Fix negative stability (#2731)

* fix

* FMT

fuzzers/baby/baby_fuzzer/src/main.rs

@@ -9,7 +9,7 @@ use libafl::monitors::SimpleMonitor;
 use libafl::{
     corpus::{InMemoryCorpus, OnDiskCorpus},
     events::SimpleEventManager,
-    executors::{InProcessExecutor, ExitKind},
+    executors::{ExitKind, InProcessExecutor},
     feedbacks::{CrashFeedback, MaxMapFeedback},
     fuzzer::{Fuzzer, StdFuzzer},
     generators::RandPrintablesGenerator,

libafl/src/stages/calibrate.rs

@@ -329,15 +329,19 @@ where
                     map_first_filled_count, 0,
                     "The map's filled count must never be 0"
                 );
+                // In theory `map_first_filled_count - unstable_entries` could be negative.
+                // Because `map_first_filled_count` is the filled count of just one single run.
+                // While the `unstable_entries` is the number of all the unstable entries across multiple runs.
+                // If the target is very unstable (~100%) then this would hit more edges than `map_first_filled_count`.
+                // But even in that case, we don't allow negative stability and just show 0% here.
+                let stable_count: u64 =
+                    map_first_filled_count.saturating_sub(unstable_entries) as u64;
                 mgr.fire(
                     state,
                     Event::UpdateUserStats {
                         name: Cow::from("stability"),
                         value: UserStats::new(
-                            UserStatsValue::Ratio(
-                                (map_first_filled_count - unstable_entries) as u64,
-                                map_first_filled_count as u64,
-                            ),
+                            UserStatsValue::Ratio(stable_count, map_first_filled_count as u64),
                             AggregatorOps::Avg,
                         ),
                         phantom: PhantomData,