af_unix: Allocate struct unix_edge for each inflight AF_UNIX fd.

As with the previous patch, we preallocate to skb's scm_fp_list an
array of struct unix_edge in the number of inflight AF_UNIX fds.

There we just preallocate memory and do not use immediately because
sendmsg() could fail after this point.  The actual use will be in
the next patch.

When we queue skb with inflight edges, we will set the inflight
socket's unix_sock as unix_edge->predecessor and the receiver's
unix_sock as successor, and then we will link the edge to the
inflight socket's unix_vertex.edges.

Note that we set NULL to cloned scm_fp_list.edges in scm_fp_dup()
so that MSG_PEEK does not change the shape of the directed graph.

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Acked-by: Paolo Abeni <pabeni@redhat.com>
Link: https://lore.kernel.org/r/20240325202425.60930-3-kuniyu@amazon.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

include/net/af_unix.h

@@ -33,6 +33,12 @@ struct unix_vertex {
 	unsigned long out_degree;
 };
 
+struct unix_edge {
+	struct unix_sock *predecessor;
+	struct unix_sock *successor;
+	struct list_head vertex_entry;
+};
+
 struct sock *unix_peer_get(struct sock *sk);
 
 #define UNIX_HASH_MOD	(256 - 1)

include/net/scm.h

@@ -23,12 +23,17 @@ struct scm_creds {
 	kgid_t	gid;
 };
 
+#ifdef CONFIG_UNIX
+struct unix_edge;
+#endif
+
 struct scm_fp_list {
 	short			count;
 	short			count_unix;
 	short			max;
 #ifdef CONFIG_UNIX
 	struct list_head	vertices;
+	struct unix_edge	*edges;
 #endif
 	struct user_struct	*user;
 	struct file		*fp[SCM_MAX_FD];

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->edges = NULL;
 		INIT_LIST_HEAD(&fpl->vertices);
 #endif
 	}
@@ -383,6 +384,7 @@ 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->edges = NULL;
 		INIT_LIST_HEAD(&new_fpl->vertices);
 #endif
 	}

net/unix/garbage.c

@@ -127,6 +127,11 @@ int unix_prepare_fpl(struct scm_fp_list *fpl)
 		list_add(&vertex->entry, &fpl->vertices);
 	}
 
+	fpl->edges = kvmalloc_array(fpl->count_unix, sizeof(*fpl->edges),
+				    GFP_KERNEL_ACCOUNT);
+	if (!fpl->edges)
+		goto err;
+
 	return 0;
 
 err:
@@ -136,6 +141,7 @@ int unix_prepare_fpl(struct scm_fp_list *fpl)
 
 void unix_destroy_fpl(struct scm_fp_list *fpl)
 {
+	kvfree(fpl->edges);
 	unix_free_vertices(fpl);
 }