Re: [Qemu-devel] [PATCH 1/6] qemu-bridge-helper: Fix misuse of isspace()

[Daniel P . Berrangé]

On Wed, May 15, 2019 at 04:54:04PM +0200, Markus Armbruster wrote:
> Daniel P. Berrangé <address@hidden> writes:
> 
> > On Wed, May 15, 2019 at 08:34:17AM +0200, Markus Armbruster wrote:
> >> Jason Wang <address@hidden> writes:
> >> 
> >> > On 2019/5/14 下午8:18, Markus Armbruster wrote:
> >> >> Peter Maydell <address@hidden> writes:
> >> >>
> >> >>> On Mon, 13 May 2019 at 14:21, Markus Armbruster <address@hidden> 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
> >> >> "$prefix/libexec/qemu-bridge-helper".  Second argument is parameter
> >> >> "br", default "br0".
> >> >>
> >> >> If @helper contains space or tab, net_bridge_run_helper() guesses its a
> >> >> full command, else it guesses its the name of the executable.  Bad
> >> >> magic.
> >> >>
> >> >> If it guesses name of executable, it execv()s this executable with
> >> >> arguments "--use-vnet", "--fd=FD", "address@hidden".
> >> >>
> >> >> If it guesses full command, it appends "--use-vnet --fd=FD", where FD is
> >> >> the helper's half of the socketpair used to connect QEMU and the helper.
> >> >> It further appends "address@hidden", unless @helper contains "--br=".
> >> >> More bad magic.
> >> >>
> >> >> It executes the resulting string with sh -c.  Magic cherry on top.
> >> >>
> >> >> When the helper fails, netdev creation fails.
> >> >>
> >> >> The helper we ship with QEMU unconditionally tries to read
> >> >> "$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?

Yes, this is for when using libvirt as an unpriv user - typically the
desktop virt use case.

> > 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.

No objection to that.

> 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.

I'm fine with that either way.

Regards,
Daniel
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