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/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 #include <linux/wait.h>
  84
  85 #include <net/sock.h>
  86 #include <net/af_unix.h>
  87 #include <net/scm.h>
  88 #include <net/tcp_states.h>
  89
  90 /* Internal data structures and random procedures: */
  91
  92 static LIST_HEAD(gc_inflight_list);
  93 static LIST_HEAD(gc_candidates);
  94 static DEFINE_SPINLOCK(unix_gc_lock);
  95 static DECLARE_WAIT_QUEUE_HEAD(unix_gc_wait);
  96
  97 unsigned int unix_tot_inflight;
  98
  99
 100 static struct sock *unix_get_socket(struct file *filp)
 101 {
 102         struct sock *u_sock = NULL;
 103         struct inode *inode = filp->f_path.dentry->d_inode;
 104
 105         /*
 106          *      Socket ?
 107          */
 108         if (S_ISSOCK(inode->i_mode)) {
 109                 struct socket *sock = SOCKET_I(inode);
 110                 struct sock *s = sock->sk;
 111
 112                 /*
 113                  *      PF_UNIX ?
 114                  */
 115                 if (s && sock->ops && sock->ops->family == PF_UNIX)
 116                         u_sock = s;
 117         }
 118         return u_sock;
 119 }
 120
 121 /*
 122  *      Keep the number of times in flight count for the file
 123  *      descriptor if it is for an AF_UNIX socket.
 124  */
 125
 126 void unix_inflight(struct file *fp)
 127 {
 128         struct sock *s = unix_get_socket(fp);
 129         if (s) {
 130                 struct unix_sock *u = unix_sk(s);
 131                 spin_lock(&unix_gc_lock);
 132                 if (atomic_long_inc_return(&u->inflight) == 1) {
 133                         BUG_ON(!list_empty(&u->link));
 134                         list_add_tail(&u->link, &gc_inflight_list);
 135                 } else {
 136                         BUG_ON(list_empty(&u->link));
 137                 }
 138                 unix_tot_inflight++;
 139                 spin_unlock(&unix_gc_lock);
 140         }
 141 }
 142
 143 void unix_notinflight(struct file *fp)
 144 {
 145         struct sock *s = unix_get_socket(fp);
 146         if (s) {
 147                 struct unix_sock *u = unix_sk(s);
 148                 spin_lock(&unix_gc_lock);
 149                 BUG_ON(list_empty(&u->link));
 150                 if (atomic_long_dec_and_test(&u->inflight))
 151                         list_del_init(&u->link);
 152                 unix_tot_inflight--;
 153                 spin_unlock(&unix_gc_lock);
 154         }
 155 }
 156
 157 static inline struct sk_buff *sock_queue_head(struct sock *sk)
 158 {
 159         return (struct sk_buff *)&sk->sk_receive_queue;
 160 }
 161
 162 #define receive_queue_for_each_skb(sk, next, skb) \
 163         for (skb = sock_queue_head(sk)->next, next = skb->next; \
 164              skb != sock_queue_head(sk); skb = next, next = skb->next)
 165
 166 static void scan_inflight(struct sock *x, void (*func)(struct unix_sock *),
 167                           struct sk_buff_head *hitlist)
 168 {
 169         struct sk_buff *skb;
 170         struct sk_buff *next;
 171
 172         spin_lock(&x->sk_receive_queue.lock);
 173         receive_queue_for_each_skb(x, next, skb) {
 174                 /*
 175                  *      Do we have file descriptors ?
 176                  */
 177                 if (UNIXCB(skb).fp) {
 178                         bool hit = false;
 179                         /*
 180                          *      Process the descriptors of this socket
 181                          */
 182                         int nfd = UNIXCB(skb).fp->count;
 183                         struct file **fp = UNIXCB(skb).fp->fp;
 184                         while (nfd--) {
 185                                 /*
 186                                  *      Get the socket the fd matches
 187                                  *      if it indeed does so
 188                                  */
 189                                 struct sock *sk = unix_get_socket(*fp++);
 190                                 if (sk) {
 191                                         struct unix_sock *u = unix_sk(sk);
 192
 193                                         /*
 194                                          * Ignore non-candidates, they could
 195                                          * have been added to the queues after
 196                                          * starting the garbage collection
 197                                          */
 198                                         if (u->gc_candidate) {
 199                                                 hit = true;
 200                                                 func(u);
 201                                         }
 202                                 }
 203                         }
 204                         if (hit && hitlist != NULL) {
 205                                 __skb_unlink(skb, &x->sk_receive_queue);
 206                                 __skb_queue_tail(hitlist, skb);
 207                         }
 208                 }
 209         }
 210         spin_unlock(&x->sk_receive_queue.lock);
 211 }
 212
 213 static void scan_children(struct sock *x, void (*func)(struct unix_sock *),
 214                           struct sk_buff_head *hitlist)
 215 {
 216         if (x->sk_state != TCP_LISTEN)
 217                 scan_inflight(x, func, hitlist);
 218         else {
 219                 struct sk_buff *skb;
 220                 struct sk_buff *next;
 221                 struct unix_sock *u;
 222                 LIST_HEAD(embryos);
 223
 224                 /*
 225                  * For a listening socket collect the queued embryos
 226                  * and perform a scan on them as well.
 227                  */
 228                 spin_lock(&x->sk_receive_queue.lock);
 229                 receive_queue_for_each_skb(x, next, skb) {
 230                         u = unix_sk(skb->sk);
 231
 232                         /*
 233                          * An embryo cannot be in-flight, so it's safe
 234                          * to use the list link.
