[libvirt PATCH 1/3] util: make locking versions of virNetDevMacVLan(Reserve|Release)Name()

[Michal Privoznik]

On 8/24/20 6:23 AM, Laine Stump wrote:
> When these functions are called from within virnetdevmacvlan.c, they
> are usually called with virNetDevMacVLanCreateMutex held, but when
> virNetDevMacVLanReserveName() is called from other places (hypervisor
> drivers keeping track of already-in-use macvlan/macvtap devices) the
> lock isn't acquired. This could lead to a situation where one thread
> is setting a bit in the bitmap to notify of a device already in-use,
> while another thread is checking/setting/clearing a bit while creating
> a new macvtap device.
> 
> In practice this *probably* doesn't happen, because the external calls
> to virNetDevMacVLan() only happen during hypervisor driver init
> routines when libvirtd is restarted, but there's no harm in protecting
> ourselves.
> 
> (NB: virNetDevMacVLanReleaseName() is actually never called from
> outside virnetdevmacvlan.c, so it could just as well be static, but
> I'm leaving it as-is for now. This locking version *is* called
> from within virnetdevmacvlan.c, since there are a couple places that
> we used to call the unlocked version after the lock was already
> released.)
> 
> Signed-off-by: Laine Stump <laine at redhat.com>
> ---
>   src/util/virnetdevmacvlan.c | 42 ++++++++++++++++++++++++++++++-------
>   1 file changed, 34 insertions(+), 8 deletions(-)
> 
> diff --git a/src/util/virnetdevmacvlan.c b/src/util/virnetdevmacvlan.c
> index dcea93a5fe..69a9c784bb 100644
> --- a/src/util/virnetdevmacvlan.c
> +++ b/src/util/virnetdevmacvlan.c
> @@ -196,7 +196,7 @@ virNetDevMacVLanReleaseID(int id, unsigned int flags)
>   
>   
>   /**
> - * virNetDevMacVLanReserveName:
> + * virNetDevMacVLanReserveNameInternal:
>    *
>    *  @name: already-known name of device
>    *  @quietFail: don't log an error if this name is already in-use
> @@ -208,8 +208,8 @@ virNetDevMacVLanReleaseID(int id, unsigned int flags)
>    *  Returns reserved ID# on success, -1 on failure, -2 if the name
>    *  doesn't fit the auto-pattern (so not reserveable).
>    */
> -int
> -virNetDevMacVLanReserveName(const char *name, bool quietFail)
> +static int
> +virNetDevMacVLanReserveNameInternal(const char *name, bool quietFail)
>   {
>       unsigned int id;
>       unsigned int flags = 0;
> @@ -237,8 +237,21 @@ virNetDevMacVLanReserveName(const char *name, bool quietFail)
>   }
>   
>   
> +int
> +virNetDevMacVLanReserveName(const char *name, bool quietFail)
> +{
> +    /* Call the internal function after locking the macvlan mutex */
> +    int ret;
> +
> +    virMutexLock(&virNetDevMacVLanCreateMutex);
> +    ret = virNetDevMacVLanReserveNameInternal(name, quietFail);
> +    virMutexUnlock(&virNetDevMacVLanCreateMutex);
> +    return ret;
> +}

Hopefully, we won't use any of these in a forked off process because 
these are not async-signal safe anymore.

Michal