v8: only remove prefix when mapping is removed

prior to this change we were tracking generation number (i.e.
count of SynchronizeMappings call where the object was found) for
both mappings and prefixes.

if the mapping did not change, we would not re-calculate prefixes
for the mapping, hence we would not increase the generation of the
prefix object.

as such, we would end up cleaning the prefix on the second call of
SynchronizeMappings (if mappings did not change), removing the prefix
from the ebpf map, and making unwinding fail for PC in the JIT area.

to fix this, we only track the generation for mappings. when mappings
are removed, we computed all the prefixes associated to those mappings
and we remove them. this should be safe because different mappings
should have different prefixes.

interpreter/nodev8/v8.go

@@ -494,8 +494,8 @@ type v8Instance struct {
 
 	// mappings is indexed by the Mapping to its generation
 	mappings map[process.Mapping]uint32
-	// prefixes is indexed by the prefix added to ebpf maps (to be cleaned up) to its generation
-	prefixes map[lpm.Prefix]uint32
+	// prefixes is indexed by the prefix added to ebpf maps (to be cleaned up)
+	prefixes map[lpm.Prefix]struct{}
 	// mappingGeneration is the current generation (so old entries can be pruned)
 	mappingGeneration uint32
 }
@@ -573,30 +573,29 @@ func (i *v8Instance) SynchronizeMappings(ebpf interpreter.EbpfHandler,
 
 		for _, prefix := range prefixes {
 			if _, exists := i.prefixes[prefix]; exists {
-				i.prefixes[prefix] = i.mappingGeneration
 				continue
 			}
 			err := ebpf.UpdatePidInterpreterMapping(pid, prefix, support.ProgUnwindV8, 0, 0)
 			if err != nil {
 				return err
 			}
-			i.prefixes[prefix] = i.mappingGeneration
+			i.prefixes[prefix] = struct{}{}
 		}
 	}
 
-	// Remove prefixes not seen
-	for prefix, generation := range i.prefixes {
-		if generation == i.mappingGeneration {
-			continue
-		}
-		_ = ebpf.DeletePidInterpreterMapping(pid, prefix)
-		delete(i.prefixes, prefix)
-	}
 	for m, generation := range i.mappings {
 		if generation == i.mappingGeneration {
 			continue
 		}
 		log.Debugf("Disabling V8 for %#x/%#x", m.Vaddr, m.Length)
+		prefixes, err := lpm.CalculatePrefixList(m.Vaddr, m.Vaddr+m.Length)
+		if err != nil {
+			return fmt.Errorf("new anonymous mapping lpm failure %#x/%#x", m.Vaddr, m.Length)
+		}
+		for _, prefix := range prefixes {
+			_ = ebpf.DeletePidInterpreterMapping(pid, prefix)
+			delete(i.prefixes, prefix)
+		}
 		delete(i.mappings, m)
 	}
 
@@ -1820,7 +1819,7 @@ func (d *v8Data) Attach(ebpf interpreter.EbpfHandler, pid util.PID, _ libpf.Addr
 		d:            d,
 		rm:           rm,
 		mappings:     make(map[process.Mapping]uint32),
-		prefixes:     make(map[lpm.Prefix]uint32),
+		prefixes:     make(map[lpm.Prefix]struct{}),
 		addrToString: addrToString,
 		addrToCode:   addrToCode,
 		addrToSFI:    addrToSFI,