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

[Markus Armbruster]

Markus Armbruster <address@hidden> writes:

> 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.

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?

[...]