fix(perf): purge inherited mappings on execve to fix unknown symbols (#392)

A child forked from a parent (e.g. bash) inherits the parent's memory
mappings. When it then execve's its own binary, the inherited PIE
mappings remain and can collide with the new binary's mappings at the
same base address, leading to wrong symbolication and unknown symbols.

Handle COMM records flagged as execve and purge the exec'ing process's
inherited mappings so only its post-exec mappings are used.

Refs COD-2779

Author: GuillaumeLagrange

src/executor/wall_time/profiler/perf/parse_perf_file.rs

@@ -39,6 +39,7 @@ pub fn parse_for_memmap2<P: AsRef<Path>>(
         mut record_iter,
     } = PerfFileReader::parse_pipe(reader)?;
 
+    // This loop relies on the events being in chronological order, which is guaranteed by the perf file format.
     while let Some(record) = record_iter.next_record(&mut perf_file)? {
         let PerfFileRecord::EventRecord { record, .. } = record else {
             continue;
@@ -75,6 +76,30 @@ pub fn parse_for_memmap2<P: AsRef<Path>>(
                     fork_record.pid,
                 );
             }
+            RecordType::COMM => {
+                // An execve() replaces the entire address space: the mappings inherited from the
+                // parent on fork (and any pre-exec mappings of this pid) are no longer valid.
+                // TODO(COD-1377):  Manage timing in module mapping to improve symbolication.
+                let Ok(parsed_record) = record.parse() else {
+                    continue;
+                };
+
+                let EventRecord::Comm(comm_record) = parsed_record else {
+                    continue;
+                };
+
+                if !comm_record.is_execve {
+                    continue;
+                }
+
+                if pid_filter.should_include(comm_record.pid) {
+                    trace!(
+                        "Exec: Purging inherited mappings for PID {}",
+                        comm_record.pid
+                    );
+                    purge_process_mappings(&mut loaded_modules_by_path, comm_record.pid);
+                }
+            }
             RecordType::MMAP2 => {
                 let Ok(parsed_record) = record.parse() else {
                     continue;
@@ -176,6 +201,13 @@ fn inherit_parent_mappings(
     }
 }
 
+/// Drop every mapping recorded for `pid` across all modules.
+fn purge_process_mappings(loaded_modules_by_path: &mut HashMap<PathBuf, LoadedModule>, pid: pid_t) {
+    for loaded_module in loaded_modules_by_path.values_mut() {
+        loaded_module.process_loaded_modules.remove(&pid);
+    }
+}
+
 /// Process a single MMAP2 record and add it to the symbols and unwind data maps
 fn process_mmap2_record(
     record: linux_perf_data::linux_perf_event_reader::Mmap2Record,
@@ -319,4 +351,51 @@ mod tests {
             .unwrap();
         assert_eq!(child.symbols_load_bias, Some(0xcafe));
     }
+
+    #[test]
+    fn purge_process_mappings_removes_only_the_target_pid() {
+        // Reproduces the fork+exec collision: child 200 forked from bash (100) and inherited
+        // bash's PIE mapping at 0xaaaaaaaa0000, then exec'd its own binary at the same base.
+        let mut modules: HashMap<PathBuf, LoadedModule> = HashMap::new();
+
+        // Inherited-from-bash module that must be dropped on exec.
+        let mut bash = make_module_with_parent(100, 0xaaaaaaaa0000);
+        bash.process_loaded_modules.insert(
+            200,
+            ProcessLoadedModule {
+                symbols_load_bias: Some(0xaaaaaaaa0000),
+                process_unwind_data: None,
+            },
+        );
+        modules.insert(PathBuf::from("/usr/bin/bash"), bash);
+
+        // The real post-exec binary, also based at 0xaaaaaaaa0000 for pid 200.
+        modules.insert(
+            PathBuf::from("/cpp/build/fractal_main"),
+            make_module_with_parent(200, 0xaaaaaaaa0000),
+        );
+
+        purge_process_mappings(&mut modules, 200);
+
+        // Pid 200 is gone from every module...
+        assert!(
+            !modules[&PathBuf::from("/usr/bin/bash")]
+                .process_loaded_modules
+                .contains_key(&200)
+        );
+        assert!(
+            !modules[&PathBuf::from("/cpp/build/fractal_main")]
+                .process_loaded_modules
+                .contains_key(&200)
+        );
+        // ...but bash's own pid 100 mapping is untouched.
+        assert_eq!(
+            modules[&PathBuf::from("/usr/bin/bash")]
+                .process_loaded_modules
+                .get(&100)
+                .unwrap()
+                .symbols_load_bias,
+            Some(0xaaaaaaaa0000)
+        );
+    }
 }