11:12 a.m.
On 8/26/20 4:21 PM, Ján Tomko wrote:
On a Wednesday in 2020, Laine Stump wrote:
> On 8/26/20 9:00 AM, Michal Privoznik wrote:
>> On 8/26/20 7:22 AM, Laine Stump wrote:
>>> There have been some reports that, due to libvirt always trying to
>>> assign the lowest numbered macvtap / tap device name possible, a new
>>> guest would sometimes be started using the same tap device name as
>>> previously used by another guest that is in the process of being
>>> destroyed *as the new guest is starting.
>>>
>>> In some cases this has led to, for example, the old guest's
>>> qemuProcessStop() code deleting a port from an OVS switch that had
>>> just been re-added by the new guest (because the port name is based on
>>> only the device name using the port). Similar problems can happen (and
>>> I believe have) with nwfilter rules and bandwidth rules (which are
>>> both instantiated based on the name of the tap device).
>>>
>>> A couple patches have been previously proposed to change the ordering
>>> of startup and shutdown processing, or to put a mutex around
>>> everything related to the tap/macvtap device name usage, but in the
>>> end no matter what you do there will still be possible holes, because
>>> the device could be deleted outside libvirt's control (for example,
>>> regular tap devices are automatically deleted when the qemu process
>>> terminates, and that isn't always initiated by libvirt but could
>>> instead happen completely asynchronously - libvirt then has no control
>>> over the ordering of shutdown operations, and no opportunity to
>>> protect it with a mutex.)
>>>
>>> But this only happens if a new device is created at the same time as
>>> one is being deleted. We can effectively eliminate the chance of this
>>> happening if we end the practice of always looking for the lowest
>>> numbered available device name, and instead just keep an integer that
>>> is incremented each time we need a new device name. At some point it
>>> will need to wrap back around to 0 (in order to avoid the IFNAMSIZ 15
>>> character limit if nothing else), and we can't guarantee that the new
>>> name really will be the *least* recently used name, but "math"
>>> suggests that it will be *much* less common that we'll try to re-use
>>> the *most* recently used name.
>>>
>>> This patch implements such a counter for macvtap/macvlan, replacing
>>> the existing, and much more complicated, "ID reservation" system.
The
>>> counter is set according to whatever macvtap/macvlan devices are
>>> already in use by guests when libvirtd is started, incremented each
>>> time a new device name is needed, and wraps back to 0 when either
>>> INT_MAX is reached, or when the resulting device name would be longer
>>> than IFNAMSIZ-1 characters (which actually is what happens when the
>>> template for the device name is "maccvtap%d"). The result is that
no
>>> macvtap name will be re-used until the host has created (and possibly
>>> destroyed) 99,999,999 devices.
>>>
>>> Signed-off-by: Laine Stump <laine(a)redhat.com>
>>> ---
>>> src/libvirt_private.syms | 1 -
>>> src/libxl/libxl_driver.c | 2 +-
>>> src/lxc/lxc_process.c | 2 +-
>>> src/qemu/qemu_process.c | 2 +-
>>> src/util/virnetdevmacvlan.c | 402
>>> +++++++++++++-----------------------
>>> src/util/virnetdevmacvlan.h | 6 +-
>>> 6 files changed, 145 insertions(+), 270 deletions(-)
>>>
>>
>>> diff --git a/src/util/virnetdevmacvlan.c b/src/util/virnetdevmacvlan.c
>>> index dcea93a5fe..dc4db2c844 100644
>>> --- a/src/util/virnetdevmacvlan.c
>>> +++ b/src/util/virnetdevmacvlan.c
>>
>>> +static int
>>> +virNetDevMacVLanGenerateName(char **ifname, unsigned int flags)
>>> {
>>> - unsigned int id;
>>> - unsigned int flags = 0;
>>> - const char *idstr = NULL;
>>> -
>>> - if (virNetDevMacVLanInitialize() < 0)
>>> - return -1;
>>> + const char *prefix;
>>> + const char *iftemplate;
>>> + int *lastID;
>>> + int id;
>>> + double maxIDd;
>>> + int maxID = INT_MAX;
>>> + int attempts = 0;
>>> - if (STRPREFIX(name, VIR_NET_GENERATED_MACVTAP_PREFIX)) {
>>> - idstr = name + strlen(VIR_NET_GENERATED_MACVTAP_PREFIX);
>>> - flags |= VIR_NETDEV_MACVLAN_CREATE_WITH_TAP;
>>> - } else if (STRPREFIX(name, VIR_NET_GENERATED_MACVLAN_PREFIX)) {
>>> - idstr = name + strlen(VIR_NET_GENERATED_MACVLAN_PREFIX);
>>> + if (flags & VIR_NETDEV_MACVLAN_CREATE_WITH_TAP) {
>>> + prefix = VIR_NET_GENERATED_MACVTAP_PREFIX;
>>> + iftemplate = VIR_NET_GENERATED_MACVTAP_PREFIX "%d";
>>> + lastID = &virNetDevMacVTapLastID;
>>> } else {
>>> - return -2;
>>> + prefix = VIR_NET_GENERATED_MACVLAN_PREFIX;
>>> + iftemplate = VIR_NET_GENERATED_MACVLAN_PREFIX "%d";
>>> + lastID = &virNetDevMacVLanLastID;
>>> }
>>> - if (virStrToLong_ui(idstr, NULL, 10, &id) < 0) {
>>> - virReportError(VIR_ERR_INTERNAL_ERROR,
>>> - _("couldn't get id value from macvtap device
>>> name %s"),
>>> - name);
>>> - return -1;
>>> - }
>>> - return virNetDevMacVLanReserveID(id, flags, quietFail, false);
>>> -}
>>> + maxIDd = pow(10, IFNAMSIZ - 1 - strlen(prefix));
>>> + if (maxIDd <= (double)INT_MAX)
>>> + maxID = (int)maxIDd;
>>
>> pow() requires -lm. We need this to be squashed in:
>
>
> Dan had said yesterday in IRC that we already link in libm, and it's
> been building correctly. Are there other targets where that isn't the
> case, and I'm just lucky?
>
libxml2 is linking to it, at least.
Anyway, we already use ldexp in virrandom and isnan in virxml.c so
I'd consider the -lm change a separate issue from this commit.
It is, but we are not linking libxml2 to virt_util_lib rather than
libvirt.so. Hence, when linking virt_util_lib we get this linking error.
I'm okay with making the change in a separate commit.
Michal