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 struct sock *unix_get_socket(struct file *filp) 99 { 100 struct sock *u_sock = NULL; 101 struct inode *inode = file_inode(filp); 102 103 /* Socket ? */ 104 if (S_ISSOCK(inode->i_mode) && !(filp->f_mode & FMODE_PATH)) { 105 struct socket *sock = SOCKET_I(inode); 106 struct sock *s = sock->sk; 107 108 /* PF_UNIX ? */ 109 if (s && sock->ops && sock->ops->family == PF_UNIX) 110 u_sock = s; 111 } 112 return u_sock; 113 } 114 115 /* Keep the number of times in flight count for the file 116 * descriptor if it is for an AF_UNIX socket. 117 */ 118 119 void unix_inflight(struct user_struct *user, struct file *fp) 120 { 121 struct sock *s = unix_get_socket(fp); 122 123 spin_lock(&unix_gc_lock); 124 125 if (s) { 126 struct unix_sock *u = unix_sk(s); 127 128 if (atomic_long_inc_return(&u->inflight) == 1) { 129 BUG_ON(!list_empty(&u->link)); 130 list_add_tail(&u->link, &gc_inflight_list); 131 } else { 132 BUG_ON(list_empty(&u->link)); 133 } 134 unix_tot_inflight++; 135 } 136 user->unix_inflight++; 137 spin_unlock(&unix_gc_lock); 138 } 139 140 void unix_notinflight(struct user_struct *user, struct file *fp) 141 { 142 struct sock *s = unix_get_socket(fp); 143 144 spin_lock(&unix_gc_lock); 145 146 if (s) { 147 struct unix_sock *u = unix_sk(s); 148 149 BUG_ON(!atomic_long_read(&u->inflight)); 150 BUG_ON(list_empty(&u->link)); 151 152 if (atomic_long_dec_and_test(&u->inflight)) 153 list_del_init(&u->link); 154 unix_tot_inflight--; 155 } 156 user->unix_inflight--; 157 spin_unlock(&unix_gc_lock); 158 } 159 160 static void scan_inflight(struct sock *x, void (*func)(struct unix_sock *), 161 struct sk_buff_head *hitlist) 162 { 163 struct sk_buff *skb; 164 struct sk_buff *next; 165 166 spin_lock(&x->sk_receive_queue.lock); 167 skb_queue_walk_safe(&x->sk_receive_queue, skb, next) { 168 /* Do we have file descriptors ? */ 169 if (UNIXCB(skb).fp) { 170 bool hit = false; 171 /* Process the descriptors of this socket */ 172 int nfd = UNIXCB(skb).fp->count; 173 struct file **fp = UNIXCB(skb).fp->fp; 174 175 while (nfd--) { 176 /* Get the socket the fd matches if it indeed does so */ 177 struct sock *sk = unix_get_socket(*fp++); 178 179 if (sk) { 180 struct unix_sock *u = unix_sk(sk); 181 182 /* Ignore non-candidates, they could 183 * have been added to the queues after 184 * starting the garbage collection 185 */ 186 if (test_bit(UNIX_GC_CANDIDATE, &u->gc_flags)) { 187 hit = true; 188 189 func(u); 190 } 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 } 201 202 static 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 /* For a listening socket collect the queued embryos 214 * and perform a scan on them as well. 215 */ 216 spin_lock(&x->sk_receive_queue.lock); 217 skb_queue_walk_safe(&x->sk_receive_queue, skb, next) { 218 u = unix_sk(skb->sk); 219 220 /* An embryo cannot be in-flight, so it's safe 221 * to use the list link. 222 */ 223 BUG_ON(!list_empty(&u->link)); 224 list_add_tail(&u->link, &embryos); 225 } 226 spin_unlock(&x->sk_receive_queue.lock); 227 228 while (!list_empty(&embryos)) { 229 u = list_entry(embryos.next, struct unix_sock, link); 230 scan_inflight(&u->sk, func, hitlist); 231 list_del_init(&u->link); 232 } 233 } 234 } 235 236 static void dec_inflight(struct unix_sock *usk) 237 { 238 atomic_long_dec(&usk->inflight); 239 } 240 241 static void inc_inflight(struct unix_sock *usk) 242 { 243 atomic_long_inc(&usk->inflight); 244 } 245 246 static void inc_inflight_move_tail(struct unix_sock *u) 247 { 248 atomic_long_inc(&u->inflight); 249 /* If this still might be part of a cycle, move it to the end 250 * of the list, so that it's checked even if it was already 251 * passed over 252 */ 253 if (test_bit(UNIX_GC_MAYBE_CYCLE, &u->gc_flags)) 254 