Markus Armbruster <arm...@redhat.com> writes: > 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.
Correction: $prefix/etc/qemu/bridge.conf >>> >> 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. Paolo and Richard objected to deprecating -netdev bridge, so that's out, too. Proposal: 1. Add qemu-bridge-helper.c to Jason's "Network device backends" 2. Deprecate -netdev tap parameter "helper" 3. Improve documentation of -netdev bridge 4. Create a manual page for qemu-bridge-helper that also covers /etc/qemu/bridge.conf. 5. Fix the nutty error handling in parse_acl_file() Objections? [...]