Merge pull request #21 from pythonbpf/globals

Adds support for globals
SO......
*I'm not merging this because it's complete, but because I don't want it to diverge from master too much.
*Stuff I still need to complete:
-> Structs and eval expressions in these globals.
-> handling the global keyword.
-> assigning back to the global and reading from inside a function.
-> Basically, `global` keyword in Python is used to write only and reading can be done directly without declaring as global as a direct assign without global declaration is going to diverge from Python.
-> The above logic is going to be supported by `global_sym_tab` generated using the new order of passes that we are doing.
-> This needs to be fixed and done ASAP to avoid conflicts. so yes, im  gonna do it soon.

tests/c-form/globals.bpf.c

@@ -0,0 +1,27 @@
+// SPDX-License-Identifier: GPL-2.0 OR BSD-3-Clause
+#include <linux/bpf.h>
+#include <bpf/bpf_helpers.h>
+#include <bpf/bpf_tracing.h>
+#include <linux/types.h>
+
+struct test_struct {
+    __u64 a;
+    __u64 b;
+};
+
+struct test_struct w = {};
+volatile __u64 prev_time = 0;
+
+SEC("tracepoint/syscalls/sys_enter_execve")
+int trace_execve(void *ctx)
+{
+    bpf_printk("previous %ul now %ul", w.b, w.a);
+    __u64 ts = bpf_ktime_get_ns();
+    bpf_printk("prev %ul now %ul", prev_time, ts);
+    w.a = ts;
+    w.b = prev_time;
+    prev_time = ts;
+    return 0;
+}
+
+char LICENSE[] SEC("license") = "GPL";