[libvirt PATCH v2 1/2] util: replace macvtap name reservation bitmap with a simple counter

[Michal Privoznik]

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 at 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:

diff --git i/meson.build w/meson.build
index dabd4196e6..81668a6681 100644
--- i/meson.build
+++ w/meson.build
@@ -1176,6 +1176,9 @@ endif
  libxml_version = '2.9.1'
  libxml_dep = dependency('libxml-2.0', version: '>=' + libxml_version)

+cc = meson.get_compiler('c')
+m_dep = cc.find_library('m', required : false)
+
  use_macvtap = false
  if not get_option('macvtap').disabled()
    if (cc.has_header_symbol('linux/if_link.h', 'MACVLAN_MODE_BRIDGE') and
diff --git i/src/util/meson.build w/src/util/meson.build
index a7017f459f..f7092cc3f1 100644
--- i/src/util/meson.build
+++ w/src/util/meson.build
@@ -188,6 +188,7 @@ virt_util_lib = static_library(
      devmapper_dep,
      gnutls_dep,
      libnl_dep,
+    m_dep,
      numactl_dep,
      secdriver_dep,
      src_dep,


NOTE: Doesn't come from my head.
https://mesonbuild.com/howtox.html#add-math-library-lm-portably

Michal