net/unix/garbage.c

   1 // SPDX-License-Identifier: GPL-2.0-or-later
   2 /*
   3  * NET3:        Garbage Collector For AF_UNIX sockets
   4  *
   5  * Garbage Collector:
   6  *      Copyright (C) Barak A. Pearlmutter.
   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  *  Fixes:
  28  *      Alan Cox        07 Sept 1997    Vmalloc internal stack as needed.
  29  *                                      Cope with changing max_files.
  30  *      Al Viro         11 Oct 1998
  31  *              Graph may have cycles. That is, we can send the descriptor
  32  *              of foo to bar and vice versa. Current code chokes on that.
  33  *              Fix: move SCM_RIGHTS ones into the separate list and then
  34  *              skb_free() them all instead of doing explicit fput's.
  35  *              Another problem: since fput() may block somebody may
  36  *              create a new unix_socket when we are in the middle of sweep
  37  *              phase. Fix: revert the logic wrt MARKED. Mark everything
  38  *              upon the beginning and unmark non-junk ones.
  39  *
  40  *              [12 Oct 1998] AAARGH! New code purges all SCM_RIGHTS
  41  *              sent to connect()'ed but still not accept()'ed sockets.
  42  *              Fixed. Old code had slightly different problem here:
  43  *              extra fput() in situation when we passed the descriptor via
  44  *              such socket and closed it (descriptor). That would happen on
  45  *              each unix_gc() until the accept(). Since the struct file in
  46  *              question would go to the free list and might be reused...
  47  *              That might be the reason of random oopses on filp_close()
  48  *              in unrelated processes.
  49  *
  50  *      AV              28 Feb 1999
  51  *              Kill the explicit allocation of stack. Now we keep the tree
  52  *              with root in dummy + pointer (gc_current) to one of the nodes.
  53  *              Stack is represented as path from gc_current to dummy. Unmark
  54  *              now means "add to tree". Push == "make it a son of gc_current".
  55  *              Pop == "move gc_current to parent". We keep only pointers to
  56  *              parents (->gc_tree).
  57  *      AV              1 Mar 1999
  58  *              Damn. Added missing check for ->dead in listen queues scanning.
  59  *
  60  *      Miklos Szeredi 25 Jun 2007
  61  *              Reimplement with a cycle collecting algorithm. This should
  62  *              solve several problems with the previous code, like being racy
  63  *              wrt receive and holding up unrelated socket operations.
  64  */
  65
  66 #include <linux/kernel.h>
  67 #include <linux/string.h>
  68 #include <linux/socket.h>
  69 #include <linux/un.h>
  70 #include <linux/net.h>
  71 #include <linux/fs.h>
  72 #include <linux/skbuff.h>
  73 #include <linux/netdevice.h>
  74 #include <linux/file.h>
  75 #include <linux/proc_fs.h>
  76 #include <linux/mutex.h>
  77 #include <linux/wait.h>
  78
  79 #include <net/sock.h>
  80 #include <net/af_unix.h>
  81 #include <net/scm.h>
  82 #include <net/tcp_states.h>
  83
  84 #include "scm.h"
  85
  86 /* Internal data structures and random procedures: */
  87
  88 static LIST_HEAD(gc_candidates);
  89 static DECLARE_WAIT_QUEUE_HEAD(unix_gc_wait);
  90
  91 static void scan_inflight(struct sock *x, void (*func)(struct unix_sock *),
  92                           struct sk_buff_head *hitlist)
  93 {
  94         struct sk_buff *skb;
  95         struct sk_buff *next;
  96
  97         spin_lock(&x->sk_receive_queue.lock);
  98         skb_queue_walk_safe(&x->sk_receive_queue, skb, next) {
  99                 /* Do we have file descriptors ? */
 100                 if (UNIXCB(skb).fp) {
 101                         bool hit = false;
 102                         /* Process the descriptors of this socket */
 103                         int nfd = UNIXCB(skb).fp->count;
 104                         struct file **fp = UNIXCB(skb).fp->fp;
 105
 106                         while (nfd--) {
 107                                 /* Get the socket the fd matches if it indeed does so */
 108                                 struct sock *sk = unix_get_socket(*fp++);
 109
 110                                 if (sk) {
