Merge branch 'sctp-avoid-redundant-initialisation-in-sctp_accept-and-sctp_do_peeloff'

Kuniyuki Iwashima says:

====================
sctp: Avoid redundant initialisation in sctp_accept() and sctp_do_peeloff().

When sctp_accept() and sctp_do_peeloff() allocates a new socket,
somehow sk_alloc() is used, and the new socket goes through full
initialisation, but most of the fields are overwritten later.

  1)
  sctp_accept()
  |- sctp_v[46]_create_accept_sk()
  |  |- sk_alloc()
  |  |- sock_init_data()
  |  |- sctp_copy_sock()
  |  `- newsk->sk_prot->init() / sctp_init_sock()
  |
  `- sctp_sock_migrate()
     `- sctp_copy_descendant(newsk, oldsk)

  sock_init_data() initialises struct sock, but many fields are
  overwritten by sctp_copy_sock(), which inherits fields of struct
  sock and inet_sock from the parent socket.

  sctp_init_sock() fully initialises struct sctp_sock, but later
  sctp_copy_descendant() inherits most fields from the parent's
  struct sctp_sock by memcpy().

  2)
  sctp_do_peeloff()
  |- sock_create()
  |  |
  |  ...
  |      |- sk_alloc()
  |      |- sock_init_data()
  |  ...
  |    `- newsk->sk_prot->init() / sctp_init_sock()
  |
  |- sctp_copy_sock()
  `- sctp_sock_migrate()
     `- sctp_copy_descendant(newsk, oldsk)

  sock_create() creates a brand new socket, but sctp_copy_sock()
  and sctp_sock_migrate() overwrite most of the fields.

So, sk_alloc(), sock_init_data(), sctp_copy_sock(), and
sctp_copy_descendant() can be replaced with a single function
like sk_clone_lock().

This series does the conversion and removes TODO comment added
by commit 4a997d49d92ad ("tcp: Save lock_sock() for memcg in
inet_csk_accept().").

Tested accept() and SCTP_SOCKOPT_PEELOFF and both work properly.

  socket(AF_INET, SOCK_STREAM, IPPROTO_SCTP) = 3
  bind(3, {sa_family=AF_INET, sin_port=htons(0), sin_addr=inet_addr("127.0.0.1")}, 16) = 0
  listen(3, -1)                           = 0
  getsockname(3, {sa_family=AF_INET, sin_port=htons(49460), sin_addr=inet_addr("127.0.0.1")}, [16]) = 0
  socket(AF_INET, SOCK_STREAM, IPPROTO_SCTP) = 4
  connect(4, {sa_family=AF_INET, sin_port=htons(49460), sin_addr=inet_addr("127.0.0.1")}, 16) = 0
  accept(3, NULL, NULL)                   = 5
  ...
  socket(AF_INET, SOCK_SEQPACKET, IPPROTO_SCTP) = 3
  bind(3, {sa_family=AF_INET, sin_port=htons(0), sin_addr=inet_addr("127.0.0.1")}, 16) = 0
  listen(3, -1)                           = 0
  getsockname(3, {sa_family=AF_INET, sin_port=htons(48240), sin_addr=inet_addr("127.0.0.1")}, [16]) = 0
  socket(AF_INET, SOCK_SEQPACKET, IPPROTO_SCTP) = 4
  connect(4, {sa_family=AF_INET, sin_port=htons(48240), sin_addr=inet_addr("127.0.0.1")}, 16) = 0
  getsockopt(3, SOL_SCTP, SCTP_SOCKOPT_PEELOFF, "*\0\0\0\5\0\0\0", [8]) = 5

v1: https://lore.kernel.org/20251021214422.1941691-1-kuniyu@google.com
====================

Link: https://patch.msgid.link/20251023231751.4168390-1-kuniyu@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>