bpf: Add ksock kfuncs

Add BPF kfuncs that allow BPF LSM programs to create and use sockets for
sending data. This provides a mechanism for BPF programs to emit
telemetry. For this first patch set, it's restricted to SOCK_DGRAM
socket types with IPPROTO_UDP protocol but could be easily extended to
SOCK_STREAM and IPPROTO_TCP in the future.

The API consists of five kfuncs:

  bpf_ksock_create()   - Create a socket (sleepable)
  bpf_ksock_connect()  - Connect socket to remote address (sleepable)
  bpf_ksock_send()     - Send data through the socket (sleepable)
  bpf_ksock_acquire()  - Acquire a reference to a socket context
  bpf_ksock_release()  - Release a reference (cleanup via
                         queue_rcu_work since sock_release sleeps)

The setup kfuncs bpf_ksock_create, bpf_ksock_connect, can be called from
SYSCALL programs only. While bpf_ksock_acquire, bpf_ksock_release and
bpf_ksock_send can be called from SYSCALL and LSM programs.

The implementation follows the established kfunc lifecycle pattern
(create/acquire/release with refcounting, kptr map storage, dtor
registration). The kernel socket is wrapped in a refcounted bpf_ksock
struct. Cleanup is deferred via queue_rcu_work() because sock_release()
may sleep.

The kfuncs are only compiled when CONFIG_INET is enabled, as they
specifically support AF_INET and AF_INET6 sockets.

The socket operations go through the expected LSM hooks instead of
by-passing them like many kernel sockets since those are created by BPF
programs and thus system users. Thus, the bpf_ksock_send() kfunc, which
is exposed to LSM progs has a verifier filter protection to avoid
recursion so that the whole bpf_kfunc_set kfunc set cannot be called in
a program attached to security_socket_sendmsg(). Also, because of the
LSM checks, we prevent the use of the kfuncs from asynchronous workqueue
as the current value would then be invalid.

In bpf_ksock_create(), we copy the arg values to avoid TOCTOU races
since the kfunc can sleep and the arg values could be stored in a map
that could be re-written by BPF progs or even userspace programs if the
map is mmaped.

Signed-off-by: Mahe Tardy <mahe.tardy@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Jiayuan Chen <jiayuan.chen@linux.dev>
Acked-by: Stanislav Fomichev <sdf@fomichev.me>
Acked-by: Song Liu <song@kernel.org>
Link: https://lore.kernel.org/bpf/20260813110540.103550-3-mahe.tardy@gmail.com

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