bpftool: Stop emitting static variables in BPF skeleton

As discussed in [0], stop emitting static variables in BPF skeletons to avoid
issues with name-conflicting static variables across multiple
statically-linked BPF object files.

Users using static variables to pass data between BPF programs and user-space
should do a trivial one-time switch according to the following simple rules:
  - read-only `static volatile const` variables should be converted to
    `volatile const`;
  - read/write `static volatile` variables should just drop `static volatile`
    modifiers to become global variables/symbols. To better handle older Clang
    versions, such newly converted global variables should be explicitly
    initialized with a specific value or `= 0`/`= {}`, whichever is
    appropriate.

  [0] https://lore.kernel.org/bpf/CAEf4BzZo7_r-hsNvJt3w3kyrmmBJj7ghGY8+k4nvKF0KLjma=w@mail.gmail.com/T/#m664d4b0d6b31ac8b2669360e0fc2d6962e9f5ec1

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20210507054119.270888-5-andrii@kernel.org

tools/bpf/bpftool/gen.c

@@ -131,6 +131,10 @@ static int codegen_datasec_def(struct bpf_object *obj,
 		int need_off = sec_var->offset, align_off, align;
 		__u32 var_type_id = var->type;
 
+		/* static variables are not exposed through BPF skeleton */
+		if (btf_var(var)->linkage == BTF_VAR_STATIC)
+			continue;
+
 		if (off > need_off) {
 			p_err("Something is wrong for %s's variable #%d: need offset %d, already at %d.\n",
 			      sec_name, i, need_off, off);