dynamic: Streamline trampoline life cycle

Install a trampoline for each loaded module map, on initialization. Keep
the trampolines in memory to allow for dynamic patching at runtime.
Clear the trampolines on libmcount exit.

Co-authored-by: Gabriel-Andrew Pollo-Guilbert <gabrielpolloguilbert@gmail.com>
Signed-off-by: Clément Guidi <cguidi@ciena.com>

libmcount/dynamic.c

@@ -294,8 +294,15 @@ static struct mcount_dynamic_info *create_mdi(struct dl_phdr_info *info)
  return mdi;
 }
 
-/* callback for dl_iterate_phdr() */
-static int find_dynamic_module(struct dl_phdr_info *info, size_t sz, void *data)
+/**
+ * prepare_dynamic_module - store dynamic module info and install trampoline;
+ * callback for dl_iterate_phdr()
+ * @info - module info
+ * @sz - data size (unused)
+ * @data - callback data: symbol info and module parsing flag
+ * @return - stop iteration on non-zero value
+ */
+static int prepare_dynamic_module(struct dl_phdr_info *info, size_t sz, void *data)
 {
  struct mcount_dynamic_info *mdi;
  struct find_module_data *fmd = data;
@@ -315,6 +322,9 @@ static int find_dynamic_module(struct dl_phdr_info *info, size_t sz, void *data)
  mdi->map = map;
  mcount_arch_find_module(mdi, &map->mod->symtab);
 
+ if (mcount_setup_trampoline(mdi) < 0)
+ mdi->trampoline = 0;
+
  mdi->next = mdinfo;
  mdinfo = mdi;
  pr_dbg3("load dynamic info for '%s'\n", mdi->map->libname);
@@ -352,7 +362,7 @@ static void prepare_dynamic_update(struct uftrace_sym_info *sinfo, bool needs_mo
  else
  return;
 
- dl_iterate_phdr(find_dynamic_module, &fmd);
+ dl_iterate_phdr(prepare_dynamic_module, &fmd);
 }
 
 /**
@@ -629,10 +639,18 @@ static void patch_func_matched(struct mcount_dynamic_info *mdi, struct uftrace_m
  patch_normal_func_matched(mdi, map);
 }
 
+/**
+ * do_dynamic_update - apply (un)patching across loaded modules' maps
+ * @sinfo - dynamic symbol info
+ * @patch_funcs - spec of symbols to (un)patch
+ * @ptype - matching pattern type
+ * @return - 0 (unused)
+ */
 static int do_dynamic_update(struct uftrace_sym_info *sinfo, char *patch_funcs,
  enum uftrace_pattern_type ptype)
 {
  struct uftrace_mmap *map;
+ struct mcount_dynamic_info *mdi;
  char *def_mod;
 
  if (patch_funcs == NULL)
@@ -641,15 +659,11 @@ static int do_dynamic_update(struct uftrace_sym_info *sinfo, char *patch_funcs,
  def_mod = basename(sinfo->exec_map->libname);
  parse_pattern_list(patch_funcs, def_mod, ptype);
 
- for_each_map(sinfo, map) {
- struct mcount_dynamic_info *mdi;
-
- /* TODO: filter out unsuppported libs */
- mdi = setup_trampoline(map);
- if (mdi == NULL)
- continue;
-
- patch_func_matched(mdi, map);
+ /* TODO: filter out unsupported libs */
+ for (mdi = mdinfo; mdi != NULL; mdi = mdi->next) {
+ map = mdi->map;
+ if (mdi->trampoline)
+ patch_func_matched(mdi, map);
  }
 
  release_pattern_list();
@@ -662,24 +676,6 @@ static int do_dynamic_update(struct uftrace_sym_info *sinfo, char *patch_funcs,
  return 0;
 }
 
-static void freeze_dynamic_update(void)
-{
- struct mcount_dynamic_info *mdi, *tmp;
-
- mdi = mdinfo;
- while (mdi) {
- tmp = mdi->next;
-
- mcount_arch_dynamic_recover(mdi, &disasm);
- mcount_cleanup_trampoline(mdi);
- free(mdi);
-
- mdi = tmp;
- }
-
- mcount_freeze_code();
-}
-
 /* do not use floating-point in libmcount */
 static int calc_percent(int n, int total, int *rem)
 {
@@ -724,7 +720,7 @@ int mcount_dynamic_update(struct uftrace_sym_info *sinfo, char *patch_funcs,
  pr_dbg("no match: %8d\n", stats.nomatch);
  }
 
- freeze_dynamic_update();
+ mcount_freeze_code();
  return ret;
 }
 
@@ -770,9 +766,29 @@ void mcount_dynamic_dlopen(struct uftrace_sym_info *sinfo, struct dl_phdr_info *
  mcount_freeze_code();
 }
 
+/**
+ * mcount_free_mdinfo - free all dynamic info structures
+ */
+static void mcount_free_mdinfo(void)
+{
+ struct mcount_dynamic_info *mdi, *tmp;
+
+ mdi = mdinfo;
+ while (mdi) {
+ tmp = mdi->next;
+
+ mcount_arch_dynamic_recover(mdi, &disasm);
+ mcount_cleanup_trampoline(mdi);
+ free(mdi);
+
+ mdi = tmp;
+ }
+}
+
 void mcount_dynamic_finish(void)
 {
  release_pattern_list();
+ mcount_free_mdinfo();
  mcount_disasm_finish(&disasm);
 }