
  87#include <net/tcp_states.h>
  88
  89/* Internal data structures and random procedures: */
  90
  91static LIST_HEAD(gc_inflight_list);
  92static LIST_HEAD(gc_candidates);
  93static DEFINE_SPINLOCK(unix_gc_lock);
  94
  95unsigned int unix_tot_inflight;
  96
  97
  98static struct sock *unix_get_socket(struct file *filp)
  99{
 100        struct sock *u_sock = NULL;
 101        struct inode *inode = filp->f_path.dentry->d_inode;
 102
 103        /*
 104         *      Socket ?
 105         */
 106        if (S_ISSOCK(inode->i_mode)) {
 107                struct socket * sock = SOCKET_I(inode);
 108                struct sock * s = sock->sk;
 109
 110                /*
 111                 *      PF_UNIX ?
 112                 */
 113                if (s && sock->ops && sock->ops->family == PF_UNIX)
 114                        u_sock = s;
 115        }
 116        return u_sock;
 117}
 118
 119/*
 120 *      Keep the number of times in flight count for the file
 121 *      descriptor if it is for an AF_UNIX socket.
 122 */
 123
 124void unix_inflight(struct file *fp)
 125{
 126        struct sock *s = unix_get_socket(fp);
 127        if(s) {
 128                struct unix_sock *u = unix_sk(s);
 129                spin_lock(&unix_gc_lock);
 130                if (atomic_inc_return(&u->inflight) == 1) {
 131                        BUG_ON(!list_empty(&u->link));
 132                        list_add_tail(&u->link, &gc_inflight_list);
 133                } else {
 134                        BUG_ON(list_empty(&u->link));
 135                }
 136                unix_tot_inflight++;
 137                spin_unlock(&unix_gc_lock);
 138        }
 139}
 140
 141void unix_notinflight(struct file *fp)
 142{
 143        struct sock *s = unix_get_socket(fp);
 144        if(s) {
 145                struct unix_sock *u = unix_sk(s);
 146                spin_lock(&unix_gc_lock);
 147                BUG_ON(list_empty(&u->link));
 148                if (atomic_dec_and_test(&u->inflight))
 149                        list_del_init(&u->link);
 150                unix_tot_inflight--;
 151                spin_unlock(&unix_gc_lock);
 152        }
 153}
 154
 155static inline struct sk_buff *sock_queue_head(struct sock *sk)
 156{
 157        return (struct sk_buff *) &sk->sk_receive_queue;
 158}
 159
 160#define receive_queue_for_each_skb(sk, next, skb) \
 161        for (skb = sock_queue_head(sk)->next, next = skb->next; \
 162             skb != sock_queue_head(sk); skb = next, next = skb->next)
 163
 164static void scan_inflight(struct sock *x, void (*func)(struct unix_sock *),
 165                          struct sk_buff_head *hitlist)
 166{
 167        struct sk_buff *skb;
 168        struct sk_buff *next;
 169
 170        spin_lock(&x->sk_receive_queue.lock);
 171        receive_queue_for_each_skb(x, next, skb) {
 172                /*
 173                 *      Do we have file descriptors ?
 174                 */
 175                if (UNIXCB(skb).fp) {
 176                        bool hit = false;
 177                        /*
 178                         *      Process the descriptors of this socket
 179                         */
 180                        int nfd = UNIXCB(skb).fp->count;
 181                        struct file **fp = UNIXCB(skb).fp->fp;
 182                        while (nfd--) {
 183                                /*
 184                                 *      Get the socket the fd matches
 185                                 *      if it indeed does so
 186                                 */
 187                                struct sock *sk = unix_get_socket(*fp++);
 188                                if (sk) {
 189                                        hit = true;
 190                                        func(unix_sk(sk));
 191                                }
 192                        }
 193                        if (hit && hitlist != NULL) {
 194                                __skb_unlink(skb, &x->sk_receive_queue);
 195                                __skb_queue_tail(hitlist, skb);
 196                        }
 197                }
 198        }
 199        spin_unlock(&x->sk_receive_queue.lock);
 200}
 201
 202static void scan_children(struct sock *x, void (*func)(struct unix_sock *),
 203                          struct sk_buff_head *hitlist)
 204{
 205        if (x->sk_state != TCP_LISTEN)
 206                scan_inflight(x, func, hitlist);
 207        else {
 208                struct sk_buff *skb;
 209                struct sk_buff *next;
 210                struct unix_sock *u;
 211                LIST_HEAD(embryos);
 212
 213                /*
 214                 * For a listening socket collect the queued embryos
 215                 * and perform a scan on them as well.
 216                 */
 217                spin_lock(&x->sk_receive_queue.lock);
 218                receive_queue_for_each_skb(x, next, skb) {
 219                        u = unix_sk(skb->sk);
 220
 221                        /*
 222                         * An embryo cannot be in-flight, so it's safe
 223                         * to use the list link.
