Passing multiple control messages at once over a Unix socket

output-native-armv6.txt

creating socketpair
fds: 3 4
encoded cmsgs into 40 byte buf:
18 00 00 00 01 00 00 00 02 00 00 00 a7 2c 00 00 e8 03 00 00 e8 03 00 00 10 00 00 00 01 00 00 00 01 00 00 00 01 00 00 00 
re-decode msgs without going through socket:
raw control buffer size: 40 bytes
cmsg 1:
cmsg lvl: 1 (SOL_SOCKET = 1)
cmsg type: 2 (SCM_RIGHTS = 1, SCM_CREDENTIALS = 2)
cmsg len: 24 (for 1 int = 16, for ucred = 24)
cmsg 2:
cmsg lvl: 1 (SOL_SOCKET = 1)
cmsg type: 1 (SCM_RIGHTS = 1, SCM_CREDENTIALS = 2)
cmsg len: 16 (for 1 int = 16, for ucred = 24)
flags = 0x00
received msgs:
raw control buffer size: 40 bytes
cmsg 1:
cmsg lvl: 1 (SOL_SOCKET = 1)
cmsg type: 2 (SCM_RIGHTS = 1, SCM_CREDENTIALS = 2)
cmsg len: 24 (for 1 int = 16, for ucred = 24)
cmsg 2:
cmsg lvl: 1 (SOL_SOCKET = 1)
cmsg type: 1 (SCM_RIGHTS = 1, SCM_CREDENTIALS = 2)
cmsg len: 16 (for 1 int = 16, for ucred = 24)

output-native-x86-64.txt

creating socketpair
fds: 3 4
encoded cmsgs into 56 byte buf:
1c 00 00 00 00 00 00 00 01 00 00 00 02 00 00 00 f3 21 00 00 e8 03 00 00 e8 03 00 00 00 00 00 00 14 00 00 00 00 00 00 00 01 00 00 00 01 00 00 00 01 00 00 00 00 00 00 00 
re-decode msgs without going through socket:
raw control buffer size: 56 bytes
cmsg 1:
cmsg lvl: 1 (SOL_SOCKET = 1)
cmsg type: 2 (SCM_RIGHTS = 1, SCM_CREDENTIALS = 2)
cmsg len: 28 (for 1 int = 20, for ucred = 28)
cmsg 2:
cmsg lvl: 1 (SOL_SOCKET = 1)
cmsg type: 1 (SCM_RIGHTS = 1, SCM_CREDENTIALS = 2)
cmsg len: 20 (for 1 int = 20, for ucred = 28)
flags = 0x00
received msgs:
raw control buffer size: 56 bytes
cmsg 1:
cmsg lvl: 1 (SOL_SOCKET = 1)
cmsg type: 2 (SCM_RIGHTS = 1, SCM_CREDENTIALS = 2)
cmsg len: 28 (for 1 int = 20, for ucred = 28)
cmsg 2:
cmsg lvl: 1 (SOL_SOCKET = 1)
cmsg type: 1 (SCM_RIGHTS = 1, SCM_CREDENTIALS = 2)
cmsg len: 20 (for 1 int = 20, for ucred = 28)

output-qemu-x86_64.txt

creating socketpair
fds: 3 4
encoded cmsgs into 56 byte buf:
1c 00 00 00 00 00 00 00 01 00 00 00 02 00 00 00 e1 22 00 00 e8 03 00 00 e8 03 00 00 00 00 00 00 14 00 00 00 00 00 00 00 01 00 00 00 01 00 00 00 01 00 00 00 00 00 00 00 
re-decode msgs without going through socket:
raw control buffer size: 56 bytes
cmsg 1:
cmsg lvl: 1 (SOL_SOCKET = 1)
cmsg type: 2 (SCM_RIGHTS = 1, SCM_CREDENTIALS = 2)
cmsg len: 28 (for 1 int = 20, for ucred = 28)
cmsg 2:
cmsg lvl: 1 (SOL_SOCKET = 1)
cmsg type: 1 (SCM_RIGHTS = 1, SCM_CREDENTIALS = 2)
cmsg len: 20 (for 1 int = 20, for ucred = 28)
flags = 0x00
received msgs:
raw control buffer size: 32 bytes
cmsg 1:
cmsg lvl: 1 (SOL_SOCKET = 1)
cmsg type: 2 (SCM_RIGHTS = 1, SCM_CREDENTIALS = 2)
cmsg len: 28 (for 1 int = 20, for ucred = 28)

test.c

#define _GNU_SOURCE
#include <sys/socket.h>
#include <sys/types.h>
#include <sys/un.h>
#include <sys/time.h>
#include <unistd.h>
#include <stdio.h>
#include <errno.h>
#include <fcntl.h>
#include <string.h>

