Add basic version of bridge helper

This patch adds a helper that can be used to create a tap device attached to
a bridge device.  Since this helper is minimal in what it does, it can be
given CAP_NET_ADMIN which allows qemu to avoid running as root while still
satisfying the majority of what users tend to want to do with tap devices.

The way this all works is that qemu launches this helper passing a bridge
name and the name of an inherited file descriptor.  The descriptor is one
end of a socketpair() of domain sockets.  This domain socket is used to
transmit a file descriptor of the opened tap device from the helper to qemu.

The helper can then exit and let qemu use the tap device.

Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
Signed-off-by: Richa Marwaha <rmarwah@linux.vnet.ibm.com>
Signed-off-by: Corey Bryant <coreyb@linux.vnet.ibm.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

Makefile

@@ -36,6 +36,8 @@ $(call set-vpath, $(SRC_PATH):$(SRC_PATH)/hw)
 
 LIBS+=-lz $(LIBS_TOOLS)
 
+HELPERS-$(CONFIG_LINUX) = qemu-bridge-helper$(EXESUF)
+
 ifdef BUILD_DOCS
 DOCS=qemu-doc.html qemu-tech.html qemu.1 qemu-img.1 qemu-nbd.8 QMP/qmp-commands.txt
 DOCS+=fsdev/virtfs-proxy-helper.1
@@ -77,7 +79,7 @@ defconfig:
 
 -include config-all-devices.mak
 
-build-all: $(DOCS) $(TOOLS) recurse-all
+build-all: $(DOCS) $(TOOLS) $(HELPERS-y) recurse-all
 
 config-host.h: config-host.h-timestamp
 config-host.h-timestamp: config-host.mak
@@ -158,6 +160,8 @@ qemu-img$(EXESUF): qemu-img.o $(tools-obj-y) $(block-obj-y)
 qemu-nbd$(EXESUF): qemu-nbd.o $(tools-obj-y) $(block-obj-y)
 qemu-io$(EXESUF): qemu-io.o cmd.o $(tools-obj-y) $(block-obj-y)
 
+qemu-bridge-helper$(EXESUF): qemu-bridge-helper.o
+
 fsdev/virtfs-proxy-helper$(EXESUF): fsdev/virtfs-proxy-helper.o fsdev/virtio-9p-marshal.o oslib-posix.o $(trace-obj-y)
 fsdev/virtfs-proxy-helper$(EXESUF): LIBS += -lcap
 
@@ -207,7 +211,7 @@ clean:
 # avoid old build problems by removing potentially incorrect old files
 	rm -f config.mak op-i386.h opc-i386.h gen-op-i386.h op-arm.h opc-arm.h gen-op-arm.h
 	rm -f qemu-options.def
-	rm -f *.o *.d *.a *.lo $(TOOLS) qemu-ga TAGS cscope.* *.pod *~ */*~
+	rm -f *.o *.d *.a *.lo $(TOOLS) $(HELPERS-y) qemu-ga TAGS cscope.* *.pod *~ */*~
 	rm -Rf .libs
 	rm -f slirp/*.o slirp/*.d audio/*.o audio/*.d block/*.o block/*.d net/*.o net/*.d fsdev/*.o fsdev/*.d ui/*.o ui/*.d qapi/*.o qapi/*.d qga/*.o qga/*.d
 	rm -f qemu-img-cmds.h
@@ -281,6 +285,10 @@ install: all $(if $(BUILD_DOCS),install-doc) install-sysconfig
 ifneq ($(TOOLS),)
 	$(INSTALL_PROG) $(STRIP_OPT) $(TOOLS) "$(DESTDIR)$(bindir)"
 endif
+ifneq ($(HELPERS-y),)
+	$(INSTALL_DIR) "$(DESTDIR)$(libexecdir)"
+	$(INSTALL_PROG) $(STRIP_OPT) $(HELPERS-y) "$(DESTDIR)$(libexecdir)"
+endif
 ifneq ($(BLOBS),)
 	$(INSTALL_DIR) "$(DESTDIR)$(datadir)"
 	set -e; for x in $(BLOBS); do \

