net/unix/garbage.c

   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
  91 static LIST_HEAD(gc_inflight_list);
  92 static LIST_HEAD(gc_candidates);
  93 static DEFINE_SPINLOCK(unix_gc_lock);
  94 static DECLARE_WAIT_QUEUE_HEAD(unix_gc_wait);
  95
  96 unsigned int unix_tot_inflight;
  97
  98
  99 static 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
 125 void 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
 142 void 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
 156 static 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
 203 static 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
 239 static void dec_inflight(struct unix_sock *usk)
 240 {
 241         atomic_long_dec(&usk->inflight);
 242 }
 243
 244 static void inc_inflight(struct unix_sock *usk)
 245 {
 246         atomic_long_inc(&usk->inflight);
 247 }
 248
 249 static 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
 261 static bool gc_in_progress = false;
 262
 263 void 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() */
 269 void 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 }