{f,m}_bpf: allow updates on program arrays

{f,m}_bpf: allow updates on program arrays

Since we have all infrastructure in place now, allow atomic live updates
on program arrays. This can be very useful e.g. in case programs that are
being tail-called need to be replaced, f.e. when classifier functionality
needs to be changed, new protocols added/removed during runtime, etc.

Thus, provide a way for in-place code updates, minimal example: Given is
an object file cls.o that contains the entry point in section 'classifier',
has a globally pinned program array 'jmp' with 2 slots and id of 0, and
two tail called programs under section '0/0' (prog array key 0) and '0/1'
(prog array key 1), the section encoding for the loader is <id/key>.
Adding the filter loads everything into cls_bpf:

  tc filter add dev foo parent ffff: bpf da obj cls.o

Now, the program under section '0/1' needs to be replaced with an updated
version that resides in the same section (also full path to tc's subfolder
of the mount point can be passed, e.g. /sys/fs/bpf/tc/globals/jmp):

  tc exec bpf graft m:globals/jmp obj cls.o sec 0/1

In case the program resides under a different section 'foo', it can also
be injected into the program array like:

  tc exec bpf graft m:globals/jmp key 1 obj cls.o sec foo

If the new tail called classifier program is already available as a pinned
object somewhere (here: /sys/fs/bpf/tc/progs/parser), it can be injected
into the prog array like:

  tc exec bpf graft m:globals/jmp key 1 fd m:progs/parser

In the kernel, the program on key 1 is being atomically replaced and the
old one's refcount dropped.

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Alexei Starovoitov <ast@kernel.org>