Re: Re: [PATCH bpf-next v3 2/5] af_unix: add unix_stream_proto for sockmap

On Wed, Aug 4, 2021 at 9:59 AM Jakub Sitnicki [off-list ref] wrote:

On Mon, Aug 02, 2021 at 11:19 PM CEST, Jiang Wang wrote:

[...]

quoted

diff --git a/net/core/sock_map.c b/net/core/sock_map.c
index ae5fa4338..42f50ea7a 100644
--- a/net/core/sock_map.c
+++ b/net/core/sock_map.c

@@ -517,9 +517,15 @@ static bool sk_is_tcp(const struct sock *sk)
             sk->sk_protocol == IPPROTO_TCP;
 }

+static bool sk_is_unix_stream(const struct sock *sk)
+{
+     return sk->sk_type == SOCK_STREAM &&
+            sk->sk_protocol == PF_UNIX;
+}
+
 static bool sock_map_redirect_allowed(const struct sock *sk)
 {
-     if (sk_is_tcp(sk))
+     if (sk_is_tcp(sk) || sk_is_unix_stream(sk))
              return sk->sk_state != TCP_LISTEN;
      else
              return sk->sk_state == TCP_ESTABLISHED;

Let me provide some context.

The reason why we check != TCP_LISTEN for TCP sockets is that we want to
allow redirect redirect to sockets that are about to transition from
TCP_SYN_RECV to TCP_ESTABLISHED, in addition to sockets already in
TCP_ESTABLISHED state.

That's because BPF_SOCK_OPS_PASSIVE_ESTABLISHED_CB callback happens when
socket is still in TCP_SYN_RECV state. With BPF sockops program, we can
insert such socket into a sockmap. Hence, there is a short window of
opportunity when we could redirect to a socket in TCP_SYN_RECV.

UNIX sockets can be only in TCP_{CLOSE,LISTEN,ESTABLISHED} state,
AFAIK. So it is sufficient to rely on the default == TCP_ESTABLISHED
check.

Got it. Thanks for the explanation. I will change unix sockets to only
check == TCP_ESTABLISHED condition.

quoted

diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
index 0ae3fc4c8..9c1711c67 100644
--- a/net/unix/af_unix.c
+++ b/net/unix/af_unix.c

@@ -791,17 +791,35 @@ static void unix_close(struct sock *sk, long timeout)
       */
 }

-struct proto unix_proto = {
-     .name                   = "UNIX",
+static void unix_unhash(struct sock *sk)
+{
+     /* Nothing to do here, unix socket does not need a ->unhash().
+      * This is merely for sockmap.
+      */
+}
+
+struct proto unix_dgram_proto = {
+     .name                   = "UNIX-DGRAM",
+     .owner                  = THIS_MODULE,
+     .obj_size               = sizeof(struct unix_sock),
+     .close                  = unix_close,
+#ifdef CONFIG_BPF_SYSCALL
+     .psock_update_sk_prot   = unix_dgram_bpf_update_proto,
+#endif
+};
+
+struct proto unix_stream_proto = {
+     .name                   = "UNIX-STREAM",
      .owner                  = THIS_MODULE,
      .obj_size               = sizeof(struct unix_sock),
      .close                  = unix_close,
+     .unhash                 = unix_unhash,
 #ifdef CONFIG_BPF_SYSCALL
-     .psock_update_sk_prot   = unix_bpf_update_proto,
+     .psock_update_sk_prot   = unix_stream_bpf_update_proto,
 #endif
 };

-static struct sock *unix_create1(struct net *net, struct socket *sock, int kern)
+static struct sock *unix_create1(struct net *net, struct socket *sock, int kern, int type)
 {
      struct sock *sk = NULL;
      struct unix_sock *u;

@@ -810,7 +828,11 @@ static struct sock *unix_create1(struct net *net, struct socket *sock, int kern)
      if (atomic_long_read(&unix_nr_socks) > 2 * get_max_files())
              goto out;

-     sk = sk_alloc(net, PF_UNIX, GFP_KERNEL, &unix_proto, kern);
+     if (type == SOCK_STREAM)
+             sk = sk_alloc(net, PF_UNIX, GFP_KERNEL, &unix_stream_proto, kern);
+     else /*dgram and  seqpacket */
+             sk = sk_alloc(net, PF_UNIX, GFP_KERNEL, &unix_dgram_proto, kern);
+
      if (!sk)
              goto out;

@@ -872,7 +894,7 @@ static int unix_create(struct net *net, struct socket *sock, int protocol,
              return -ESOCKTNOSUPPORT;
      }

-     return unix_create1(net, sock, kern) ? 0 : -ENOMEM;
+     return unix_create1(net, sock, kern, sock->type) ? 0 : -ENOMEM;
 }

 static int unix_release(struct socket *sock)

@@ -1286,7 +1308,7 @@ static int unix_stream_connect(struct socket *sock, struct sockaddr *uaddr,
      err = -ENOMEM;

      /* create new sock for complete connection */
-     newsk = unix_create1(sock_net(sk), NULL, 0);
+     newsk = unix_create1(sock_net(sk), NULL, 0, sock->type);
      if (newsk == NULL)
              goto out;

@@ -2214,7 +2236,7 @@ static int unix_dgram_recvmsg(struct socket *sock, struct msghdr *msg, size_t si
      struct sock *sk = sock->sk;

 #ifdef CONFIG_BPF_SYSCALL
-     if (sk->sk_prot != &unix_proto)
+     if (sk->sk_prot != &unix_dgram_proto)
              return sk->sk_prot->recvmsg(sk, msg, size, flags & MSG_DONTWAIT,
                                          flags & ~MSG_DONTWAIT, NULL);
 #endif

KASAN might be unhappy about access to sk->sk_prot not annotated with
READ_ONCE. In unix_bpf we have WRITE_ONCE(sk->sk_prot, ...) [1]

Got it.  Will check and add READ_ONCE if necessary.

[...]

[1] https://github.com/google/ktsan/wiki/READ_ONCE-and-WRITE_ONCE#why-kernel-code-should-use-read_once-and-write_once-for-shared-memory-accesses