Re: [PATCH 4/4] af_unix: convert to lock_cmp_fn

[Kuniyuki Iwashima <kuniyu@amazon.com>]

From: Kent Overstreet <kent.overstreet@linux.dev>
Date: Sun, 28 Jan 2024 14:38:02 -0500
> On Sun, Jan 28, 2024 at 12:28:38AM -0800, Kuniyuki Iwashima wrote:
> > From: Kent Overstreet <kent.overstreet@linux.dev>
> > Date: Fri, 26 Jan 2024 21:08:31 -0500
> > > Kill
> > >  - unix_state_lock_nested
> > >  - _nested usage for net->unx.table.locks[].
> > > 
> > > replace both with lock_set_cmp_fn_ptr_order(&u->lock).
> > > 
> > > The lock ordering in sk_diag_dump_icons() looks suspicious; this may
> > > turn up a real issue.
> > 
> > Yes, you cannot use lock_cmp_fn() for unix_state_lock_nested().
> > 
> > The lock order in sk_diag_dump_icons() is
> > 
> >   listening socket -> child socket in the listener's queue
> > 
> > , and the inverse order never happens.  ptr comparison does not make
> > sense in this case, and lockdep will complain about false positive.
> 
> Is that a real lock ordering? Is this parent -> child relationship well
> defined?
> 
> If it is, we should be able to write a lock_cmp_fn for it, as long as
> it's not some handwavy "this will never happen but _nested won't check
> for it" like I saw elsewhere in the net code... :)

The problem would be there's no handy way to detect the relationship
except for iterating the queue again.

---8<---
static int unix_state_lock_cmp_fn(const struct lockdep_map *_a,
				  const struct lockdep_map *_b)
{
	const struct unix_sock *a = container_of(_a, struct unix_sock, lock.dep_map);
	const struct unix_sock *b = container_of(_b, struct unix_sock, lock.dep_map);

	if (a->sk.sk_state == TCP_LISTEN && b->sk.sk_state == TCP_ESTABLISHED) {
		/* check if b is a's cihld */
	}

	/* otherwise, ptr comparison here. */
}
---8<---


This can be resolved by a patch like this, which is in my local tree
for another series.

So, after posting the series, I can revisit this and write lock_cmp_fn
for u->lock.

---8<---
commit 12d39766b06068fda5987f4e7b4827e8c3273137
Author: Kuniyuki Iwashima <kuniyu@amazon.com>
Date:   Thu Jan 11 22:36:58 2024 +0000

    af_unix: Save listener for embryo socket.
    
    Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>

diff --git a/include/net/af_unix.h b/include/net/af_unix.h
index 9ea04653c7c9..d0c0d81bcb1d 100644
--- a/include/net/af_unix.h
+++ b/include/net/af_unix.h
@@ -82,6 +82,7 @@ struct scm_stat {
 struct unix_sock {
 	/* WARNING: sk has to be the first member */
 	struct sock		sk;
+#define usk_listener		sk.__sk_common.skc_unix_listener
 	struct unix_address	*addr;
 	struct path		path;
 	struct mutex		iolock, bindlock;
diff --git a/include/net/sock.h b/include/net/sock.h
index a9d99a9c583f..3df3d068345a 100644
--- a/include/net/sock.h
+++ b/include/net/sock.h
@@ -142,6 +142,8 @@ typedef __u64 __bitwise __addrpair;
  *		[union with @skc_incoming_cpu]
  *	@skc_tw_rcv_nxt: (aka tw_rcv_nxt) TCP window next expected seq number
  *		[union with @skc_incoming_cpu]
+ *	@skc_unix_listener: connection request listener socket for AF_UNIX
+ *		[union with @skc_rxhash]
  *	@skc_refcnt: reference count
  *
  *	This is the minimal network layer representation of sockets, the header
@@ -227,6 +229,7 @@ struct sock_common {
 		u32		skc_rxhash;
 		u32		skc_window_clamp;
 		u32		skc_tw_snd_nxt; /* struct tcp_timewait_sock */
+		struct sock	*skc_unix_listener; /* struct unix_sock */
 	};
 	/* public: */
 };
diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
index 5f9871555ec6..4a41bb727c32 100644
--- a/net/unix/af_unix.c
+++ b/net/unix/af_unix.c
@@ -991,6 +991,7 @@ static struct sock *unix_create1(struct net *net, struct socket *sock, int kern,
 	sk->sk_max_ack_backlog	= net->unx.sysctl_max_dgram_qlen;
 	sk->sk_destruct		= unix_sock_destructor;
 	u = unix_sk(sk);
+	u->usk_listener = NULL;
 	u->inflight = 0;
 	u->path.dentry = NULL;
 	u->path.mnt = NULL;
@@ -1612,6 +1613,7 @@ static int unix_stream_connect(struct socket *sock, struct sockaddr *uaddr,
 	newsk->sk_type		= sk->sk_type;
 	init_peercred(newsk);
 	newu = unix_sk(newsk);
+	newu->usk_listener = other;
 	RCU_INIT_POINTER(newsk->sk_wq, &newu->peer_wq);
 	otheru = unix_sk(other);
 
@@ -1707,8 +1709,8 @@ static int unix_accept(struct socket *sock, struct socket *newsock, int flags,
 		       bool kern)
 {
 	struct sock *sk = sock->sk;
-	struct sock *tsk;
 	struct sk_buff *skb;
+	struct sock *tsk;
 	int err;
 
 	err = -EOPNOTSUPP;
@@ -1733,6 +1735,7 @@ static int unix_accept(struct socket *sock, struct socket *newsock, int flags,
 	}
 
 	tsk = skb->sk;
+	unix_sk(tsk)->usk_listener = NULL;
 	skb_free_datagram(sk, skb);
 	wake_up_interruptible(&unix_sk(sk)->peer_wait);
 
---8<---