fd-passing

Fri, 05 Oct 2012 13:30:16 -0700
tags/fdo

FD passing for DRI.Next

Using the DMA-BUF interfaces to pass DRI objects between the client and server, as discussed in my previous blog posting on DRI-Next, requires that we successfully pass file descriptors over the X protocol socket.

Rumor has it that this has been tried and found to be difficult, and so I decided to do a bit of experimentation to see how this could be made to work within the existing X implementation.

(All of the examples shown here are licensed under the GPL, version 2 and are available from git://keithp.com/git/fdpassing)

Basics of FD passing

The kernel internals that support FD passing are actually quite simple -- POSIX already require that two processes be able to share the same underlying reference to a file because of the semantics of the fork(2) call. Adding some ability to share arbitrary file descriptors between two processes then is far more about how you ask the kernel than the actual file descriptor sharing operation.

In Linux, file descriptors can be passed through local network sockets. The sender constructs a mystic-looking sendmsg(2) call, placing the file descriptor in the control field of that operation. The kernel pulls the file descriptor out of the control field, allocates a file descriptor in the target process which references the same file object and then sticks the file descriptor in a queue for the receiving process to fetch.

The receiver then constructs a matching call to recvmsg that provides a place for the kernel to stick the new file descriptor.

A helper API for testing

I first write a stand-alone program that created a socketpair, forked and then passed an fd from the parent to the child. Once that was working, I decided that some short helper functions would make further testing a whole lot easier.

Here's a function that writes some data and an optional file descriptor:

ssize_t
sock_fd_write(int sock, void *buf, ssize_t buflen, int fd)
{
    ssize_t     size;
    struct msghdr   msg;
    struct iovec    iov;
    union {
        struct cmsghdr  cmsghdr;
        char        control[CMSG_SPACE(sizeof (int))];
    } cmsgu;
    struct cmsghdr  *cmsg;

    iov.iov_base = buf;
    iov.iov_len = buflen;

    msg.msg_name = NULL;
    msg.msg_namelen = 0;
    msg.msg_iov = &iov;
    msg.msg_iovlen = 1;

    if (fd != -1) {
        msg.msg_control = cmsgu.control;
        msg.msg_controllen = sizeof(cmsgu.control);

        cmsg = CMSG_FIRSTHDR(&msg);
        cmsg->cmsg_len = CMSG_LEN(sizeof (int));
        cmsg->cmsg_level = SOL_SOCKET;
        cmsg->cmsg_type = SCM_RIGHTS;

        printf ("passing fd %d\n", fd);
        *((int *) CMSG_DATA(cmsg)) = fd;
    } else {
        msg.msg_control = NULL;
        msg.msg_controllen = 0;
        printf ("not passing fd\n");
    }

    size = sendmsg(sock, &msg, 0);

    if (size < 0)
        perror ("sendmsg");
    return size;
}

And here's the matching receiver function:

ssize_t
sock_fd_read(int sock, void *buf, ssize_t bufsize, int *fd)
{
    ssize_t     size;

    if (fd) {
        struct msghdr   msg;
        struct iovec    iov;
        union {
            struct cmsghdr  cmsghdr;
            char        control[CMSG_SPACE(sizeof (int))];
        } cmsgu;
        struct cmsghdr  *cmsg;

        iov.iov_base = buf;
        iov.iov_len = bufsize;

        msg.msg_name = NULL;
        msg.msg_namelen = 0;
        msg.msg_iov = &iov;
        msg.msg_iovlen = 1;
        msg.msg_control = cmsgu.control;
        msg.msg_controllen = sizeof(cmsgu.control);
        size = recvmsg (sock, &msg, 0);
        if (size < 0) {
            perror ("recvmsg");
            exit(1);
        }
        cmsg = CMSG_FIRSTHDR(&msg);
        if (cmsg && cmsg->cmsg_len == CMSG_LEN(sizeof(int))) {
            if (cmsg->cmsg_level != SOL_SOCKET) {
                fprintf (stderr, "invalid cmsg_level %d\n",
                     cmsg->cmsg_level);
                exit(1);
            }
            if (cmsg->cmsg_type != SCM_RIGHTS) {
                fprintf (stderr, "invalid cmsg_type %d\n",
                     cmsg->cmsg_type);
                exit(1);
            }

