af_unix: Allocate struct unix_vertex for each inflight AF_UNIX fd.

We will replace the garbage collection algorithm for AF_UNIX, where
we will consider each inflight AF_UNIX socket as a vertex and its file
descriptor as an edge in a directed graph.

This patch introduces a new struct unix_vertex representing a vertex
in the graph and adds its pointer to struct unix_sock.

When we send a fd using the SCM_RIGHTS message, we allocate struct
scm_fp_list to struct scm_cookie in scm_fp_copy().  Then, we bump
each refcount of the inflight fds' struct file and save them in
scm_fp_list.fp.

After that, unix_attach_fds() inexplicably clones scm_fp_list of
scm_cookie and sets it to skb.  (We will remove this part after
replacing GC.)

Here, we add a new function call in unix_attach_fds() to preallocate
struct unix_vertex per inflight AF_UNIX fd and link each vertex to
skb's scm_fp_list.vertices.

When sendmsg() succeeds later, if the socket of the inflight fd is
still not inflight yet, we will set the preallocated vertex to struct
unix_sock.vertex and link it to a global list unix_unvisited_vertices
under spin_lock(&unix_gc_lock).

If the socket is already inflight, we free the preallocated vertex.
This is to avoid taking the lock unnecessarily when sendmsg() could
fail later.

In the following patch, we will similarly allocate another struct
per edge, which will finally be linked to the inflight socket's
unix_vertex.edges.

And then, we will count the number of edges as unix_vertex.out_degree.

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Acked-by: Paolo Abeni <pabeni@redhat.com>
Link: https://lore.kernel.org/r/20240325202425.60930-2-kuniyu@amazon.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>