 224                         */
 225                        BUG_ON(!list_empty(&u->link));
 226                        list_add_tail(&u->link, &embryos);
 227                }
 228                spin_unlock(&x->sk_receive_queue.lock);
 229
 230                while (!list_empty(&embryos)) {
 231                        u = list_entry(embryos.next, struct unix_sock, link);
 232                        scan_inflight(&u->sk, func, hitlist);
 233                        list_del_init(&u->link);
 234                }
 235        }
 236}
 237
 238static void dec_inflight(struct unix_sock *usk)
 239{
 240        atomic_dec(&usk->inflight);
 241}
 242
 243static void inc_inflight(struct unix_sock *usk)
 244{
 245        atomic_inc(&usk->inflight);
 246}
 247
 248static void inc_inflight_move_tail(struct unix_sock *u)
 249{
 250        atomic_inc(&u->inflight);
 251        /*
 252         * If this is still a candidate, move it to the end of the
 253         * list, so that it's checked even if it was already passed
 254         * over
 255         */
 256        if (u->gc_candidate)
 257                list_move_tail(&u->link, &gc_candidates);
 258}
 259
 260/* The external entry point: unix_gc() */
 261
 262void unix_gc(void)
 263{
 264        static bool gc_in_progress = false;
 265
 266        struct unix_sock *u;
 267        struct unix_sock *next;
 268        struct sk_buff_head hitlist;
 269        struct list_head cursor;
 270
 271        spin_lock(&unix_gc_lock);
 272
 273        /* Avoid a recursive GC. */
 274        if (gc_in_progress)
 275                goto out;
 276
 277        gc_in_progress = true;
 278        /*
 279         * First, select candidates for garbage collection.  Only
 280         * in-flight sockets are considered, and from those only ones
 281         * which don't have any external reference.
 282         *
 283         * Holding unix_gc_lock will protect these candidates from
 284         * being detached, and hence from gaining an external
 285         * reference.  This also means, that since there are no
 286         * possible receivers, the receive queues of these sockets are
 287         * static during the GC, even though the dequeue is done
 288         * before the detach without atomicity guarantees.
 289         */
 290        list_for_each_entry_safe(u, next, &gc_inflight_list, link) {
 291                int total_refs;
 292                int inflight_refs;
 293
 294                total_refs = file_count(u->sk.sk_socket->file);
 295                inflight_refs = atomic_read(&u->inflight);
 296
 297                BUG_ON(inflight_refs < 1);
 298                BUG_ON(total_refs < inflight_refs);
 299                if (total_refs == inflight_refs) {
 300                        list_move_tail(&u->link, &gc_candidates);
 301                        u->gc_candidate = 1;
 302                }
 303        }
 304
 305        /*
 306         * Now remove all internal in-flight reference to children of
 307         * the candidates.
 308         */
 309        list_for_each_entry(u, &gc_candidates, link)
 310                scan_children(&u->sk, dec_inflight, NULL);
 311
 312        /*
 313         * Restore the references for children of all candidates,
 314         * which have remaining references.  Do this recursively, so
 315         * only those remain, which form cyclic references.
 316         *
 317         * Use a "cursor" link, to make the list traversal safe, even
 318         * though elements might be moved about.
 319         */
 320        list_add(&cursor, &gc_candidates);
 321        while (cursor.next != &gc_candidates) {
 322                u = list_entry(cursor.next, struct unix_sock, link);
 323
 324                /* Move cursor to after the current position. */
 325                list_move(&cursor, &u->link);
 326
 327                if (atomic_read(&u->inflight) > 0) {
 328                        list_move_tail(&u->link, &gc_inflight_list);
 329                        u->gc_candidate = 0;
 330                        scan_children(&u->sk, inc_inflight_move_tail, NULL);
 331                }
 332        }
 333        list_del(&cursor);
 334
 335        /*
 336         * Now gc_candidates contains only garbage.  Restore original
 337         * inflight counters for these as well, and remove the skbuffs
 338         * which are creating the cycle(s).
 339         */
 340        skb_queue_head_init(&hitlist);
 341        list_for_each_entry(u, &gc_candidates, link)
 342                scan_children(&u->sk, inc_inflight, &hitlist);
 343
 344        spin_unlock(&unix_gc_lock);
 345
 346        /* Here we are. Hitlist is filled. Die. */
 347        __skb_queue_purge(&hitlist);
 348
 349        spin_lock(&unix_gc_lock);
 350
 351        /* All candidates should have been detached by now. */
 352        BUG_ON(!list_empty(&gc_candidates));
 353        gc_in_progress = false;
 354
 355 out:
 356        spin_unlock(&unix_gc_lock);
 357}
 358
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