selftests/bpf: xdp_hw_metadata cleanup cause segfault

Using xdp_hw_metadata I experince Segmentation fault after seeing
"detaching bpf program....".

On my system the segfault happened when accessing bpf_obj->skeleton
in xdp_hw_metadata__destroy(bpf_obj) call. That doesn't make any sense
as this memory have not been freed by program at this point in time.

Prior to calling xdp_hw_metadata__destroy(bpf_obj) the function
close_xsk() is called for each RX-queue xsk.  The real bug lays
in close_xsk() that unmap via munmap() the wrong memory pointer.
The call xsk_umem__delete(xsk->umem) will free xsk->umem, thus
the call to munmap(xsk->umem, UMEM_SIZE) will have unpredictable
behavior. And man page explain subsequent references to these
pages will generate SIGSEGV.

Unmapping xsk->umem_area instead removes the segfault.

Fixes: 297a3f12 ("selftests/bpf: Simple program to dump XDP RX metadata")
Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Stanislav Fomichev <sdf@google.com>
Link: https://lore.kernel.org/bpf/167527271533.937063.5717065138099679142.stgit@firesoul

tools/testing/selftests/bpf/xdp_hw_metadata.c

@@ -121,7 +121,7 @@ static void close_xsk(struct xsk *xsk)
 		xsk_umem__delete(xsk->umem);
 	if (xsk->socket)
 		xsk_socket__delete(xsk->socket);
-	munmap(xsk->umem, UMEM_SIZE);
+	munmap(xsk->umem_area, UMEM_SIZE);
 }
 
 static void refill_rx(struct xsk *xsk, __u64 addr)