            *fd = *((int *) CMSG_DATA(cmsg));
            printf ("received fd %d\n", *fd);
        } else
            *fd = -1;
    } else {
        size = read (sock, buf, bufsize);
        if (size < 0) {
            perror("read");
            exit(1);
        }
    }
    return size;
}

With these two functions, I rewrote the simple example as follows:

void
child(int sock)
{
    int fd;
    char    buf[16];
    ssize_t size;

    sleep(1);
    for (;;) {
        size = sock_fd_read(sock, buf, sizeof(buf), &fd);
        if (size <= 0)
            break;
        printf ("read %d\n", size);
        if (fd != -1) {
            write(fd, "hello, world\n", 13);
            close(fd);
        }
    }
}

void
parent(int sock)
{
    ssize_t size;
    int i;
    int fd;

    fd = 1;
    size = sock_fd_write(sock, "1", 1, 1);
    printf ("wrote %d\n", size);
}

int
main(int argc, char **argv)
{
    int sv[2];
    int pid;

    if (socketpair(AF_LOCAL, SOCK_STREAM, 0, sv) < 0) {
        perror("socketpair");
        exit(1);
    }
    switch ((pid = fork())) {
    case 0:
        close(sv[0]);
        child(sv[1]);
        break;
    case -1:
        perror("fork");
        exit(1);
    default:
        close(sv[1]);
        parent(sv[0]);
        break;
    }
    return 0;
}

Experimenting with multiple writes

I wanted to know what would happen if multiple writes were made, some with file descriptors and some without. So I changed the simple example parent function to look like:

void
parent(int sock)
{
    ssize_t size;
    int i;
    int fd;

    fd = 1;
    size = sock_fd_write(sock, "1", 1, -1);
    printf ("wrote %d without fd\n", size);
    size = sock_fd_write(sock, "1", 1, 1);
    printf ("wrote %d with fd\n", size);
    size = sock_fd_write(sock, "1", 1, -1);
    printf ("wrote %d without fd\n", size);
}

When run, this demonstrates that the reader gets two bytes in the first read along with a file descriptor followed by one byte in a second read, without a file descriptor. This demonstrates that a file descriptor message forms a barrier within the socket; multiple messages will be merged together, but not past a message containing a file descriptor.

Reading without accepting a file descriptor

What happens when the reader isn't expecting a file descriptor? Does it just get lost? Does the reader not get the message until it asks for the file descriptor? What about the boundary issue described above?

Here's my test case:

void
child(int sock)
{
    int fd;
    char    buf[16];
    ssize_t size;

    sleep(1);
    size = sock_fd_read(sock, buf, sizeof(buf), NULL);
    if (size <= 0)
        return;
    printf ("read %d\n", size);
    size = sock_fd_read(sock, buf, sizeof(buf), &fd);
    if (size <= 0)
        return;
    printf ("read %d\n", size);
    if (fd != -1) {
        write(fd, "hello, world\n", 13);
        close(fd);
    }
}

void
parent(int sock)
{
    ssize_t size;
    int i;
    int fd;

    fd = 1;
    size = sock_fd_write(sock, "1", 1, 1);
    printf ("wrote %d without fd\n", size);
    size = sock_fd_write(sock, "1", 1, 2);
    printf ("wrote %d with fd\n", size);
}

This shows that the first passed file descriptor is picked up by the first sock_fd_read call, but the file descriptor is closed. The second file descriptor passed is picked up by the second sock_fd_read call.

Zero-length writes

Can a file descriptor be passed without sending any data?

void
parent(int sock)
{
    ssize_t size;
    int i;
    int fd;

    fd = 1;
    size = sock_fd_write(sock, "1", 1, -1);
    printf ("wrote %d without fd\n", size);
    size = sock_fd_write(sock, NULL, 0, 1);
    printf ("wrote %d with fd\n", size);
    size = sock_fd_write(sock, "1", 1, -1);
    printf ("wrote %d without fd\n", size);
}

And the answer is clearly "no" -- the file descriptor is not passed when no data are included in the write.

A summary of results

1. read and recvmsg don't merge data across a file descriptor message boundary.

2. failing to accept an fd in the receiver results in the fd being closed by the kernel.

