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// SPDX-License-Identifier: GPL-2.0-or-later
* NET3: Garbage Collector For AF_UNIX sockets
* Garbage Collector:
* Copyright (C) Barak A. Pearlmutter.
* Chopped about by Alan Cox 22/3/96 to make it fit the AF_UNIX socket problem.
* If it doesn't work blame me, it worked when Barak sent it.
* Assumptions:
* - object w/ a bit
* - free list
* Current optimizations:
* - explicit stack instead of recursion
* - tail recurse on first born instead of immediate push/pop
* - we gather the stuff that should not be killed into tree
* and stack is just a path from root to the current pointer.
* Future optimizations:
* - don't just push entire root set; process in place
* Fixes:
* Alan Cox 07 Sept 1997 Vmalloc internal stack as needed.
* Cope with changing max_files.
* Al Viro 11 Oct 1998
* Graph may have cycles. That is, we can send the descriptor
* of foo to bar and vice versa. Current code chokes on that.
* Fix: move SCM_RIGHTS ones into the separate list and then
* skb_free() them all instead of doing explicit fput's.
* Another problem: since fput() may block somebody may
* create a new unix_socket when we are in the middle of sweep
* phase. Fix: revert the logic wrt MARKED. Mark everything
* upon the beginning and unmark non-junk ones.
* [12 Oct 1998] AAARGH! New code purges all SCM_RIGHTS
* sent to connect()'ed but still not accept()'ed sockets.
* Fixed. Old code had slightly different problem here:
* extra fput() in situation when we passed the descriptor via
* such socket and closed it (descriptor). That would happen on
* each unix_gc() until the accept(). Since the struct file in
* question would go to the free list and might be reused...
* That might be the reason of random oopses on filp_close()
* in unrelated processes.
* AV 28 Feb 1999
* Kill the explicit allocation of stack. Now we keep the tree
* with root in dummy + pointer (gc_current) to one of the nodes.
* Stack is represented as path from gc_current to dummy. Unmark
* now means "add to tree". Push == "make it a son of gc_current".
* Pop == "move gc_current to parent". We keep only pointers to
* parents (->gc_tree).
* AV 1 Mar 1999
* Damn. Added missing check for ->dead in listen queues scanning.
* Miklos Szeredi 25 Jun 2007
* Reimplement with a cycle collecting algorithm. This should
* solve several problems with the previous code, like being racy
* wrt receive and holding up unrelated socket operations.
#include <linux/kernel.h>
#include <linux/string.h>
#include <linux/socket.h>
#include <linux/un.h>
#include <linux/net.h>
#include <linux/fs.h>
#include <linux/skbuff.h>
#include <linux/netdevice.h>
#include <linux/file.h>
#include <linux/proc_fs.h>
#include <linux/mutex.h>
#include <linux/wait.h>
#include <net/sock.h>
#include <net/af_unix.h>
#include <net/scm.h>
#include <net/tcp_states.h>
#include "scm.h"
/* Internal data structures and random procedures: */
static LIST_HEAD(gc_candidates);
static DECLARE_WAIT_QUEUE_HEAD(unix_gc_wait);
static void scan_inflight(struct sock *x, void (*func)(struct unix_sock *),
struct sk_buff_head *hitlist)
struct sk_buff *skb;
struct sk_buff *next;
skb_queue_walk_safe(&x->sk_receive_queue, skb, next) {
/* Do we have file descriptors ? */
if (UNIXCB(skb).fp) {
bool hit = false;
/* Process the descriptors of this socket */
int nfd = UNIXCB(skb).fp->count;
struct file **fp = UNIXCB(skb).fp->fp;
while (nfd--) {
/* Get the socket the fd matches if it indeed does so */
struct sock *sk = unix_get_socket(*fp++);
if (sk) {
struct unix_sock *u = unix_sk(sk);
/* Ignore non-candidates, they could
* have been added to the queues after
* starting the garbage collection
if (test_bit(UNIX_GC_CANDIDATE, &u->gc_flags)) {
hit = true;
if (hit && hitlist != NULL) {
__skb_unlink(skb, &x->sk_receive_queue);
__skb_queue_tail(hitlist, skb);
static void scan_children(struct sock *x, void (*func)(struct unix_sock *),
struct sk_buff_head *hitlist)
if (x->sk_state != TCP_LISTEN) {
scan_inflight(x, func, hitlist);
} else {
struct sk_buff *skb;
struct sk_buff *next;
struct unix_sock *u;
/* For a listening socket collect the queued embryos
* and perform a scan on them as well.
