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?
