af_unix: Link struct unix_edge when queuing skb.

Just before queuing skb with inflight fds, we call scm_stat_add(),
which is a good place to set up the preallocated struct unix_vertex
and struct unix_edge in UNIXCB(skb).fp.

Then, we call unix_add_edges() and construct the directed graph
as follows:

  1. Set the inflight socket's unix_sock to unix_edge.predecessor.
  2. Set the receiver's unix_sock to unix_edge.successor.
  3. Set the preallocated vertex to inflight socket's unix_sock.vertex.
  4. Link inflight socket's unix_vertex.entry to unix_unvisited_vertices.
  5. Link unix_edge.vertex_entry to the inflight socket's unix_vertex.edges.

Let's say we pass the fd of AF_UNIX socket A to B and the fd of B
to C.  The graph looks like this:

  +-------------------------+
  | unix_unvisited_vertices | <-------------------------.
  +-------------------------+                           |
  +                                                     |
  |     +--------------+             +--------------+   |         +--------------+
  |     |  unix_sock A | <---. .---> |  unix_sock B | <-|-. .---> |  unix_sock C |
  |     +--------------+     | |     +--------------+   | | |     +--------------+
  | .-+ |    vertex    |     | | .-+ |    vertex    |   | | |     |    vertex    |
  | |   +--------------+     | | |   +--------------+   | | |     +--------------+
  | |                        | | |                      | | |
  | |   +--------------+     | | |   +--------------+   | | |
  | '-> |  unix_vertex |     | | '-> |  unix_vertex |   | | |
  |     +--------------+     | |     +--------------+   | | |
  `---> |    entry     | +---------> |    entry     | +-' | |
        |--------------|     | |     |--------------|     | |
        |    edges     | <-. | |     |    edges     | <-. | |
        +--------------+   | | |     +--------------+   | | |
                           | | |                        | | |
    .----------------------' | | .----------------------' | |
    |                        | | |                        | |
    |   +--------------+     | | |   +--------------+     | |
    |   |   unix_edge  |     | | |   |   unix_edge  |     | |
    |   +--------------+     | | |   +--------------+     | |
    `-> | vertex_entry |     | | `-> | vertex_entry |     | |
        |--------------|     | |     |--------------|     | |
        |  predecessor | +---' |     |  predecessor | +---' |
        |--------------|       |     |--------------|       |
        |   successor  | +-----'     |   successor  | +-----'
        +--------------+             +--------------+

Henceforth, we denote such a graph as A -> B (-> C).

Now, we can express all inflight fd graphs that do not contain
embryo sockets.  We will support the particular case later.

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Acked-by: Paolo Abeni <pabeni@redhat.com>
Link: https://lore.kernel.org/r/20240325202425.60930-4-kuniyu@amazon.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

1 files changed, 2 insertions, 0 deletions

diff --git a/net/core/scm.c b/net/core/scm.c
index 1bcc8a2d65e3..5763f3320358 100644
--- a/net/core/scm.c
+++ b/net/core/scm.c
@@ -90,6 +90,7 @@ static int scm_fp_copy(struct cmsghdr *cmsg, struct scm_fp_list **fplp)
 		fpl->max = SCM_MAX_FD;
 		fpl->user = NULL;
 #if IS_ENABLED(CONFIG_UNIX)
+		fpl->inflight = false;
 		fpl->edges = NULL;
 		INIT_LIST_HEAD(&fpl->vertices);
 #endif
@@ -384,6 +385,7 @@ struct scm_fp_list *scm_fp_dup(struct scm_fp_list *fpl)
 		new_fpl->max = new_fpl->count;
 		new_fpl->user = get_uid(fpl->user);
 #if IS_ENABLED(CONFIG_UNIX)
+		new_fpl->inflight = false;
 		new_fpl->edges = NULL;
 		INIT_LIST_HEAD(&new_fpl->vertices);
 #endif