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/skbuff.h>
  78#include <linux/netdevice.h>
  79#include <linux/file.h>
  80#include <linux/proc_fs.h>
  81#include <linux/mutex.h>
  82#include <linux/wait.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);
  94static DECLARE_WAIT_QUEUE_HEAD(unix_gc_wait);
  95
  96unsigned int unix_tot_inflight;
  97
  98
  99static struct sock *unix_get_socket(struct file *filp)
 100{
 101        struct sock *u_sock = NULL;
 102        struct inode *inode = filp->f_path.dentry->d_inode;
 103
 104        /*
 105         *      Socket ?
 106         */
 107        if (S_ISSOCK(inode->i_mode)) {
 108                struct socket *sock = SOCKET_I(inode);
 109                struct sock *s = sock->sk;
 110
 111                /*
 112                 *      PF_UNIX ?
 113                 */
 114                if (s && sock->ops && sock->ops->family == PF_UNIX)
 115                        u_sock = s;
 116        }
 117        return u_sock;
 118}
 119
 120/*
 121 *      Keep the number of times in flight count for the file
 122 *      descriptor if it is for an AF_UNIX socket.
 123 */
 124
 125void unix_inflight(struct file *fp)
 126{
 127        struct sock *s = unix_get_socket(fp);
 128        if (s) {
 129                struct unix_sock *u = unix_sk(s);
 130                spin_lock(&unix_gc_lock);
 131                if (atomic_long_inc_return(&u->inflight) == 1) {
 132                        BUG_ON(!list_empty(&u->link));
 133                        list_add_tail(&u->link, &gc_inflight_list);
 134                } else {
 135                        BUG_ON(list_empty(&u->link));
 136                }
 137                unix_tot_inflight++;
 138                spin_unlock(&unix_gc_lock);
 139        }
 140}
 141
 142void unix_notinflight(struct file *fp)
 143{
 144        struct sock *s = unix_get_socket(fp);
 145        if (s) {
 146                struct unix_sock *u = unix_sk(s);
 147                spin_lock(&unix_gc_lock);
 148                BUG_ON(list_empty(&u->link));
 149                if (atomic_long_dec_and_test(&u->inflight))
 150                        list_del_init(&u->link);
 151                unix_tot_inflight--;
 152                spin_unlock(&unix_gc_lock);
 153        }
 154}
 155
 156static void scan_inflight(struct sock *x, void (*func)(struct unix_sock *),
 157                          struct sk_buff_head *hitlist)
 158{
 159        struct sk_buff *skb;
 160        struct sk_buff *next;
 161
 162        spin_lock(&x->sk_receive_queue.lock);
 163        skb_queue_walk_safe(&x->sk_receive_queue, skb, next) {
 164                /*
 165                 *      Do we have file descriptors ?
 166                 */
 167                if (UNIXCB(skb).fp) {
 168                        bool hit = false;
 169                        /*
 170                         *      Process the descriptors of this socket
 171                         */
 172                        int nfd = UNIXCB(skb).fp->count;
 173                        struct file **fp = UNIXCB(skb).fp->fp;
 174                        while (nfd--) {
 175                                /*
 176                                 *      Get the socket the fd matches
 177                                 *      if it indeed does so
 178                                 */
 179                                struct sock *sk = unix_get_socket(*fp++);
 180                                if (sk) {
 181                                        struct unix_sock *u = unix_sk(sk);
 182
 183                                        /*
 184                                         * Ignore non-candidates, they could
 185                                         * have been added to the queues after
 186                                         * starting the garbage collection
 187                                         */
 188                                        if (u->gc_candidate) {
 189                                                hit = true;
 190                                                func(u);
 191                                        }
 192                                }
 193                        }
 194                        if (hit && hitlist != NULL) {
 195                                __skb_unlink(skb, &x->sk_receive_queue);
 196                                __skb_queue_tail(hitlist, skb);
 197                        }
 198                }
 199        }
 200        spin_unlock(&x->sk_receive_queue.lock);
 201}
 202
 203static void scan_children(struct sock *x, void (*func)(struct unix_sock *),
 204                          struct sk_buff_head *hitlist)
 205{
 206        if (x->sk_state != TCP_LISTEN)
 207                scan_inflight(x, func, hitlist);
 208        else {
 209                struct sk_buff *skb;
 210                struct sk_buff *next;
 211                struct unix_sock *u;
 212                LIST_HEAD(embryos);
 213
 214                /*
 215                 * For a listening socket collect the queued embryos
 216                 * and perform a scan on them as well.
 217                 */
 218                spin_lock(&x->sk_receive_queue.lock);
 219                skb_queue_walk_safe(&x->sk_receive_queue, skb, next) {
 220                        u = unix_sk(skb->sk);
 221
 222                        /*
 223                         * An embryo cannot be in-flight, so it's safe
 224                         * to use the list link.