#define MSG_SPACE    CMSG_SPACE(sizeof(int)) + CMSG_SPACE(sizeof(struct ucred))

void decode_msgs(struct msghdr* hdr);

void encode_msgs(struct msghdr* hdr)
{
    struct cmsghdr* cmsg = CMSG_FIRSTHDR(hdr);

    cmsg->cmsg_level = SOL_SOCKET;
    cmsg->cmsg_type = SCM_CREDENTIALS;
    cmsg->cmsg_len = CMSG_LEN(sizeof(struct ucred));
    struct ucred* cred = (struct ucred*) CMSG_DATA(cmsg);
    cred->pid = getpid();
    cred->uid = getuid();
    cred->gid = getgid();

    cmsg = CMSG_NXTHDR(hdr, cmsg);
    
    cmsg->cmsg_level = SOL_SOCKET;
    cmsg->cmsg_type = SCM_RIGHTS;
    cmsg->cmsg_len = CMSG_LEN(sizeof(int));
    int* fd = (int*) CMSG_DATA(cmsg);
    *fd = STDOUT_FILENO;


    printf("encoded cmsgs into %zd byte buf:\n", hdr->msg_controllen);
    for (int i = 0; i < hdr->msg_controllen; i++)
    {
        printf("%.2x ", ((unsigned char*) hdr->msg_control)[i]);
    }
    printf("\n");

    printf("re-decode msgs without going through socket:\n");
    decode_msgs(hdr);
}

void decode_msgs(struct msghdr* hdr)
{
    printf("raw control buffer size: %zd bytes\n", hdr->msg_controllen);
    struct cmsghdr* cmsg = CMSG_FIRSTHDR(hdr);
    for (int i = 1; cmsg; i++)
    {
        printf("cmsg %d:\n", i);
        printf("cmsg lvl: %d (SOL_SOCKET = %d)\n", cmsg->cmsg_level, SOL_SOCKET);
        printf("cmsg type: %d (SCM_RIGHTS = %d, SCM_CREDENTIALS = %d)\n", cmsg->cmsg_type, SCM_RIGHTS, SCM_CREDENTIALS);
        printf("cmsg len: %zd (for 1 int = %zd, for ucred = %zd)\n", cmsg->cmsg_len, CMSG_LEN(sizeof(int)), CMSG_LEN(sizeof(struct ucred)));
        cmsg = CMSG_NXTHDR(hdr, cmsg);
    }
}

int main()
{
    char sendbuf[MSG_SPACE];
    char recvbuf[MSG_SPACE];
    memset(sendbuf, 0, MSG_SPACE);
    memset(recvbuf, 0, MSG_SPACE);

    int fds[2];

    printf("creating socketpair\n");
    if (socketpair(PF_UNIX, SOCK_STREAM, 0, fds) == -1)
    {
        perror("socketpair");
        return 1;
    }
    printf("fds: %d %d\n", fds[0], fds[1]);

    // 1-byte dummy transfer
    char dummy = '!';
    struct iovec iov;
    iov.iov_base = &dummy;
    iov.iov_len = 1;

    struct msghdr hdr;
    hdr.msg_name = NULL;
    hdr.msg_flags = 0;
    hdr.msg_namelen = 0;
    hdr.msg_iov = &iov;
    hdr.msg_iovlen = 1;
    hdr.msg_control = sendbuf;
    hdr.msg_controllen = sizeof(sendbuf);
    encode_msgs(&hdr);

    int val = 1;
    if (setsockopt(fds[1], SOL_SOCKET, SO_PASSCRED, &val, sizeof(int)) != 0)
    {
        perror("setsockopt");
        return 1;
    }

    ssize_t sent = sendmsg(fds[0], &hdr, 0);
    switch (sent)
    {
    case 1:
        break;
    case -1:
        perror("sendmsg");
        return 1;
    default:
        printf("expected to send 1 byte, actual: %zd\n", sent);
        return 1;
    }

    // recv from other fd back into same buffer
    hdr.msg_control = recvbuf;
    hdr.msg_controllen = sizeof(recvbuf);
    ssize_t recvd = recvmsg(fds[1], &hdr, 0);
    switch (recvd)
    {
    case 1:
        break;
    case -1:
        perror("recvmsg");
        return 1;
    default:
        printf("expected to recv 1 byte, got %zd\n", recvd);
        return 1;
    }

    printf("flags = 0x%.2x\n", hdr.msg_flags);
    if ((hdr.msg_flags & MSG_TRUNC != 0) || (hdr.msg_flags & MSG_CTRUNC != 0))
    {
        printf("truncated! flags = %x\n", hdr.msg_flags);
        return 1;
    }

    printf("received msgs:\n");
    decode_msgs(&hdr);

    return 0;
}