 111                                         struct unix_sock *u = unix_sk(sk);
 112
 113                                         /* Ignore non-candidates, they could
 114                                          * have been added to the queues after
 115                                          * starting the garbage collection
 116                                          */
 117                                         if (test_bit(UNIX_GC_CANDIDATE, &u->gc_flags)) {
 118                                                 hit = true;
 119
 120                                                 func(u);
 121                                         }
 122                                 }
 123                         }
 124                         if (hit && hitlist != NULL) {
 125                                 __skb_unlink(skb, &x->sk_receive_queue);
 126                                 __skb_queue_tail(hitlist, skb);
 127                         }
 128                 }
 129         }
 130         spin_unlock(&x->sk_receive_queue.lock);
 131 }
 132
 133 static void scan_children(struct sock *x, void (*func)(struct unix_sock *),
 134                           struct sk_buff_head *hitlist)
 135 {
 136         if (x->sk_state != TCP_LISTEN) {
 137                 scan_inflight(x, func, hitlist);
 138         } else {
 139                 struct sk_buff *skb;
 140                 struct sk_buff *next;
 141                 struct unix_sock *u;
 142                 LIST_HEAD(embryos);
 143
 144                 /* For a listening socket collect the queued embryos
 145                  * and perform a scan on them as well.
 146                  */
 147                 spin_lock(&x->sk_receive_queue.lock);
 148                 skb_queue_walk_safe(&x->sk_receive_queue, skb, next) {
 149                         u = unix_sk(skb->sk);
 150
 151                         /* An embryo cannot be in-flight, so it's safe
 152                          * to use the list link.
 153                          */
 154                         BUG_ON(!list_empty(&u->link));
 155                         list_add_tail(&u->link, &embryos);
 156                 }
 157                 spin_unlock(&x->sk_receive_queue.lock);
 158
 159                 while (!list_empty(&embryos)) {
 160                         u = list_entry(embryos.next, struct unix_sock, link);
 161                         scan_inflight(&u->sk, func, hitlist);
 162                         list_del_init(&u->link);
 163                 }
 164         }
 165 }
 166
 167 static void dec_inflight(struct unix_sock *usk)
 168 {
 169         atomic_long_dec(&usk->inflight);
 170 }
 171
 172 static void inc_inflight(struct unix_sock *usk)
 173 {
 174         atomic_long_inc(&usk->inflight);
 175 }
 176
 177 static void inc_inflight_move_tail(struct unix_sock *u)
 178 {
 179         atomic_long_inc(&u->inflight);
 180         /* If this still might be part of a cycle, move it to the end
 181          * of the list, so that it's checked even if it was already
 182          * passed over
 183          */
 184         if (test_bit(UNIX_GC_MAYBE_CYCLE, &u->gc_flags))
 185                 list_move_tail(&u->link, &gc_candidates);
 186 }
 187
 188 static bool gc_in_progress;
 189 #define UNIX_INFLIGHT_TRIGGER_GC 16000
 190
 191 void wait_for_unix_gc(void)
 192 {
 193         /* If number of inflight sockets is insane,
 194          * force a garbage collect right now.
 195          */
 196         if (unix_tot_inflight > UNIX_INFLIGHT_TRIGGER_GC && !gc_in_progress)
 197                 unix_gc();
 198         wait_event(unix_gc_wait, gc_in_progress == false);
 199 }
 200
 201 /* The external entry point: unix_gc() */
 202 void unix_gc(void)
 203 {
 204         struct unix_sock *u;
 205         struct unix_sock *next;
 206         struct sk_buff_head hitlist;
 207         struct list_head cursor;
 208         LIST_HEAD(not_cycle_list);
 209
 210         spin_lock(&unix_gc_lock);
 211
 212         /* Avoid a recursive GC. */
 213         if (gc_in_progress)
 214                 goto out;
 215
 216         gc_in_progress = true;
 217         /* First, select candidates for garbage collection.  Only
 218          * in-flight sockets are considered, and from those only ones
 219          * which don't have any external reference.