list_move_tail(&u->link, &gc_candidates); 255 } 256 257 static bool gc_in_progress; 258 #define UNIX_INFLIGHT_TRIGGER_GC 16000 259 260 void wait_for_unix_gc(void) 261 { 262 /* If number of inflight sockets is insane, 263 * force a garbage collect right now. 264 */ 265 if (unix_tot_inflight > UNIX_INFLIGHT_TRIGGER_GC && !gc_in_progress) 266 unix_gc(); 267 wait_event(unix_gc_wait, gc_in_progress == false); 268 } 269 270 /* The external entry point: unix_gc() */ 271 void unix_gc(void) 272 { 273 struct unix_sock *u; 274 struct unix_sock *next; 275 struct sk_buff_head hitlist; 276 struct list_head cursor; 277 LIST_HEAD(not_cycle_list); 278 279 spin_lock(&unix_gc_lock); 280 281 /* Avoid a recursive GC. */ 282 if (gc_in_progress) 283 goto out; 284 285 gc_in_progress = true; 286 /* First, select candidates for garbage collection. Only 287 * in-flight sockets are considered, and from those only ones 288 * which don't have any external reference. 289 * 290 * Holding unix_gc_lock will protect these candidates from 291 * being detached, and hence from gaining an external 292 * reference. Since there are no possible receivers, all 293 * buffers currently on the candidates' queues stay there 294 * during the garbage collection. 295 * 296 * We also know that no new candidate can be added onto the 297 * receive queues. Other, non candidate sockets _can_ be 298 * added to queue, so we must make sure only to touch 299 * candidates. 300 */ 301 list_for_each_entry_safe(u, next, &gc_inflight_list, link) { 302 long total_refs; 303 long inflight_refs; 304 305 total_refs = file_count(u->sk.sk_socket->file); 306 inflight_refs = atomic_long_read(&u->inflight); 307 308 BUG_ON(inflight_refs < 1); 309 BUG_ON(total_refs < inflight_refs); 310 if (total_refs == inflight_refs) { 311 list_move_tail(&u->link, &gc_candidates); 312 __set_bit(UNIX_GC_CANDIDATE, &u->gc_flags); 313 __set_bit(UNIX_GC_MAYBE_CYCLE, &u->gc_flags); 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 /* Restore the references for children of all candidates, 324 * which have remaining references. Do this recursively, so 325 * only those remain, which form cyclic references. 326 * 327 * Use a "cursor" link, to make the list traversal safe, even 328 * though elements might be moved about. 329 */ 330 list_add(&cursor, &gc_candidates); 331 while (cursor.next != &gc_candidates) { 332 u = list_entry(cursor.next, struct unix_sock, link); 333 334 /* Move cursor to after the current position. */ 335 list_move(&cursor, &u->link); 336 337 if (atomic_long_read(&u->inflight) > 0) { 338 list_move_tail(&u->link, ¬_cycle_list); 339 __clear_bit(UNIX_GC_MAYBE_CYCLE, &u->gc_flags); 340 scan_children(&u->sk, inc_inflight_move_tail, NULL); 341 } 342 } 343 list_del(&cursor); 344 345 /* Now gc_candidates contains only garbage. Restore original 346 * inflight counters for these as well, and remove the skbuffs 347 * which are creating the cycle(s). 348 */ 349 skb_queue_head_init(&hitlist); 350 list_for_each_entry(u, &gc_candidates, link) 351 scan_children(&u->sk, inc_inflight, &hitlist); 352 353 /* not_cycle_list contains those sockets which do not make up a 354 * cycle. Restore these to the inflight list. 355 */ 356 while (!list_empty(¬_cycle_list)) { 357 u = list_entry(not_cycle_list.next, struct unix_sock, link); 358 __clear_bit(UNIX_GC_CANDIDATE, &u->gc_flags); 359 list_move_tail(&u->link, &gc_inflight_list); 360 } 361 362 spin_unlock(&unix_gc_lock); 363 364 /* Here we are. Hitlist is filled. Die. */ 365 __skb_queue_purge(&hitlist); 366 367 spin_lock(&unix_gc_lock); 368 369 /* All candidates should have been detached by now. */ 370 BUG_ON(!list_empty(&gc_candidates)); 371 gc_in_progress = false; 372 wake_up(&unix_gc_wait); 373 374 out: 375 spin_unlock(&unix_gc_lock); 376 } 377