 225                         */
 226                        BUG_ON(!list_empty(&u->link));
 227                        list_add_tail(&u->link, &embryos);
 228                }
 229                spin_unlock(&x->sk_receive_queue.lock);
 230
 231                while (!list_empty(&embryos)) {
 232                        u = list_entry(embryos.next, struct unix_sock, link);
 233                        scan_inflight(&u->sk, func, hitlist);
 234                        list_del_init(&u->link);
 235                }
 236        }
 237}
 238
 239static void dec_inflight(struct unix_sock *usk)
 240{
 241        atomic_long_dec(&usk->inflight);
 242}
 243
 244static void inc_inflight(struct unix_sock *usk)
 245{
 246        atomic_long_inc(&usk->inflight);
 247}
 248
 249static void inc_inflight_move_tail(struct unix_sock *u)
 250{
 251        atomic_long_inc(&u->inflight);
 252        /*
 253         * If this still might be part of a cycle, move it to the end
 254         * of the list, so that it's checked even if it was already
 255         * passed over
 256         */
 257        if (u->gc_maybe_cycle)
 258                list_move_tail(&u->link, &gc_candidates);
 259}
 260
 261static bool gc_in_progress = false;
 262
 263void wait_for_unix_gc(void)
 264{
 265        wait_event(unix_gc_wait, gc_in_progress == false);
 266}
 267
 268/* The external entry point: unix_gc() */
 269void unix_gc(void)
 270{
 271        struct unix_sock *u;
 272        struct unix_sock *next;
 273        struct sk_buff_head hitlist;
 274        struct list_head cursor;
 275        LIST_HEAD(not_cycle_list);
 276
 277        spin_lock(&unix_gc_lock);
 278
 279        /* Avoid a recursive GC. */
 280        if (gc_in_progress)
 281                goto out;
 282
 283        gc_in_progress = true;
 284        /*
 285         * First, select candidates for garbage collection.  Only
 286         * in-flight sockets are considered, and from those only ones
 287         * which don't have any external reference.
 288         *
 289         * Holding unix_gc_lock will protect these candidates from
 290         * being detached, and hence from gaining an external
 291         * reference.  Since there are no possible receivers, all
 292         * buffers currently on the candidates' queues stay there
 293         * during the garbage collection.
 294         *
 295         * We also know that no new candidate can be added onto the
 296         * receive queues.  Other, non candidate sockets _can_ be
 297         * added to queue, so we must make sure only to touch
 298         * candidates.
 299         */
 300        list_for_each_entry_safe(u, next, &gc_inflight_list, link) {
 301                long total_refs;
 302                long inflight_refs;
 303
 304                total_refs = file_count(u->sk.sk_socket->file);
 305                inflight_refs = atomic_long_read(&u->inflight);
 306
 307                BUG_ON(inflight_refs < 1);
 308                BUG_ON(total_refs < inflight_refs);
 309                if (total_refs == inflight_refs) {
 310                        list_move_tail(&u->link, &gc_candidates);
 311                        u->gc_candidate = 1;
 312                        u->gc_maybe_cycle = 1;
 313                }
 314        }
 315
 316        /*
 317         * Now remove all internal in-flight reference to children of
 318         * the candidates.
 319         */
 320        list_for_each_entry(u, &gc_candidates, link)
 321                scan_children(&u->sk, dec_inflight, NULL);
 322
 323        /*
 324         * Restore the references for children of all candidates,
 325         * which have remaining references.  Do this recursively, so
 326         * only those remain, which form cyclic references.
 327         *
 328         * Use a "cursor" link, to make the list traversal safe, even
 329         * though elements might be moved about.
 330         */
 331        list_add(&cursor, &gc_candidates);
 332        while (cursor.next != &gc_candidates) {
 333                u = list_entry(cursor.next, struct unix_sock, link);
 334
 335                /* Move cursor to after the current position. */
 336                list_move(&cursor, &u->link);
 337
 338                if (atomic_long_read(&u->inflight) > 0) {
 339                        list_move_tail(&u->link, &not_cycle_list);
 340                        u->gc_maybe_cycle = 0;
 341                        scan_children(&u->sk, inc_inflight_move_tail, NULL);
 342                }
 343        }
 344        list_del(&cursor);
 345
 346        /*
 347         * not_cycle_list contains those sockets which do not make up a
 348         * cycle.  Restore these to the inflight list.
 349         */
 350        while (!list_empty(&not_cycle_list)) {
 351                u = list_entry(not_cycle_list.next, struct unix_sock, link);
 352                u->gc_candidate = 0;
 353                list_move_tail(&u->link, &gc_inflight_list);
 354        }
 355
 356        /*
 357         * Now gc_candidates contains only garbage.  Restore original
 358         * inflight counters for these as well, and remove the skbuffs
 359         * which are creating the cycle(s).
 360         */
 361        skb_queue_head_init(&hitlist);
 362        list_for_each_entry(u, &gc_candidates, link)
 363        scan_children(&u->sk, inc_inflight, &hitlist);
 364
 365        spin_unlock(&unix_gc_lock);
 366
 367        /* Here we are. Hitlist is filled. Die. */
 368        __skb_queue_purge(&hitlist);
 369
 370        spin_lock(&unix_gc_lock);
 371
 372        /* All candidates should have been detached by now. */
 373        BUG_ON(!list_empty(&gc_candidates));
 374        gc_in_progress = false;
 375        wake_up(&unix_gc_wait);
 376
 377 out:
 378        spin_unlock(&unix_gc_lock);
 379}
 380
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