3. a file descriptor must be accompanied by some data.

Make X pass file descriptors

I'd like to get X to pass a file descriptor without completely rewriting the internals of both the library and the X server. Ideally, without making any changes to the existing code paths for regular request processing at all.

On the sending side, this seems pretty straightforward -- we just need to get the X connection file descriptor and call sendmsg directly, passing the desired file descriptor along. In XCB, this could be done by using the xcb_take_socket interface to temporarily hijack the protocol as Xlib does.

It's the receiving side where things are messier. Because a bare read will discard any delivered file descriptor, we must make sure to use recvmsg whenever we want to actually capture the file descriptor.

Kludge X server fd receiving

Because a passed fd creates a barrier in the bytestream, when the X server reads requests from a client, the read will stop sending data after the message with the file descriptor is consumed.

Of course, this process consumes the passed file descriptor, and if that call isn't made with recvmsg set up to receive it, the fd will be lost.

As a simple kludge, if we pass a meaningless fd with the X request and then the 'real' fd with a following XNoOperation request, the existing request reading code will get the request, discard the meaningless fd and then stop reading at that point due to the barrier. Once into the request processing code, recvmsg can be called to get the real file descriptor and the associated XNoOperation request.

I wrote a test for this that demonstrates how this works:

static void
child(int sock)
{
    uint8_t xreq[1024];
    uint8_t xnop[4];
    uint8_t req;
    int i, reqlen;
    ssize_t size, fdsize;
    int fd = -1, *fdp;
    int j;

    sleep (1);
    for (j = 0;; j++) {
        size = sock_fd_read(sock, xreq, sizeof (xreq), NULL);
        printf ("got %d\n", size);
        if (size == 0)
            break;
        i = 0;
        while (i < size) {
            req = xreq[i];
            reqlen = xreq[i+1];
            i += reqlen;
            switch (req) {
            case 0:
                break;
            case 1:
                if (i != size) {
                    fprintf (stderr, "Got fd req, but not at end of input %d < %d\n",
                         i, size);
                }
                fdsize = sock_fd_read(sock, xnop, sizeof (xnop), &fd);
                if (fd == -1) {
                    fprintf (stderr, "no fd received\n");
                } else {
                    FILE    *f = fdopen (fd, "w");
                    fprintf(f, "hello %d\n", j);
                    fflush(f);
                    fclose(f);
                    close(fd);
                    fd = -1;
                }
                break;
            case 2:
                fprintf (stderr, "Unexpected FD passing req\n");
                break;
            }
        }
    }
}

int
tmp_file(int j) {
    char    name[64];

    sprintf (name, "tmp-file-%d", j);
    return creat(name, 0666);
}

static void
parent(int sock)
{
    uint8_t xreq[32];
    uint8_t xnop[4];
    int i, j;
    int fd;

    for (j = 0; j < 4; j++) {
        /* Write a bunch of regular requests */
        for (i = 0; i < 8; i++) {
            xreq[0] = 0;
            xreq[1] = sizeof (xreq);
            sock_fd_write(sock, xreq, sizeof (xreq), -1);
        }

        /* Write our 'pass an fd' request with a 'useless' FD to block the receiver */
        xreq[0] = 1;
        xreq[1] = sizeof(xreq);
        sock_fd_write(sock, xreq, sizeof (xreq), 1);

        /* Pass an fd */
        xnop[0] = 2;
        xnop[1] = sizeof (xnop);
        fd = tmp_file(j);
        sock_fd_write(sock, xnop, sizeof (xnop), fd);
        close(fd);
    }
}

Fixing XCB to receive file descriptors

Multiple threads may be trying to get replies and events back from the X server at the same time, which means the kludge of having the real fd follow the message will likely lead to the wrong thread getting the file descriptor.

Instead, I suspect the best plan will be to fix XCB to internally capture passed file descriptors and save them with the associated reply. Because the file descriptor message will form a barrier in the read stream, xcb can associate any received file descriptor with the last reply in the read data. The X server would then send the reply with an explicit sendmsg call to pass both reply and file descriptor together.

Next steps

The next thing to do is code up a simple fd passing extension and try to get it working, passing descriptors back and forth to the X server. Once that works, design of the rest of the DRM-Next extension should be pretty straightforward.