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

[Markus Armbruster]

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?