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@xxxxxxxxxx> --- 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. Jano
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