configure

@@ -2910,6 +2910,7 @@ echo "datadir=$datadir" >> $config_host_mak
 echo "sysconfdir=$sysconfdir" >> $config_host_mak
 echo "docdir=$docdir" >> $config_host_mak
 echo "confdir=$confdir" >> $config_host_mak
+echo "libexecdir=\${prefix}/libexec" >> $config_host_mak
 
 echo "ARCH=$ARCH" >> $config_host_mak
 if test "$debug_tcg" = "yes" ; then

qemu-bridge-helper.c

@@ -0,0 +1,221 @@
+/*
+ * QEMU Bridge Helper
+ *
+ * Copyright IBM, Corp. 2011
+ *
+ * Authors:
+ * Anthony Liguori   <aliguori@us.ibm.com>
+ * Richa Marwaha     <rmarwah@linux.vnet.ibm.com>
+ * Corey Bryant      <coreyb@linux.vnet.ibm.com>
+ *
+ * This work is licensed under the terms of the GNU GPL, version 2.  See
+ * the COPYING file in the top-level directory.
+ *
+ */
+
+#include "config-host.h"
+
+#include <stdio.h>
+#include <errno.h>
+#include <fcntl.h>
+#include <unistd.h>
+#include <string.h>
+#include <stdlib.h>
+#include <stdbool.h>
+#include <ctype.h>
+
+#include <sys/types.h>
+#include <sys/ioctl.h>
+#include <sys/socket.h>
+#include <sys/un.h>
+#include <sys/prctl.h>
+
+#include <net/if.h>
+
+#include <linux/sockios.h>
+
+#include "net/tap-linux.h"
+
+static void usage(void)
+{
+    fprintf(stderr,
+            "Usage: qemu-bridge-helper [--use-vnet] --br=bridge --fd=unixfd\n");
+}
+
+static bool has_vnet_hdr(int fd)
+{
+    unsigned int features = 0;
+
+    if (ioctl(fd, TUNGETFEATURES, &features) == -1) {
+        return false;
+    }
+
+    if (!(features & IFF_VNET_HDR)) {
+        return false;
+    }
+
+    return true;
+}
+
+static void prep_ifreq(struct ifreq *ifr, const char *ifname)
+{
+    memset(ifr, 0, sizeof(*ifr));
+    snprintf(ifr->ifr_name, IFNAMSIZ, "%s", ifname);
+}
+
+static int send_fd(int c, int fd)
+{
+    char msgbuf[CMSG_SPACE(sizeof(fd))];
+    struct msghdr msg = {
+        .msg_control = msgbuf,
+        .msg_controllen = sizeof(msgbuf),
+    };
+    struct cmsghdr *cmsg;
+    struct iovec iov;
+    char req[1] = { 0x00 };
+
+    cmsg = CMSG_FIRSTHDR(&msg);
+    cmsg->cmsg_level = SOL_SOCKET;
+    cmsg->cmsg_type = SCM_RIGHTS;
+    cmsg->cmsg_len = CMSG_LEN(sizeof(fd));
+    msg.msg_controllen = cmsg->cmsg_len;
+
+    iov.iov_base = req;
+    iov.iov_len = sizeof(req);
+
+    msg.msg_iov = &iov;
+    msg.msg_iovlen = 1;
+    memcpy(CMSG_DATA(cmsg), &fd, sizeof(fd));
+
+    return sendmsg(c, &msg, 0);
+}
+
+int main(int argc, char **argv)
+{
+    struct ifreq ifr;
+    int fd, ctlfd, unixfd = -1;
+    int use_vnet = 0;
+    int mtu;
+    const char *bridge = NULL;
+    char iface[IFNAMSIZ];
+    int index;
+    int ret = EXIT_SUCCESS;
+
+    /* parse arguments */
+    for (index = 1; index < argc; index++) {
+        if (strcmp(argv[index], "--use-vnet") == 0) {
+            use_vnet = 1;
+        } else if (strncmp(argv[index], "--br=", 5) == 0) {
+            bridge = &argv[index][5];
+        } else if (strncmp(argv[index], "--fd=", 5) == 0) {
+            unixfd = atoi(&argv[index][5]);
+        } else {
+            usage();
+            return EXIT_FAILURE;
+        }
+    }
+
