sockcmsg_unix.go

     1  // Copyright 2011 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  //go:build unix
     6  
     7  // Socket control messages
     8  
     9  package syscall
    10  
    11  import (
    12  	"unsafe"
    13  )
    14  
    15  // CmsgLen returns the value to store in the Len field of the [Cmsghdr]
    16  // structure, taking into account any necessary alignment.
    17  func CmsgLen(datalen int) int {
    18  	return cmsgAlignOf(SizeofCmsghdr) + datalen
    19  }
    20  
    21  // CmsgSpace returns the number of bytes an ancillary element with
    22  // payload of the passed data length occupies.
    23  func CmsgSpace(datalen int) int {
    24  	return cmsgAlignOf(SizeofCmsghdr) + cmsgAlignOf(datalen)
    25  }
    26  
    27  func (h *Cmsghdr) data(offset uintptr) unsafe.Pointer {
    28  	return unsafe.Pointer(uintptr(unsafe.Pointer(h)) + uintptr(cmsgAlignOf(SizeofCmsghdr)) + offset)
    29  }
    30  
    31  // SocketControlMessage represents a socket control message.
    32  type SocketControlMessage struct {
    33  	Header Cmsghdr
    34  	Data   []byte
    35  }
    36  
    37  // ParseSocketControlMessage parses b as an array of socket control
    38  // messages.
    39  func ParseSocketControlMessage(b []byte) ([]SocketControlMessage, error) {
    40  	var msgs []SocketControlMessage
    41  	i := 0
    42  	for i+CmsgLen(0) <= len(b) {
    43  		h, dbuf, err := socketControlMessageHeaderAndData(b[i:])
    44  		if err != nil {
    45  			return nil, err
    46  		}
    47  		m := SocketControlMessage{Header: *h, Data: dbuf}
    48  		msgs = append(msgs, m)
    49  		i += cmsgAlignOf(int(h.Len))
    50  	}
    51  	return msgs, nil
    52  }
    53  
    54  func socketControlMessageHeaderAndData(b []byte) (*Cmsghdr, []byte, error) {
    55  	h := (*Cmsghdr)(unsafe.Pointer(&b[0]))
    56  	if h.Len < SizeofCmsghdr || uint64(h.Len) > uint64(len(b)) {
    57  		return nil, nil, EINVAL
    58  	}
    59  	return h, b[cmsgAlignOf(SizeofCmsghdr):h.Len], nil
    60  }
    61  
    62  // UnixRights encodes a set of open file descriptors into a socket
    63  // control message for sending to another process.
    64  func UnixRights(fds ...int) []byte {
    65  	datalen := len(fds) * 4
    66  	b := make([]byte, CmsgSpace(datalen))
    67  	h := (*Cmsghdr)(unsafe.Pointer(&b[0]))
    68  	h.Level = SOL_SOCKET
    69  	h.Type = SCM_RIGHTS
    70  	h.SetLen(CmsgLen(datalen))
    71  	for i, fd := range fds {
    72  		*(*int32)(h.data(4 * uintptr(i))) = int32(fd)
    73  	}
    74  	return b
    75  }
    76  
    77  // ParseUnixRights decodes a socket control message that contains an
    78  // integer array of open file descriptors from another process.
    79  func ParseUnixRights(m *SocketControlMessage) ([]int, error) {
    80  	if m.Header.Level != SOL_SOCKET {
    81  		return nil, EINVAL
    82  	}
    83  	if m.Header.Type != SCM_RIGHTS {
    84  		return nil, EINVAL
    85  	}
    86  	fds := make([]int, len(m.Data)>>2)
    87  	for i, j := 0, 0; i < len(m.Data); i += 4 {
    88  		fds[j] = int(*(*int32)(unsafe.Pointer(&m.Data[i])))
    89  		j++
    90  	}
    91  	return fds, nil
    92  }
    93
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