 235                          */
 236                         BUG_ON(!list_empty(&u->link));
 237                         list_add_tail(&u->link, &embryos);
 238                 }
 239                 spin_unlock(&x->sk_receive_queue.lock);
 240
 241                 while (!list_empty(&embryos)) {
 242                         u = list_entry(embryos.next, struct unix_sock, link);
 243                         scan_inflight(&u->sk, func, hitlist);
 244                         list_del_init(&u->link);
 245                 }
 246         }
 247 }
 248
 249 static void dec_inflight(struct unix_sock *usk)
 250 {
 251         atomic_long_dec(&usk->inflight);
 252 }
 253
 254 static void inc_inflight(struct unix_sock *usk)
 255 {
 256         atomic_long_inc(&usk->inflight);
 257 }
 258
 259 static void inc_inflight_move_tail(struct unix_sock *u)
 260 {
 261         atomic_long_inc(&u->inflight);
 262         /*
 263          * If this still might be part of a cycle, move it to the end
 264          * of the list, so that it's checked even if it was already
 265          * passed over
 266          */
 267         if (u->gc_maybe_cycle)
 268                 list_move_tail(&u->link, &gc_candidates);
 269 }
 270
 271 static bool gc_in_progress = false;
 272
 273 void wait_for_unix_gc(void)
 274 {
 275         wait_event(unix_gc_wait, gc_in_progress == false);
 276 }
 277
 278 /* The external entry point: unix_gc() */
 279 void unix_gc(void)
 280 {
 281         struct unix_sock *u;
 282         struct unix_sock *next;
 283         struct sk_buff_head hitlist;
 284         struct list_head cursor;
 285         LIST_HEAD(not_cycle_list);
 286
 287         spin_lock(&unix_gc_lock);
 288
 289         /* Avoid a recursive GC. */
 290         if (gc_in_progress)
 291                 goto out;
 292
 293         gc_in_progress = true;
 294         /*
 295          * First, select candidates for garbage collection.  Only
 296          * in-flight sockets are considered, and from those only ones
 297          * which don't have any external reference.
 298          *
 299          * Holding unix_gc_lock will protect these candidates from
 300          * being detached, and hence from gaining an external
 301          * reference.  Since there are no possible receivers, all
 302          * buffers currently on the candidates' queues stay there
 303          * during the garbage collection.
 304          *
 305          * We also know that no new candidate can be added onto the
 306          * receive queues.  Other, non candidate sockets _can_ be
 307          * added to queue, so we must make sure only to touch
 308          * candidates.
 309          */
 310         list_for_each_entry_safe(u, next, &gc_inflight_list, link) {
 311                 long total_refs;
 312                 long inflight_refs;
 313
 314                 total_refs = file_count(u->sk.sk_socket->file);
 315                 inflight_refs = atomic_long_read(&u->inflight);
 316
 317                 BUG_ON(inflight_refs < 1);
 318                 BUG_ON(total_refs < inflight_refs);
 319                 if (total_refs == inflight_refs) {
 320                         list_move_tail(&u->link, &gc_candidates);
 321                         u->gc_candidate = 1;
 322                         u->gc_maybe_cycle = 1;
 323                 }
 324         }
 325
 326         /*
 327          * Now remove all internal in-flight reference to children of
 328          * the candidates.
 329          */
 330         list_for_each_entry(u, &gc_candidates, link)
 331                 scan_children(&u->sk, dec_inflight, NULL);
 332
 333         /*
 334          * Restore the references for children of all candidates,
 335          * which have remaining references.  Do this recursively, so
 336          * only those remain, which form cyclic references.
 337          *
 338          * Use a "cursor" link, to make the list traversal safe, even
 339          * though elements might be moved about.
 340          */
 341         list_add(&cursor, &gc_candidates);
 342         while (cursor.next != &gc_candidates) {
 343                 u = list_entry(cursor.next, struct unix_sock, link);
 344
 345                 /* Move cursor to after the current position. */
 346                 list_move(&cursor, &u->link);
 347
 348                 if (atomic_long_read(&u->inflight) > 0) {
 349                         list_move_tail(&u->link, &not_cycle_list);
 350                         u->gc_maybe_cycle = 0;
 351                         scan_children(&u->sk, inc_inflight_move_tail, NULL);
 352                 }
 353         }
 354         list_del(&cursor);
 355
 356         /*
 357          * not_cycle_list contains those sockets which do not make up a
 358          * cycle.  Restore these to the inflight list.
 359          */
 360         while (!list_empty(&not_cycle_list)) {
 361                 u = list_entry(not_cycle_list.next, struct unix_sock, link);
 362                 u->gc_candidate = 0;
 363                 list_move_tail(&u->link, &gc_inflight_list);
 364         }
 365
 366         /*
 367          * Now gc_candidates contains only garbage.  Restore original
 368          * inflight counters for these as well, and remove the skbuffs
 369          * which are creating the cycle(s).
 370          */
 371         skb_queue_head_init(&hitlist);
 372         list_for_each_entry(u, &gc_candidates, link)
 373         scan_children(&u->sk, inc_inflight, &hitlist);
 374
 375         spin_unlock(&unix_gc_lock);
 376
 377         /* Here we are. Hitlist is filled. Die. */
 378         __skb_queue_purge(&hitlist);
 379
 380         spin_lock(&unix_gc_lock);
 381
 382         /* All candidates should have been detached by now. */
 383         BUG_ON(!list_empty(&gc_candidates));
 384         gc_in_progress = false;
 385         wake_up(&unix_gc_wait);
 386
 387  out:
 388         spin_unlock(&unix_gc_lock);
 389 }