Re: [libvirt] [PATCH v2 2/4] qemu: support passing pre-opened UNIX socket listen FD

There is a race condition when spawning QEMU where libvirt has spawned QEMU but the monitor socket is not yet open. Libvirt has to repeatedly try to connect() to QEMU's monitor until eventually it succeeds, or times out. We use kill() to check if QEMU is still alive so we avoid waiting a long time if QEMU exited, but having a timeout at all is still unpleasant. With QEMU 2.12 we can pass in a pre-opened FD for UNIX domain or TCP sockets. If libvirt has called bind() and listen() on this FD, then we have a guarantee that libvirt can immediately call connect() and succeed without any race. Although we only really care about this for the monitor socket and agent socket, this patch does FD passing for all UNIX socket based character devices since there appears to be no downside to it. We don't do FD passing for TCP sockets, however, because it is only possible to pass a single FD, while some hostnames may require listening on multiple FDs to cover IPv4 and IPv6 concurrently. Reviewed-by: John Ferlan <jferlan(a)redhat.com&gt; Signed-off-by: Daniel P. Berrangé <berrange(a)redhat.com&gt; --- src/qemu/qemu_command.c | 54 +++++++++++++++++++++++++++++++++++++++++++++++-- 1 file changed, 52 insertions(+), 2 deletions(-)

diff --git a/src/qemu/qemu_command.c b/src/qemu/qemu_command.c index 0c109c63e7..9fc48eb829 100644 --- a/src/qemu/qemu_command.c +++ b/src/qemu/qemu_command.c @@ -5034,8 +5034,58 @@ qemuBuildChrChardevStr(virLogManagerPtr logManager, break; case VIR_DOMAIN_CHR_TYPE_UNIX: - virBufferAsprintf(&buf, "socket,id=%s,path=", charAlias); - virQEMUBuildBufferEscapeComma(&buf, dev->data.nix.path); + if (virQEMUCapsGet(qemuCaps, QEMU_CAPS_CHARDEV_FD_PASS)) { + struct sockaddr_un addr; + socklen_t addrlen = sizeof(addr); + int fd; + + if ((fd = socket(AF_UNIX, SOCK_STREAM, 0)) < 0) { + virReportSystemError(errno, "%s", + _("Unable to create UNIX socket")); + goto cleanup; + } + + memset(&addr, 0, sizeof(addr)); + addr.sun_family = AF_UNIX; + if (virStrcpyStatic(addr.sun_path, dev->data.nix.path) == NULL) { + virReportError(VIR_ERR_INTERNAL_ERROR, + _("UNIX socket path '%s' too long"), + dev->data.nix.path); + VIR_FORCE_CLOSE(fd); + goto cleanup; + } + + if (unlink(dev->data.nix.path) < 0 && errno != ENOENT) { + virReportSystemError(errno, + _("Unable to unlink %s"), + dev->data.nix.path); + VIR_FORCE_CLOSE(fd); + goto cleanup; + } + + if (bind(fd, (struct sockaddr *)&addr, addrlen) < 0) { + virReportSystemError(errno, + _("Unable to bind to UNIX socket path '%s'"), + dev->data.nix.path); + VIR_FORCE_CLOSE(fd); + goto cleanup; + } + + if (listen(fd, 1) < 0) { + virReportSystemError(errno, + _("Unable to listen to UNIX socket path '%s'"), + dev->data.nix.path); + VIR_FORCE_CLOSE(fd); + goto cleanup; + } + + virBufferAsprintf(&buf, "socket,id=%s,fd=%d", charAlias, fd); + + virCommandPassFD(cmd, fd, VIR_COMMAND_PASS_FD_CLOSE_PARENT); + } else { + virBufferAsprintf(&buf, "socket,id=%s,path=", charAlias); + virQEMUBuildBufferEscapeComma(&buf, dev->data.nix.path); + } if (dev->data.nix.listen) virBufferAdd(&buf, nowait ? ",server,nowait" : ",server", -1);