359 lines
10 KiB
C
359 lines
10 KiB
C
// SPDX-License-Identifier: GPL-2.0-or-later
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/*
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* NET3: Garbage Collector For AF_UNIX sockets
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*
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* Garbage Collector:
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* Copyright (C) Barak A. Pearlmutter.
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*
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* Chopped about by Alan Cox 22/3/96 to make it fit the AF_UNIX socket problem.
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* If it doesn't work blame me, it worked when Barak sent it.
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*
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* Assumptions:
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*
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* - object w/ a bit
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* - free list
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*
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* Current optimizations:
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*
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* - explicit stack instead of recursion
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* - tail recurse on first born instead of immediate push/pop
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* - we gather the stuff that should not be killed into tree
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* and stack is just a path from root to the current pointer.
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*
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* Future optimizations:
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*
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* - don't just push entire root set; process in place
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*
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* Fixes:
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* Alan Cox 07 Sept 1997 Vmalloc internal stack as needed.
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* Cope with changing max_files.
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* Al Viro 11 Oct 1998
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* Graph may have cycles. That is, we can send the descriptor
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* of foo to bar and vice versa. Current code chokes on that.
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* Fix: move SCM_RIGHTS ones into the separate list and then
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* skb_free() them all instead of doing explicit fput's.
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* Another problem: since fput() may block somebody may
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* create a new unix_socket when we are in the middle of sweep
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* phase. Fix: revert the logic wrt MARKED. Mark everything
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* upon the beginning and unmark non-junk ones.
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*
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* [12 Oct 1998] AAARGH! New code purges all SCM_RIGHTS
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* sent to connect()'ed but still not accept()'ed sockets.
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* Fixed. Old code had slightly different problem here:
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* extra fput() in situation when we passed the descriptor via
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* such socket and closed it (descriptor). That would happen on
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* each unix_gc() until the accept(). Since the struct file in
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* question would go to the free list and might be reused...
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* That might be the reason of random oopses on filp_close()
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* in unrelated processes.
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*
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* AV 28 Feb 1999
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* Kill the explicit allocation of stack. Now we keep the tree
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* with root in dummy + pointer (gc_current) to one of the nodes.
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* Stack is represented as path from gc_current to dummy. Unmark
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* now means "add to tree". Push == "make it a son of gc_current".
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* Pop == "move gc_current to parent". We keep only pointers to
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* parents (->gc_tree).
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* AV 1 Mar 1999
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* Damn. Added missing check for ->dead in listen queues scanning.
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*
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* Miklos Szeredi 25 Jun 2007
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* Reimplement with a cycle collecting algorithm. This should
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* solve several problems with the previous code, like being racy
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* wrt receive and holding up unrelated socket operations.
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*/
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#include <linux/kernel.h>
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#include <linux/string.h>
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#include <linux/socket.h>
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#include <linux/un.h>
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#include <linux/net.h>
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#include <linux/fs.h>
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#include <linux/skbuff.h>
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#include <linux/netdevice.h>
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#include <linux/file.h>
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#include <linux/proc_fs.h>
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#include <linux/mutex.h>
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#include <linux/wait.h>
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#include <net/sock.h>
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#include <net/af_unix.h>
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#include <net/scm.h>
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#include <net/tcp_states.h>
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#include "scm.h"
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/* Internal data structures and random procedures: */
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static LIST_HEAD(gc_candidates);
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static DECLARE_WAIT_QUEUE_HEAD(unix_gc_wait);
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static void scan_inflight(struct sock *x, void (*func)(struct unix_sock *),
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struct sk_buff_head *hitlist)
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{
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struct sk_buff *skb;
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struct sk_buff *next;
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spin_lock(&x->sk_receive_queue.lock);
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skb_queue_walk_safe(&x->sk_receive_queue, skb, next) {
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/* Do we have file descriptors ? */
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if (UNIXCB(skb).fp) {
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bool hit = false;
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/* Process the descriptors of this socket */
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int nfd = UNIXCB(skb).fp->count;
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struct file **fp = UNIXCB(skb).fp->fp;
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while (nfd--) {
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/* Get the socket the fd matches if it indeed does so */
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struct sock *sk = unix_get_socket(*fp++);
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if (sk) {
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struct unix_sock *u = unix_sk(sk);
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/* Ignore non-candidates, they could
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* have been added to the queues after
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* starting the garbage collection
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*/
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if (test_bit(UNIX_GC_CANDIDATE, &u->gc_flags)) {
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hit = true;
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func(u);
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}
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}
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}
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if (hit && hitlist != NULL) {
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__skb_unlink(skb, &x->sk_receive_queue);
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__skb_queue_tail(hitlist, skb);
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}
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}
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}
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spin_unlock(&x->sk_receive_queue.lock);
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}
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static void scan_children(struct sock *x, void (*func)(struct unix_sock *),
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struct sk_buff_head *hitlist)
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{
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if (x->sk_state != TCP_LISTEN) {
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scan_inflight(x, func, hitlist);
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} else {
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struct sk_buff *skb;
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struct sk_buff *next;
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struct unix_sock *u;
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LIST_HEAD(embryos);
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/* For a listening socket collect the queued embryos
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* and perform a scan on them as well.
