diff --git a/changes/bug40020 b/changes/bug40020
new file mode 100644
index 0000000000..ca6ee2b85b
--- /dev/null
+++ b/changes/bug40020
@@ -0,0 +1,9 @@
+  o Minor bugfixes (linux seccomp2 sandbox):
+    - Makes the seccomp sandbox allow the correct syscall for opendir
+      according to the running glibc version. The opendir function
+      either uses open or openat but the current code does not
+      differenciate between opendir and open calls. This adds a new
+      seccomp sandbox rule for opendir. This fixes crashes when 
+      reloading torrc with sandbox enabled when running on glibc
+      2.15 to 2.21 and 2.26. Patch from Daniel Pinto. Fixes bug 40020; 
+      bugfix on 0.3.5.11.
diff --git a/src/app/main/main.c b/src/app/main/main.c
index e04663ec1d..e09b003b42 100644
--- a/src/app/main/main.c
+++ b/src/app/main/main.c
@@ -830,6 +830,9 @@ sandbox_init_filter(void)
 #define OPEN(name)                              \
   sandbox_cfg_allow_open_filename(&cfg, tor_strdup(name))
 
+#define OPENDIR(dir)                            \
+  sandbox_cfg_allow_opendir_dirname(&cfg, tor_strdup(dir))
+
 #define OPEN_DATADIR(name)                      \
   sandbox_cfg_allow_open_filename(&cfg, get_datadir_fname(name))
 
@@ -847,7 +850,7 @@ sandbox_init_filter(void)
   } while (0)
 
 #define OPEN_KEY_DIRECTORY() \
-  sandbox_cfg_allow_open_filename(&cfg, tor_strdup(options->KeyDirectory))
+  OPENDIR(options->KeyDirectory)
 #define OPEN_CACHEDIR(name)                      \
   sandbox_cfg_allow_open_filename(&cfg, get_cachedir_fname(name))
 #define OPEN_CACHEDIR_SUFFIX(name, suffix) do {  \
@@ -861,7 +864,7 @@ sandbox_init_filter(void)
     OPEN_KEYDIR(name suffix);                    \
   } while (0)
 
-  OPEN(options->DataDirectory);
+  OPENDIR(options->DataDirectory);
   OPEN_KEY_DIRECTORY();
 
   OPEN_CACHEDIR_SUFFIX("cached-certs", ".tmp");
@@ -908,7 +911,11 @@ sandbox_init_filter(void)
   }
 
   SMARTLIST_FOREACH(options->FilesOpenedByIncludes, char *, f, {
-    OPEN(f);
+    if (file_status(f) == FN_DIR) {
+      OPENDIR(f);
+    } else {
+      OPEN(f);
+    }
   });
 
 #define RENAME_SUFFIX(name, suffix)        \
@@ -1021,7 +1028,7 @@ sandbox_init_filter(void)
      * directory that holds it. */
     char *dirname = tor_strdup(port->unix_addr);
     if (get_parent_directory(dirname) == 0) {
-      OPEN(dirname);
+      OPENDIR(dirname);
     }
     tor_free(dirname);
     sandbox_cfg_allow_chmod_filename(&cfg, tor_strdup(port->unix_addr));
diff --git a/src/lib/sandbox/sandbox.c b/src/lib/sandbox/sandbox.c
index 2a8c288c99..a6eea9daca 100644
--- a/src/lib/sandbox/sandbox.c
+++ b/src/lib/sandbox/sandbox.c
@@ -289,6 +289,12 @@ static int filter_nopar_gen[] = {
     SCMP_SYS(poll)
 };
 
+/* opendir is not a syscall but it will use either open or openat. We do not
+ * want the decision to allow open/openat to be the callers reponsability, so
+ * we create a phony syscall number for opendir and sb_opendir will choose the
+ * correct syscall. */
+#define PHONY_OPENDIR_SYSCALL -2
+
 /* These macros help avoid the error where the number of filters we add on a
  * single rule don't match the arg_cnt param. */
 #define seccomp_rule_add_0(ctx,act,call) \
@@ -469,14 +475,24 @@ is_libc_at_least(int major, int minor)
 #endif /* defined(CHECK_LIBC_VERSION) */
 }
 
-/* Return true if we think we're running with a libc that always uses
- * openat on linux. */
+/* Return true if we think we're running with a libc that uses openat for the
+ * open function on linux. */
 static int
-libc_uses_openat_for_everything(void)
+libc_uses_openat_for_open(void)
 {
   return is_libc_at_least(2, 26);
 }
 