 220          *
 221          * Holding unix_gc_lock will protect these candidates from
 222          * being detached, and hence from gaining an external
 223          * reference.  Since there are no possible receivers, all
 224          * buffers currently on the candidates' queues stay there
 225          * during the garbage collection.
 226          *
 227          * We also know that no new candidate can be added onto the
 228          * receive queues.  Other, non candidate sockets _can_ be
 229          * added to queue, so we must make sure only to touch
 230          * candidates.
 231          */
 232         list_for_each_entry_safe(u, next, &gc_inflight_list, link) {
 233                 long total_refs;
 234                 long inflight_refs;
 235
 236                 total_refs = file_count(u->sk.sk_socket->file);
 237                 inflight_refs = atomic_long_read(&u->inflight);
 238
 239                 BUG_ON(inflight_refs < 1);
 240                 BUG_ON(total_refs < inflight_refs);
 241                 if (total_refs == inflight_refs) {
 242                         list_move_tail(&u->link, &gc_candidates);
 243                         __set_bit(UNIX_GC_CANDIDATE, &u->gc_flags);
 244                         __set_bit(UNIX_GC_MAYBE_CYCLE, &u->gc_flags);
 245                 }
 246         }
 247
 248         /* Now remove all internal in-flight reference to children of
 249          * the candidates.
 250          */
 251         list_for_each_entry(u, &gc_candidates, link)
 252                 scan_children(&u->sk, dec_inflight, NULL);
 253
 254         /* Restore the references for children of all candidates,
 255          * which have remaining references.  Do this recursively, so
 256          * only those remain, which form cyclic references.
 257          *
 258          * Use a "cursor" link, to make the list traversal safe, even
 259          * though elements might be moved about.
 260          */
 261         list_add(&cursor, &gc_candidates);
 262         while (cursor.next != &gc_candidates) {
 263                 u = list_entry(cursor.next, struct unix_sock, link);
 264
 265                 /* Move cursor to after the current position. */
 266                 list_move(&cursor, &u->link);
 267
 268                 if (atomic_long_read(&u->inflight) > 0) {
 269                         list_move_tail(&u->link, &not_cycle_list);
 270                         __clear_bit(UNIX_GC_MAYBE_CYCLE, &u->gc_flags);
 271                         scan_children(&u->sk, inc_inflight_move_tail, NULL);
 272                 }
 273         }
 274         list_del(&cursor);
 275
 276         /* Now gc_candidates contains only garbage.  Restore original
 277          * inflight counters for these as well, and remove the skbuffs
 278          * which are creating the cycle(s).
 279          */
 280         skb_queue_head_init(&hitlist);
 281         list_for_each_entry(u, &gc_candidates, link)
 282                 scan_children(&u->sk, inc_inflight, &hitlist);
 283
 284         /* not_cycle_list contains those sockets which do not make up a
 285          * cycle.  Restore these to the inflight list.
 286          */
 287         while (!list_empty(&not_cycle_list)) {
 288                 u = list_entry(not_cycle_list.next, struct unix_sock, link);
 289                 __clear_bit(UNIX_GC_CANDIDATE, &u->gc_flags);
 290                 list_move_tail(&u->link, &gc_inflight_list);
 291         }
 292
 293         spin_unlock(&unix_gc_lock);
 294
 295         /* Here we are. Hitlist is filled. Die. */
 296         __skb_queue_purge(&hitlist);
 297
 298         spin_lock(&unix_gc_lock);
 299
 300         /* All candidates should have been detached by now. */
 301         BUG_ON(!list_empty(&gc_candidates));
 302         gc_in_progress = false;
 303         wake_up(&unix_gc_wait);
 304
 305  out:
 306         spin_unlock(&unix_gc_lock);
 307 }