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*/
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spin_lock(&x->sk_receive_queue.lock);
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skb_queue_walk_safe(&x->sk_receive_queue, skb, next) {
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u = unix_sk(skb->sk);
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/* An embryo cannot be in-flight, so it's safe
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* to use the list link.
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*/
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BUG_ON(!list_empty(&u->link));
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list_add_tail(&u->link, &embryos);
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}
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spin_unlock(&x->sk_receive_queue.lock);
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while (!list_empty(&embryos)) {
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u = list_entry(embryos.next, struct unix_sock, link);
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scan_inflight(&u->sk, func, hitlist);
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list_del_init(&u->link);
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}
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}
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}
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static void dec_inflight(struct unix_sock *usk)
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{
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usk->inflight--;
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}
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static void inc_inflight(struct unix_sock *usk)
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{
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usk->inflight++;
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}
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static void inc_inflight_move_tail(struct unix_sock *u)
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{
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u->inflight++;
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/* If this still might be part of a cycle, move it to the end
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* of the list, so that it's checked even if it was already
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* passed over
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*/
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if (test_bit(UNIX_GC_MAYBE_CYCLE, &u->gc_flags))
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list_move_tail(&u->link, &gc_candidates);
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}
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static bool gc_in_progress;
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#define UNIX_INFLIGHT_TRIGGER_GC 16000
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void wait_for_unix_gc(void)
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{
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/* If number of inflight sockets is insane,
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* force a garbage collect right now.
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* Paired with the WRITE_ONCE() in unix_inflight(),
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* unix_notinflight() and gc_in_progress().
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*/
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if (READ_ONCE(unix_tot_inflight) > UNIX_INFLIGHT_TRIGGER_GC &&
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!READ_ONCE(gc_in_progress))
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unix_gc();
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wait_event(unix_gc_wait, !READ_ONCE(gc_in_progress));
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}
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/* The external entry point: unix_gc() */
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void unix_gc(void)
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{
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struct sk_buff *next_skb, *skb;
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struct unix_sock *u;
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struct unix_sock *next;
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struct sk_buff_head hitlist;
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struct list_head cursor;
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LIST_HEAD(not_cycle_list);
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spin_lock(&unix_gc_lock);
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/* Avoid a recursive GC. */
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if (gc_in_progress)
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goto out;
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/* Paired with READ_ONCE() in wait_for_unix_gc(). */
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WRITE_ONCE(gc_in_progress, true);
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/* First, select candidates for garbage collection. Only
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* in-flight sockets are considered, and from those only ones
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* which don't have any external reference.
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*
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* Holding unix_gc_lock will protect these candidates from
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* being detached, and hence from gaining an external
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* reference. Since there are no possible receivers, all
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* buffers currently on the candidates' queues stay there
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* during the garbage collection.
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*
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* We also know that no new candidate can be added onto the
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* receive queues. Other, non candidate sockets _can_ be
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* added to queue, so we must make sure only to touch
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* candidates.
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*
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* Embryos, though never candidates themselves, affect which
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* candidates are reachable by the garbage collector. Before
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* being added to a listener's queue, an embryo may already
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* receive data carrying SCM_RIGHTS, potentially making the
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* passed socket a candidate that is not yet reachable by the
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* collector. It becomes reachable once the embryo is
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* enqueued. Therefore, we must ensure that no SCM-laden
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* embryo appears in a (candidate) listener's queue between
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* consecutive scan_children() calls.
