Socket Operations

This document describes the socket operation syscalls in the Kyronix kernel. It is the child of Syscalls.

Syscall Table

• 41 socket - Create a socket endpoint
• 42 connect - Initiate a connection on a socket
• 43 accept - Accept a connection on a socket
• 44 sendto - Send a message on a socket
• 45 recvfrom - Receive a message from a socket
• 46 sendmsg - Send a message with ancillary data
• 47 recvmsg - Receive a message with ancillary data
• 48 shutdown - Shut down part of a full-duplex connection
• 49 bind - Bind a socket to an address
• 50 listen - Listen for connections on a socket
• 51 getsockname - Get local address of a socket
• 52 getpeername - Get remote address of a socket
• 53 socketpair - Create a pair of connected sockets
• 54 setsockopt - Set a socket option
• 55 getsockopt - Get a socket option
• 288 accept4 - Accept a connection with flags

Socket Types

The socket syscall routes to one of two backends based on the domain argument.

AF_UNIX (Unix Domain Sockets)

Unix domain sockets are implemented in unix_socket.c. The domain value is 1. Only SOCK_STREAM is supported. Unix domain sockets provide local inter-process communication (IPC) using pipe-backed data transfer.

The socket lifecycle follows these states:

1. SOCK_UNBOUND - Socket created but not yet bound to a path
2. SOCK_BOUND - Socket bound to a filesystem path or abstract name
3. SOCK_LISTENING - Socket in listening state, accepting connections

Abstract sockets use names prefixed with a null byte. The kernel maintains a table of MAX_ABSTRACT_SOCKS (16) entries for abstract socket names. Abstract sockets are invisible across jails when the JAILF_IPC flag is set on the originating jail.

AF_INET (Internet Sockets)

Internet sockets are implemented in inet_socket.c using the lwIP (Lightweight IP) stack. The domain value is 2. Three socket types are supported:

• SOCK_STREAM (1) - Transmission Control Protocol (TCP) connections backed by tcp_pcb
• SOCK_DGRAM (2) - User Datagram Protocol (UDP) connections backed by upcb
• SOCK_RAW (3) - Raw IP connections backed by raw_pcb

Each internet socket contains a net_conn_t structure with protocol control blocks, a 16 KiB receive ring buffer for TCP, and an 8-slot datagram queue for UDP/raw sockets.

SCM_RIGHTS (File Descriptor Passing)

The SCM_RIGHTS ancillary data type enables file descriptor passing between processes over Unix domain sockets. The constant value is 1.

1. The sender calls sendmsg with a control message containing SOL_SOCKET, SCM_RIGHTS, and an array of file descriptors
2. The kernel extracts VFS (Virtual File System) node pointers from the sender’s file descriptors via fd_get_node()
3. Nodes are queued into the pipe’s ancillary data ring via pipe_anc_send()
4. The receiver calls recvmsg and the kernel reconstructs file descriptors from the queued VFS nodes via fd_open_node()
5. A maximum of PIPE_ANC_MAXFDS file descriptors are transferred per message

SCM_CREDENTIALS (Peer Credential Passing)

The SCM_CREDENTIALS ancillary data type enables peer credential passing. The constant value is 2. The SO_PASSCRED socket option (value 16) must be enabled on the receiving socket to receive credentials.

1. The receiver enables credential passing via setsockopt(SOL_SOCKET, SO_PASSCRED, &on)
2. The receiver calls recvmsg with a control buffer large enough to hold a ucred_s structure
3. The kernel fills the control message with the sender’s process ID (PID), user ID (UID), and group ID (GID)
4. The ucred_s structure contains pid (int32), uid (uint32), and gid (uint32)

Socket Options

Socket options are managed via setsockopt (syscall 54) and getsockopt (syscall 55) at the SOL_SOCKET (level 1) layer.

Supported Options

sendmsg / recvmsg (Vectored I/O)

The sendmsg (syscall 46) and recvmsg (syscall 47) syscalls support vectored I/O through scatter-gather lists of iovec structures.

The msghdr layout in memory:

sendmsg

1. Validate the msghdr pointer and iovec array
2. For each iovec entry, call fd_write() with the iovec base and length
3. If a control buffer is present, parse cmsg_len, cmsg_level, and cmsg_type
4. For SCM_RIGHTS messages, extract file descriptors and pass their VFS nodes through the pipe’s ancillary ring

recvmsg

1. For internet sockets, delegate to inet_recvfrom() for the first iovec entry and fill msg_name with the source address
2. For Unix domain sockets, read data from iovec entries via fd_read() or fd_peek() (when MSG_PEEK is set)
3. If ancillary data is available in the pipe’s receive ring, construct an SCM_RIGHTS control message with reconstructed file descriptors
4. If SO_PASSCRED is enabled and credentials are available, construct an SCM_CREDENTIALS control message with peer PID/UID/GID

Last reviewed: 2026-07-22