Re: Can one pass file descriptors to QEMU through QMP/monitor after it has started?

[dongli . zhang]

+ qemu-discuss@nongnu.org back.


On 9/10/24 1:15 PM, DUO Labs wrote:
> Should the fd be sent through SCM_RIGHTS just before the getfd command, or can
> you somehow combine the two messages into one?

It is one simple cmsghdr message, with both the getfd qmp message, and also the
fd/SCM_RIGHTS.

I suggest read the libvirt code.

465 /**
466  * virSocketSendMsgWithFDs:
467  * @sock: socket to send payload and fds to
468  * @payload: payload to send
469  * @payload_len: length of @payload
470  * @fds: array of fds to send
471  * @fds_len: len of fds array
472
473  * Send @fds along with @payload to @sock using SCM_RIGHTS.
474  * Return number of bytes sent on success.
475  * On error, set errno and return -1.
476  */
477 int
478 virSocketSendMsgWithFDs(int sock,
479                         const char *payload,
480                         size_t payload_len,
481                         int *fds,
482                         size_t fds_len)
483 {
484     g_autofree char *control = NULL;
485     const size_t control_size = CMSG_SPACE(sizeof(int) * fds_len);
486     struct cmsghdr *cmsg;
487     struct msghdr msg = { 0 };
488     struct iovec iov[1]; /* Send a single payload, so set vector len to 1 */
489     int ret;
490
491     control = g_new0(char, control_size);
492
493     iov[0].iov_base = (void *) payload;
494     iov[0].iov_len = payload_len;
495
496     msg.msg_iov = iov;
497     msg.msg_iovlen = 1;
498
499     msg.msg_control = control;
500     msg.msg_controllen = control_size;
501
502     cmsg = CMSG_FIRSTHDR(&msg);
503     /* check to eliminate "potential null pointer dereference" errors during
build */
504     if (!cmsg) {
505         errno = ENOSPC;
506         return -1;
507     }
508
509     cmsg->cmsg_len = CMSG_LEN(sizeof(int) * fds_len);
510     cmsg->cmsg_level = SOL_SOCKET;
511     cmsg->cmsg_type = SCM_RIGHTS;
512     memcpy(CMSG_DATA(cmsg), fds, sizeof(int) * fds_len);
513
514     do {
515         ret = sendmsg(sock, &msg, 0);
516     } while (ret < 0 && errno == EINTR);
517
518     return ret;
519 }

Dongli Zhang