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*/
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list_for_each_entry_safe(u, next, &gc_inflight_list, link) {
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struct sock *sk = &u->sk;
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long total_refs;
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total_refs = file_count(sk->sk_socket->file);
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BUG_ON(!u->inflight);
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BUG_ON(total_refs < u->inflight);
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if (total_refs == u->inflight) {
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list_move_tail(&u->link, &gc_candidates);
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__set_bit(UNIX_GC_CANDIDATE, &u->gc_flags);
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__set_bit(UNIX_GC_MAYBE_CYCLE, &u->gc_flags);
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if (sk->sk_state == TCP_LISTEN) {
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unix_state_lock_nested(sk, U_LOCK_GC_LISTENER);
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unix_state_unlock(sk);
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}
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}
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}
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/* Now remove all internal in-flight reference to children of
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* the candidates.
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*/
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list_for_each_entry(u, &gc_candidates, link)
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scan_children(&u->sk, dec_inflight, NULL);
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/* Restore the references for children of all candidates,
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* which have remaining references. Do this recursively, so
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* only those remain, which form cyclic references.
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*
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* Use a "cursor" link, to make the list traversal safe, even
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* though elements might be moved about.
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*/
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list_add(&cursor, &gc_candidates);
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while (cursor.next != &gc_candidates) {
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u = list_entry(cursor.next, struct unix_sock, link);
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/* Move cursor to after the current position. */
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list_move(&cursor, &u->link);
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if (u->inflight) {
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list_move_tail(&u->link, ¬_cycle_list);
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__clear_bit(UNIX_GC_MAYBE_CYCLE, &u->gc_flags);
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scan_children(&u->sk, inc_inflight_move_tail, NULL);
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}
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}
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list_del(&cursor);
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/* Now gc_candidates contains only garbage. Restore original
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* inflight counters for these as well, and remove the skbuffs
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* which are creating the cycle(s).
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*/
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skb_queue_head_init(&hitlist);
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list_for_each_entry(u, &gc_candidates, link) {
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scan_children(&u->sk, inc_inflight, &hitlist);
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#if IS_ENABLED(CONFIG_AF_UNIX_OOB)
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if (u->oob_skb) {
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kfree_skb(u->oob_skb);
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u->oob_skb = NULL;
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}
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#endif
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}
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/* not_cycle_list contains those sockets which do not make up a
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* cycle. Restore these to the inflight list.
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*/
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while (!list_empty(¬_cycle_list)) {
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u = list_entry(not_cycle_list.next, struct unix_sock, link);
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__clear_bit(UNIX_GC_CANDIDATE, &u->gc_flags);
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list_move_tail(&u->link, &gc_inflight_list);
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}
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spin_unlock(&unix_gc_lock);
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/* We need io_uring to clean its registered files, ignore all io_uring
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* originated skbs. It's fine as io_uring doesn't keep references to
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* other io_uring instances and so killing all other files in the cycle
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* will put all io_uring references forcing it to go through normal
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* release.path eventually putting registered files.
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*/
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skb_queue_walk_safe(&hitlist, skb, next_skb) {
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if (skb->destructor == io_uring_destruct_scm) {
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__skb_unlink(skb, &hitlist);
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skb_queue_tail(&skb->sk->sk_receive_queue, skb);
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}
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}
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/* Here we are. Hitlist is filled. Die. */
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__skb_queue_purge(&hitlist);
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spin_lock(&unix_gc_lock);
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/* There could be io_uring registered files, just push them back to
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* the inflight list
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*/
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list_for_each_entry_safe(u, next, &gc_candidates, link)
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list_move_tail(&u->link, &gc_inflight_list);
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/* All candidates should have been detached by now. */
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BUG_ON(!list_empty(&gc_candidates));
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/* Paired with READ_ONCE() in wait_for_unix_gc(). */
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WRITE_ONCE(gc_in_progress, false);
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wake_up(&unix_gc_wait);
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out:
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spin_unlock(&unix_gc_lock);
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}
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