+    if (bridge == NULL || unixfd == -1) {
+        usage();
+        return EXIT_FAILURE;
+    }
+
+    /* open a socket to use to control the network interfaces */
+    ctlfd = socket(AF_INET, SOCK_STREAM, 0);
+    if (ctlfd == -1) {
+        fprintf(stderr, "failed to open control socket: %s\n", strerror(errno));
+        ret = EXIT_FAILURE;
+        goto cleanup;
+    }
+
+    /* open the tap device */
+    fd = open("/dev/net/tun", O_RDWR);
+    if (fd == -1) {
+        fprintf(stderr, "failed to open /dev/net/tun: %s\n", strerror(errno));
+        ret = EXIT_FAILURE;
+        goto cleanup;
+    }
+
+    /* request a tap device, disable PI, and add vnet header support if
+     * requested and it's available. */
+    prep_ifreq(&ifr, "tap%d");
+    ifr.ifr_flags = IFF_TAP|IFF_NO_PI;
+    if (use_vnet && has_vnet_hdr(fd)) {
+        ifr.ifr_flags |= IFF_VNET_HDR;
+    }
+
+    if (ioctl(fd, TUNSETIFF, &ifr) == -1) {
+        fprintf(stderr, "failed to create tun device: %s\n", strerror(errno));
+        ret = EXIT_FAILURE;
+        goto cleanup;
+    }
+
+    /* save tap device name */
+    snprintf(iface, sizeof(iface), "%s", ifr.ifr_name);
+
+    /* get the mtu of the bridge */
+    prep_ifreq(&ifr, bridge);
+    if (ioctl(ctlfd, SIOCGIFMTU, &ifr) == -1) {
+        fprintf(stderr, "failed to get mtu of bridge `%s': %s\n",
+                bridge, strerror(errno));
+        ret = EXIT_FAILURE;
+        goto cleanup;
+    }
+
+    /* save mtu */
+    mtu = ifr.ifr_mtu;
+
+    /* set the mtu of the interface based on the bridge */
+    prep_ifreq(&ifr, iface);
+    ifr.ifr_mtu = mtu;
+    if (ioctl(ctlfd, SIOCSIFMTU, &ifr) == -1) {
+        fprintf(stderr, "failed to set mtu of device `%s' to %d: %s\n",
+                iface, mtu, strerror(errno));
+        ret = EXIT_FAILURE;
+        goto cleanup;
+    }
+
+    /* add the interface to the bridge */
+    prep_ifreq(&ifr, bridge);
+    ifr.ifr_ifindex = if_nametoindex(iface);
+
+    if (ioctl(ctlfd, SIOCBRADDIF, &ifr) == -1) {
+        fprintf(stderr, "failed to add interface `%s' to bridge `%s': %s\n",
+                iface, bridge, strerror(errno));
+        ret = EXIT_FAILURE;
+        goto cleanup;
+    }
+
+    /* bring the interface up */
+    prep_ifreq(&ifr, iface);
+    if (ioctl(ctlfd, SIOCGIFFLAGS, &ifr) == -1) {
+        fprintf(stderr, "failed to get interface flags for `%s': %s\n",
+                iface, strerror(errno));
+        ret = EXIT_FAILURE;
+        goto cleanup;
+    }
+
+    ifr.ifr_flags |= IFF_UP;
+    if (ioctl(ctlfd, SIOCSIFFLAGS, &ifr) == -1) {
+        fprintf(stderr, "failed to bring up interface `%s': %s\n",
+                iface, strerror(errno));
+        ret = EXIT_FAILURE;
+        goto cleanup;
+    }
+
+    /* write fd to the domain socket */
+    if (send_fd(unixfd, fd) == -1) {
+        fprintf(stderr, "failed to write fd to unix socket: %s\n",
+                strerror(errno));
+        ret = EXIT_FAILURE;
+        goto cleanup;
+    }
+
+    /* ... */
+
+    /* profit! */
+
+cleanup:
+
+    return ret;
+}