skb_queue_walk_safe(&x->sk_receive_queue, skb, next) {
u = unix_sk(skb->sk);
/* An embryo cannot be in-flight, so it's safe
* to use the list link.
list_add_tail(&u->link, &embryos);
while (!list_empty(&embryos)) {
u = list_entry(, struct unix_sock, link);
scan_inflight(&u->sk, func, hitlist);
static void dec_inflight(struct unix_sock *usk)
static void inc_inflight(struct unix_sock *usk)
static void inc_inflight_move_tail(struct unix_sock *u)
/* If this still might be part of a cycle, move it to the end
* of the list, so that it's checked even if it was already
* passed over
if (test_bit(UNIX_GC_MAYBE_CYCLE, &u->gc_flags))
list_move_tail(&u->link, &gc_candidates);
static bool gc_in_progress;
void wait_for_unix_gc(void)
/* If number of inflight sockets is insane,
* force a garbage collect right now.
* Paired with the WRITE_ONCE() in unix_inflight(),
* unix_notinflight() and gc_in_progress().
if (READ_ONCE(unix_tot_inflight) > UNIX_INFLIGHT_TRIGGER_GC &&
wait_event(unix_gc_wait, gc_in_progress == false);
/* The external entry point: unix_gc() */
void unix_gc(void)
struct unix_sock *u;
struct unix_sock *next;
struct sk_buff_head hitlist;
struct list_head cursor;
/* Avoid a recursive GC. */
if (gc_in_progress)
goto out;
/* Paired with READ_ONCE() in wait_for_unix_gc(). */
WRITE_ONCE(gc_in_progress, true);
/* First, select candidates for garbage collection. Only
* in-flight sockets are considered, and from those only ones
* which don't have any external reference.
* Holding unix_gc_lock will protect these candidates from
* being detached, and hence from gaining an external
* reference. Since there are no possible receivers, all
* buffers currently on the candidates' queues stay there
* during the garbage collection.
* We also know that no new candidate can be added onto the
* receive queues. Other, non candidate sockets _can_ be
* added to queue, so we must make sure only to touch
* candidates.
list_for_each_entry_safe(u, next, &gc_inflight_list, link) {
long total_refs;
long inflight_refs;
total_refs = file_count(u->sk.sk_socket->file);
inflight_refs = atomic_long_read(&u->inflight);
BUG_ON(inflight_refs < 1);
BUG_ON(total_refs < inflight_refs);
if (total_refs == inflight_refs) {
list_move_tail(&u->link, &gc_candidates);
__set_bit(UNIX_GC_CANDIDATE, &u->gc_flags);
__set_bit(UNIX_GC_MAYBE_CYCLE, &u->gc_flags);
/* Now remove all internal in-flight reference to children of
* the candidates.
list_for_each_entry(u, &gc_candidates, link)
scan_children(&u->sk, dec_inflight, NULL);
/* Restore the references for children of all candidates,
* which have remaining references. Do this recursively, so
* only those remain, which form cyclic references.
* Use a "cursor" link, to make the list traversal safe, even
* though elements might be moved about.
list_add(&cursor, &gc_candidates);
while ( != &gc_candidates) {
u = list_entry(, struct unix_sock, link);
/* Move cursor to after the current position. */
list_move(&cursor, &u->link);
if (atomic_long_read(&u->inflight) > 0) {
list_move_tail(&u->link, &not_cycle_list);
__clear_bit(UNIX_GC_MAYBE_CYCLE, &u->gc_flags);
scan_children(&u->sk, inc_inflight_move_tail, NULL);
/* Now gc_candidates contains only garbage. Restore original
* inflight counters for these as well, and remove the skbuffs
* which are creating the cycle(s).
list_for_each_entry(u, &gc_candidates, link)
scan_children(&u->sk, inc_inflight, &hitlist);
/* not_cycle_list contains those sockets which do not make up a
* cycle. Restore these to the inflight list.
while (!list_empty(&not_cycle_list)) {
u = list_entry(, struct unix_sock, link);
__clear_bit(UNIX_GC_CANDIDATE, &u->gc_flags);
list_move_tail(&u->link, &gc_inflight_list);
/* Here we are. Hitlist is filled. Die. */
/* All candidates should have been detached by now. */
/* Paired with READ_ONCE() in wait_for_unix_gc(). */
WRITE_ONCE(gc_in_progress, false);