Daniel P. Berrangé <berra...@redhat.com> writes: > On Wed, May 15, 2019 at 08:34:17AM +0200, Markus Armbruster wrote: >> Jason Wang <jasow...@redhat.com> writes: >> >> > On 2019/5/14 下午8:18, Markus Armbruster wrote: >> >> Peter Maydell <peter.mayd...@linaro.org> writes: >> >> >> >>> On Mon, 13 May 2019 at 14:21, Markus Armbruster <arm...@redhat.com> >> >>> wrote: >> >>>> Perhaps I should do it just for this file while I touch it anyway. The >> >>>> question to ask: should parse_acl_file() obey the locale for whitespace >> >>>> recognition? >> >>> I vote for "no". >> >>> >> >>> Q: do we document the format of the ACL file anywhere ? >> >> Support for it was added in commit bdef79a2994, v1.1. Just code, no >> >> documentation. >> >> >> >> Grepping for qemu-bridge-helper finds just qemu-options.hx. Contains >> >> -help output and some .texi that goes into qemu-doc and the manual page. >> >> None of it mentions how qemu-bridge-helper is run, or the ACL file >> >> feature, let alone what its format might be. >> >> >> >> I'm afraid all we have is the commit message. Which doesn't really >> >> define the file format, it merely gives a bunch of examples. >> >> >> >> As far as I can tell, qemu-bridge-helper is for use with -netdev tap and >> >> -netdev bridge. >> >> >> >> Both variations of -netdev call net_bridge_run_helper() to run the >> >> helper. First argument is -netdev parameter "helper", default usually >> >> "$prefix/libexec/qemu-bridge-helper". Second argument is parameter >> >> "br", default "br0". >> >> >> >> If @helper contains space or tab, net_bridge_run_helper() guesses its a >> >> full command, else it guesses its the name of the executable. Bad >> >> magic. >> >> >> >> If it guesses name of executable, it execv()s this executable with >> >> arguments "--use-vnet", "--fd=FD", "--br=@bridge". >> >> >> >> If it guesses full command, it appends "--use-vnet --fd=FD", where FD is >> >> the helper's half of the socketpair used to connect QEMU and the helper. >> >> It further appends "--br=@bridge", unless @helper contains "--br=". >> >> More bad magic. >> >> >> >> It executes the resulting string with sh -c. Magic cherry on top. >> >> >> >> When the helper fails, netdev creation fails. >> >> >> >> The helper we ship with QEMU unconditionally tries to read >> >> "$prefix/etc/bridge.conf". Fatal error if this file doesn't exist. >> >> Errors in this file are fatal. Errors in files it includes are not >> >> fatal; instead, the remainder of the erroneous file is ignored. >> >> *Boggle* >> >> >> >> As far as I can tell, libvirt runs qemu-bridge-helper itself (Paolo's >> >> commit 2d80fbb14df). Makes sense, because running QEMU with the >> >> necessary privileges would be unwise, and so would be letting it execute >> >> setuid helpers. Also bypasses the bad magic in QEMU's >> >> net_bridge_run_helper(). >> > >> > >> > I don't notice this before. Is this only for the convenience of >> > development? I guess libvirt should have native support like adding >> > port to bridge/OVS without the help any external command or script. >> >> Commit 2d80fbb14df hints at the reason: >> >> <source type='bridge'> uses a helper application to do the necessary >> TUN/TAP setup to use an existing network bridge, thus letting >> unprivileged users use TUN/TAP interfaces. >> ~~~~~~~~~~~~~~~~~~ >> >> The code confirms: >> >> /* qemuInterfaceBridgeConnect: >> * @def: the definition of the VM >> * @driver: qemu driver data >> * @net: pointer to the VM's interface description >> * @tapfd: array of file descriptor return value for the new device >> * @tapfdsize: number of file descriptors in @tapfd >> * >> ---> * Called *only* called if actualType is VIR_DOMAIN_NET_TYPE_NETWORK or >> ---> * VIR_DOMAIN_NET_TYPE_BRIDGE (i.e. if the connection is made with a tap >> * device connecting to a bridge device) >> */ >> int >> qemuInterfaceBridgeConnect(virDomainDefPtr def, >> virQEMUDriverPtr driver, >> virDomainNetDefPtr net, >> int *tapfd, >> size_t *tapfdSize) >> { >> [...] >> ---> if (virQEMUDriverIsPrivileged(driver)) { >> [...] >> } else { >> if (qemuCreateInBridgePortWithHelper(cfg, brname, >> &net->ifname, >> tapfd, tap_create_flags) < >> 0) { >> virDomainAuditNetDevice(def, net, tunpath, false); >> goto cleanup; >> } >> [...] >> } >> [...] >> } >> >> >> qemu-bridge-helper should have a manual page, and its handling of errors >> >> in ACL include files needs work. There's probably more; I just glanced >> >> at it. I'm not volunteering, though. It lacks a maintainer. Should we >> >> add it to Jason's "Network device backends"? >> > >> > >> > Yes. >> > >> >> -netdev's helper parameter is seriously underdocumented. Document or >> >> deprecate? >> > >> > >> > I believe management should only use fd parameter of TAP. If we have >> > other, it should be a duplication. So I suggest to deprecate the >> > bridge helper and -netdev bridge. >> >> Objections, anyone? > > Libvirt runs the qemu bridge helper command directly, and we have > applications using this functionality.
Specifically, when libvirt lacks the privileges to set up a TAP fd, it farms out the job to setuid qemu-bridge-helper. Correct? > I'd like libvirt to be able to avoid use of the QEMU bridge helper and > instead have unprivileged libvirt talk to privileged libvirtd to open a > TAP fd on its behalf. If we ever get that done, then libvirt would not > need the qemu bridge helper command anymore. We don't want to deprecate qemu-bridge-helper while libvirt has a sensible use for it. We can still deprecate -netdev tap parameter "helper" and -netdev bridge entirely. Once they're gone, qemu-bridge-helper wull have no user within QEMU. We could discuss moving it to libvirt then, but I doubt it'll be worth the trouble.