+/* Return true if we think we're running with a libc that uses openat for the
+ * opendir function on linux. */
+static int
+libc_uses_openat_for_opendir(void)
+{
+  // libc 2.27 and above or between 2.15 (inclusive) and 2.22 (exclusive)
+  return is_libc_at_least(2, 27) ||
+         (is_libc_at_least(2, 15) && !is_libc_at_least(2, 22));
+}
+
 /* Return true if we think we're running with a libc that needs to cast
  * negative arguments like AT_FDCWD for seccomp rules. */
 static int
@@ -510,7 +526,7 @@ sb_open(scmp_filter_ctx ctx, sandbox_cfg_t *filter)
   int rc;
   sandbox_cfg_t *elem = NULL;
 
-  int use_openat = libc_uses_openat_for_everything();
+  int use_openat = libc_uses_openat_for_open();
 
   // for each dynamic parameter filters
   for (elem = filter; elem != NULL; elem = elem->next) {
@@ -643,6 +659,38 @@ sb_openat(scmp_filter_ctx ctx, sandbox_cfg_t *filter)
   return 0;
 }
 
+static int
+sb_opendir(scmp_filter_ctx ctx, sandbox_cfg_t *filter)
+{
+  int rc;
+  sandbox_cfg_t *elem = NULL;
+
+  // for each dynamic parameter filters
+  for (elem = filter; elem != NULL; elem = elem->next) {
+    smp_param_t *param = elem->param;
+
+    if (param != NULL && param->prot == 1 && param->syscall
+        == PHONY_OPENDIR_SYSCALL) {
+      if (libc_uses_openat_for_opendir()) {
+        rc = seccomp_rule_add_3(ctx, SCMP_ACT_ALLOW, SCMP_SYS(openat),
+            SCMP_CMP_NEG(0, SCMP_CMP_EQ, AT_FDCWD),
+            SCMP_CMP_STR(1, SCMP_CMP_EQ, param->value),
+            SCMP_CMP(2, SCMP_CMP_EQ, O_RDONLY|O_NONBLOCK|O_LARGEFILE|
+                O_DIRECTORY|O_CLOEXEC));
+      } else {
+        rc = allow_file_open(ctx, 0, param->value);
+      }
+      if (rc != 0) {
+        log_err(LD_BUG,"(Sandbox) failed to add openat syscall, received "
+            "libseccomp error %d", rc);
+        return rc;
+      }
+    }
+  }
+
+  return 0;
+}
+
 /**
  * Function responsible for setting up the socket syscall for
  * the seccomp filter sandbox.
@@ -1157,6 +1205,7 @@ static sandbox_filter_func_t filter_func[] = {
     sb_chmod,
     sb_open,
     sb_openat,
+    sb_opendir,
     sb_rename,
 #ifdef __NR_fcntl64
     sb_fcntl64,
@@ -1476,6 +1525,19 @@ sandbox_cfg_allow_openat_filename(sandbox_cfg_t **cfg, char *file)
   return 0;
 }
 
+int
+sandbox_cfg_allow_opendir_dirname(sandbox_cfg_t **cfg, char *dir)
+{
+  sandbox_cfg_t *elem = NULL;
+
+  elem = new_element(PHONY_OPENDIR_SYSCALL, dir);
+
+  elem->next = *cfg;
+  *cfg = elem;
+
+  return 0;
+}
+
 /**
  * Function responsible for going through the parameter syscall filters and
  * call each function pointer in the list.
@@ -1803,6 +1865,13 @@ sandbox_cfg_allow_openat_filename(sandbox_cfg_t **cfg, char *file)
   return 0;
 }
 
+int
+sandbox_cfg_allow_opendir_dirname(sandbox_cfg_t **cfg, char *dir)
+{
+  (void)cfg; (void)dir;
+  return 0;
+}
+
 int
 sandbox_cfg_allow_stat_filename(sandbox_cfg_t **cfg, char *file)
 {
diff --git a/src/lib/sandbox/sandbox.h b/src/lib/sandbox/sandbox.h
index b50df48255..a2b3227b90 100644
--- a/src/lib/sandbox/sandbox.h
+++ b/src/lib/sandbox/sandbox.h
@@ -135,6 +135,13 @@ int sandbox_cfg_allow_rename(sandbox_cfg_t **cfg, char *file1, char *file2);
  */
 int sandbox_cfg_allow_openat_filename(sandbox_cfg_t **cfg, char *file);
 
+/**
+ * Function used to add a opendir allowed filename to a supplied configuration.
+ * The (char*) specifies the path to the allowed dir; we steal the pointer to
+ * that dir.
+ */
+int sandbox_cfg_allow_opendir_dirname(sandbox_cfg_t **cfg, char *dir);
+
 /**
  * Function used to add a stat/stat64 allowed filename to a configuration.
  * The (char*) specifies the path to the allowed file; that pointer is stolen.