bpf: Don't run arg-tracking analysis twice on main subprog

Because subprog 0, the main subprog, is considered a global function,
we end up running the arg-tracking dataflow analysis twice on it. That
results in slightly longer verification but mostly in more verbose
verifier logs. This patch fixes it by keeping only the iteration over
global subprogs.

When running over all of Cilium's programs with BPF_LOG_LEVEL2, this
reduces verbosity by ~20% on average.

Fixes: bf0c571 ("bpf: introduce forward arg-tracking dataflow analysis")
Signed-off-by: Paul Chaignon <paul.chaignon@gmail.com>

Author: pchaigno

kernel/bpf/liveness.c

@@ -1914,15 +1914,6 @@ int bpf_compute_subprog_arg_access(struct bpf_verifier_env *env)
 		return -ENOMEM;
 	}
 
-	instance = call_instance(env, NULL, 0, 0);
-	if (IS_ERR(instance)) {
-		err = PTR_ERR(instance);
-		goto out;
-	}
-	err = analyze_subprog(env, NULL, info, instance, callsites);
-	if (err)
-		goto out;
-
 	/*
 	 * Subprogs and callbacks that don't receive FP-derived arguments
 	 * cannot access ancestor stack frames, so they were skipped during
@@ -1934,6 +1925,8 @@ int bpf_compute_subprog_arg_access(struct bpf_verifier_env *env)
 	 * each subprog is analyzed before its callees, allowing the
 	 * recursive walk inside analyze_subprog() to naturally
 	 * reach nested callees that also lack FP-derived args.
+	 *
+	 * Note the main subprog is also analyzed as part of this loop.
 	 */
 	for (k = env->subprog_cnt - 1; k >= 0; k--) {
 		int sub = env->subprog_topo_order[k];