post-process: display visual warning if reachability may be off

Display visual warning in the event that covered funtions (at runtime,
from coverage analysis) is larger than the functions deemed reachable by
the static analysis component. We could insert more warnings other
places, i.e. start to enforce a "certainty of analysis" for each
analysis.

Ref: google/oss-fuzz#7593

post-processing/fuzz_html.py

@@ -826,6 +826,21 @@ def create_fuzzer_detailed_section(
                    "limitation our of our static analysis capabilities."
     html_string += "</div>"
 
+    if total_hit_functions > reachable_funcs:
+        html_string += (
+            "<div class=\"high-level-conclusions-wrapper\">"
+            "<span class=\"high-level-conclusion red-conclusion\">"
+            "<b>Warning:</b> The amount of covered functions are larger than the "
+            "amount of reachable functions. This means the functions covered at runtime "
+            "is larger than those extract using static analysis. As such, the static "
+            "analysis component is either failing to extract the right callgraph or "
+            "the coverage runtime is compiled with sanitizerse in code that the static "
+            "analysis has not analysed. This can happen if lto/gold is not used in "
+            "all places that coverage instrumentation is used."
+            "</span>"
+            "</div>"
+        )
+
     html_string += func_hit_table_string
 
     # Table showing which files this fuzzer hits.