When allowing/denying a device in devices CGroupV2 we have to
write a BPF program for it. The program we put there is merely
static and all it does it looks up a device in a hash table (also
known as map in BPF terminology). A map is referenced via an FD
which can be acquired via virBPFCreateMap() and like any other FD
it should be closed when no longer needed. However, we close it
twice: the first time in virCgroupV2DevicesAttachProg() which
closes it unconditionally, and the second time in either
virCgroupV2DevicesCreateProg() or
virCgroupV2DevicesPrepareProg(). Remove the second close.
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>
Reviewed-by: NPavel Hrdina <phrdina@redhat.com>

This function is not called outside of the source file where it's
defined. There's no need to export it.
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>
Reviewed-by: NPavel Hrdina <phrdina@redhat.com>

In virCgroupV2DevicesReallocMap() we are debug printing both
arguments passed to the function. However, the @size argument is
type of size_t but '%lu' is used to format it.
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>

Device rules are stored in BPF map that is a hash type, this function
will create a key based on major and minor id of device.
Signed-off-by: NPavel Hrdina <phrdina@redhat.com>
Reviewed-by: NJán Tomko <jtomko@redhat.com>

Signed-off-by: NPavel Hrdina <phrdina@redhat.com>
Reviewed-by: NJán Tomko <jtomko@redhat.com>

We need to close our FD that we have for BPF program and map in order
to let kernel remove all resources once the cgroup is removed as well.
Signed-off-by: NPavel Hrdina <phrdina@redhat.com>
Reviewed-by: NJán Tomko <jtomko@redhat.com>

This function will be called for every virCgroup(Allow|Deny)* API in
order to prepare BPF program for guest.  Since libvirtd can be restarted
at any point we will first try to detect existing progam, if there is
none we will create a new empty BPF program and lastly if we don't have
any space left in the existing BPF map we will create a new copy of the
BPF map with more space and attach a new program with that map into the
guest cgroup.

This solution allows us to start with reasonably small BPF map consuming
only small amount of memory and if needed we can easily extend the BPF
map if there is a lot of host devices used in guest or if user wants to
hot-plug a lot of devices once the guest is running.

Since there is no way how to reallocate existing BPF map we need to
create a new copy if we run out of space in current BPF map.

This overcomes all the limitations in BPF:

    - map used in program has to be created before the program is loaded
      into kernel

    - once map is created you cannot change its size

    - you cannot replace map in existing program

    - you cannot use an array of maps because it can store FD to maps
      of one specific size so we would not be able to use it to overcome
      the second issue
Signed-off-by: NPavel Hrdina <phrdina@redhat.com>
Reviewed-by: NJán Tomko <jtomko@redhat.com>

This function creates new BPF program with new empty BPF map with the
default size and attaches it to the guest cgroup.
Signed-off-by: NPavel Hrdina <phrdina@redhat.com>
Reviewed-by: NJán Tomko <jtomko@redhat.com>

This function will be called if libvirtd was restarted while some
domains were running.  It will try to detect existing programs attached
to the guest cgroup.
Signed-off-by: NPavel Hrdina <phrdina@redhat.com>
Reviewed-by: NJán Tomko <jtomko@redhat.com>

This function loads the BPF prog with prepared map into kernel and
attaches it into guest cgroup.  It can be also used to replace existing
program in the cgroup if we need to resize BPF map to store more rules
for devices. The old program will be closed and removed from kernel.

There are two possible ways how to create BPF program:

    - One way is to write simple C-like code which can by compiled into
      BPF object file which can be loaded into kernel using elfutils.

    - The second way is to define macros which look like assembler
      instructions and can be used directly to create BPF program that
      can be directly loaded into kernel.

Since the program is not too complex we can use the second option.

If there is no program, all devices are allowed, if there is some
program it is executed and based on the exit status the access is
denied for 0 and allowed for 1.

Our program will follow these rules:

    - first it will try to look for the specific key using major and
      minor to see if there is any rule for that specific device

    - if there is no specific rule it will try to look for any rule that
      matches only major of the device

    - if there is no match with major it will try the same but with
      minor of the device

    - as the last attempt it will try to look for rule for all devices
      and if there is no match it will return 0 to deny that access
Signed-off-by: NPavel Hrdina <phrdina@redhat.com>
Reviewed-by: NJán Tomko <jtomko@redhat.com>

There is no exact way how to figure out whether BPF devices support is
compiled into kernel.  One way is to check kernel configure options but
this is not reliable as it may not be available.  Let's try to do
syscall to which will list BPF cgroup device programs.
Signed-off-by: NPavel Hrdina <phrdina@redhat.com>
Reviewed-by: NJán Tomko <jtomko@redhat.com>