[Qemu-devel] [PATCH 1/4] Add basic version of bridge helper

[Anthony Liguori]

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 <address@hidden>
---
 Makefile             |   12 +++-
 configure            |    1 +
 qemu-bridge-helper.c |  205 ++++++++++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 216 insertions(+), 2 deletions(-)
 create mode 100644 qemu-bridge-helper.c

diff --git a/Makefile b/Makefile
index c783aa4..9b15e67 100644
--- a/Makefile
+++ b/Makefile
@@ -26,6 +26,8 @@ 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
 else
@@ -40,7 +42,7 @@ config-all-devices.mak: $(SUBDIR_DEVICES_MAK)
 
 -include config-all-devices.mak
 
-build-all: config-host.h config-all-devices.h $(DOCS) $(TOOLS)
+build-all: config-host.h config-all-devices.h $(DOCS) $(TOOLS) $(HELPERS-y)
        $(call quiet-command, $(MAKE) $(SUBDIR_MAKEFLAGS) recurse-all,)
 
 config-host.h: config-host.h-timestamp
@@ -217,6 +219,8 @@ qemu-nbd$(EXESUF):  qemu-nbd.o qemu-tool.o $(block-obj-y)
 
 qemu-io$(EXESUF):  qemu-io.o qemu-tool.o cmd.o $(block-obj-y)
 
+qemu-bridge-helper$(EXESUF): qemu-bridge-helper.o
+
 qemu-img-cmds.h: $(SRC_PATH)/qemu-img-cmds.hx
        $(call quiet-command,sh $(SRC_PATH)/hxtool -h < $< > $@,"  GEN   $@")
 
@@ -228,7 +232,7 @@ check-qlist: check-qlist.o qlist.o qint.o qemu-malloc.o
 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 *.o *.d *.a $(TOOLS) TAGS cscope.* *.pod *~ */*~
+       rm -f *.o *.d *.a $(TOOLS) $(HELPERS-y) TAGS cscope.* *.pod *~ */*~
        rm -f slirp/*.o slirp/*.d audio/*.o audio/*.d block/*.o block/*.d 
net/*.o net/*.d
        rm -f qemu-img-cmds.h
        $(MAKE) -C tests clean
@@ -274,6 +278,10 @@ install: all $(if $(BUILD_DOCS),install-doc)
 ifneq ($(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 $(BLOBS); do \
diff --git a/configure b/configure
index aa2cc43..a341e77 100755
--- a/configure
+++ b/configure
@@ -2106,6 +2106,7 @@ echo "bindir=\${prefix}$binsuffix" >> $config_host_mak
 echo "mandir=\${prefix}$mansuffix" >> $config_host_mak
 echo "datadir=\${prefix}$datasuffix" >> $config_host_mak
 echo "docdir=\${prefix}$docsuffix" >> $config_host_mak
+echo "libexecdir=\${prefix}/libexec" >> $config_host_mak
 echo "MAKE=$make" >> $config_host_mak
 echo "INSTALL=$install" >> $config_host_mak
 echo "INSTALL_DIR=$install -d -m0755 -p" >> $config_host_mak
diff --git a/qemu-bridge-helper.c b/qemu-bridge-helper.c
new file mode 100644
index 0000000..f10d37c
--- /dev/null
+++ b/qemu-bridge-helper.c
@@ -0,0 +1,205 @@
+/*
+ * QEMU Bridge Helper
+ *
+ * Copyright IBM, Corp. 2009
+ *
+ * Authors:
+ *  Anthony Liguori   <address@hidden>
+ *
+ * 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 <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 int has_vnet_hdr(int fd)
+{
+    unsigned int features;
+    struct ifreq ifreq;
+
+    if (ioctl(fd, TUNGETFEATURES, &features) == -1) {
+        return -errno;
+    }
+
+    if (!(features & IFF_VNET_HDR)) {
+        return -ENOTSUP;
+    }
+
+    if (ioctl(fd, TUNGETIFF, &ifreq) != -1 || errno != EBADFD) {
+        return -ENOTSUP;
+    }
+
+    return 1;
+}
+
+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;
+    int use_vnet = 0;
+    int mtu;
+    const char *bridge;
+    char iface[IFNAMSIZ];
+    int index;
+
+    /* parse arguments */
+    if (argc < 3 || argc > 4) {
+        fprintf(stderr, "Usage: %s [--use-vnet] BRIDGE FD\n", argv[0]);
+        return 1;
+    }
+
+    index = 1;
+    if (strcmp(argv[index], "--use-vnet") == 0) {
+        use_vnet = 1;
+        index++;
+        if (argc == 3) {
+            fprintf(stderr, "invalid number of arguments\n");
+            return -1;
+        }
+    }
+
+    bridge = argv[index++];
+    unixfd = atoi(argv[index++]);
+
+    /* 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\n");
+        return -errno;
+    }
+
+    /* open the tap device */
+    fd = open("/dev/net/tun", O_RDWR);
+    if (fd == -1) {
+        fprintf(stderr, "failed to open /dev/net/tun\n");
+        return -errno;
+    }
+
+    /* 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\n");
+        return -errno;
+    }
+
+    /* 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'\n", bridge);
+        return -errno;
+    }
+
+    /* 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\n",
+                iface, mtu);
+        return -errno;
+    }
+
+    /* 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'\n",
+                iface, bridge);
+        return -errno;
+    }
+
+    /* bring the interface up */
+    prep_ifreq(&ifr, iface);
+    if (ioctl(ctlfd, SIOCGIFFLAGS, &ifr) == -1) {
+        fprintf(stderr, "failed to get interface flags for `%s'\n", iface);
+        return -errno;
+    }
+
+    ifr.ifr_flags |= IFF_UP;
+    if (ioctl(ctlfd, SIOCSIFFLAGS, &ifr) == -1) {
+        fprintf(stderr, "failed to set bring up interface `%s'\n", iface);
+        return -errno;
+    }
+
+    /* write fd to the domain socket */
+    if (send_fd(unixfd, fd) == -1) {
+        fprintf(stderr, "failed to write fd to unix socket\n");
+        return -errno;
+    }
+
+    /* ... */
+
+    /* profit! */
+
+    close(fd);
+
+    close(ctlfd);
+
+    return 0;
+}
-- 
1.6.2.5