aarch64: Handle GCS marking

- Handle GCS marking
 - Use l_searchlist.r_list for gcs (allows using the
   same function for static exe)

Co-authored-by: Yury Khrustalev <yury.khrustalev@arm.com>
Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>

sysdeps/aarch64/dl-gcs.c

@@ -15,10 +15,101 @@
    License along with the GNU C Library; if not, see
    <https://www.gnu.org/licenses/>.  */
 
+#include <unistd.h>
 #include <ldsodefs.h>
 
-/* Used to report error when prctl system call to enabled GCS fails.  */
+/* GCS is disabled.  */
+#define GCS_POLICY_DISABLED 0
 
+/* Enable GCS, abort if unmarked binary is found.  */
+#define GCS_POLICY_ENFORCED 1
+
+/* Optionally enable GCS if all startup dependencies are marked.  */
+#define GCS_POLICY_OPTIONAL 2
+
+/* Override binary marking and always enabled GCS.  */
+#define GCS_POLICY_OVERRIDE 3
+
+static void
+fail (struct link_map *l, const char *program)
+{
+  if (program && program[0])
+    _dl_fatal_printf ("%s: %s: %s\n", program, l->l_name, "not GCS compatible");
+  else if (program)
+    _dl_fatal_printf ("%s\n", "not GCS compatible");
+  else
+    _dl_signal_error (0, l->l_name, "dlopen", "not GCS compatible");
+}
+
+static void
+unsupported (void)
+{
+  _dl_fatal_printf ("%s\n", "unsupported GCS policy");
+}
+
+/* This function is called only when binary markings are not
+   ignored and GCS is supposed to be enabled.  This occurs
+   for the GCS_POLICY_ENFORCED and GCS_POLICY_ENFORCED policies.  */
+static bool
+check_gcs (struct link_map *l, const char *program, bool enforced)
+{
+  /* Binary is marked, all good.  */
+  if (l->l_mach.gcs)
+    return true;
+  /* Binary is not marked and loaded via dlopen: abort.  */
+  if (program == NULL)
+    fail (l, program);
+  /* Binary is not marked and we enforce GCS: abort.  */
+  if (enforced)
+    fail (l, program);
+  /* Binary is not marked but GSC is optional: disable GCS.  */
+  else
+    {
+      GL(dl_aarch64_gcs) = 0;
+      return false;
+    }
+  __builtin_unreachable ();
+}
+
+/* Iterate over all dependencies and check GCS marking.
+   This function is called only when binary markings are not
+   ignored and GCS is supposed to be enabled.  This occurs
+   for the GCS_POLICY_ENFORCED and GCS_POLICY_ENFORCED policies.
+   We interrupt checking if GCS is optional and we already know
+   it is going to be disabled. */
+static void
+check_gcs_depends (struct link_map *l, const char *program, bool enforced)
+{
+  if (check_gcs (l, program, enforced))
+    for (unsigned int i = 0; i < l->l_searchlist.r_nlist; i++)
+      if (!check_gcs (l->l_searchlist.r_list[i], program, enforced))
+	break;
+}
+
+/* Apply GCS policy for L and its dependencies.
+   PROGRAM is NULL when this check is invoked for dl_open.  */
+void
+_dl_gcs_check (struct link_map *l, const char *program)
+{
+  unsigned long policy = GL (dl_aarch64_gcs);
+  switch (policy)
+    {
+    case GCS_POLICY_DISABLED:
+    case GCS_POLICY_OVERRIDE:
+      return;
+    case GCS_POLICY_ENFORCED:
+      check_gcs_depends (l, program, true);
+      return;
+    case GCS_POLICY_OPTIONAL:
+      check_gcs_depends (l, program, false);
+      return;
+    default:
+      /* All other policy values are not supported: abort.  */
+      unsupported ();
+    }
+}
+
+/* Used to report error when prctl system call to enabled GCS fails.  */
 void _dl_gcs_enable_failed (int code)
 {
   _dl_fatal_printf ("failed to enable GCS: %d\n", -code);

sysdeps/aarch64/dl-prop.h

@@ -24,16 +24,21 @@ extern void _dl_bti_protect (struct link_map *, int) attribute_hidden;
 extern void _dl_bti_check (struct link_map *, const char *)
     attribute_hidden;
 
+extern void _dl_gcs_check (struct link_map *, const char *)
+    attribute_hidden;
+
 static inline void __attribute__ ((always_inline))
 _rtld_main_check (struct link_map *m, const char *program)
 {
   _dl_bti_check (m, program);
+  _dl_gcs_check (m, program);
 }
 
 static inline void __attribute__ ((always_inline))
 _dl_open_check (struct link_map *m)
 {
   _dl_bti_check (m, NULL);
+  _dl_gcs_check (m, NULL);
 }
 
 static inline void __attribute__ ((always_inline))
@@ -45,10 +50,6 @@ static inline int
 _dl_process_gnu_property (struct link_map *l, int fd, uint32_t type,
 			  uint32_t datasz, void *data)
 {
-  if (!GLRO(dl_aarch64_cpu_features).bti)
-    /* Skip note processing.  */
-    return 0;
-
   if (type == GNU_PROPERTY_AARCH64_FEATURE_1_AND)
     {
       /* Stop if the property note is ill-formed.  */
@@ -57,7 +58,11 @@ _dl_process_gnu_property (struct link_map *l, int fd, uint32_t type,
 
       unsigned int feature_1 = *(unsigned int *) data;
       if (feature_1 & GNU_PROPERTY_AARCH64_FEATURE_1_BTI)
-	_dl_bti_protect (l, fd);
+	if (GLRO(dl_aarch64_cpu_features).bti)
+	  _dl_bti_protect (l, fd);
+
+      if (feature_1 & GNU_PROPERTY_AARCH64_FEATURE_1_GCS)
+	l->l_mach.gcs = 1;
 
       /* Stop if we processed the property note.  */
       return 0;

sysdeps/aarch64/linkmap.h

@@ -23,4 +23,5 @@ struct link_map_machine
   ElfW(Addr) plt;	  /* Address of .plt */
   void *tlsdesc_table;	  /* Address of TLS descriptor hash table.  */
   bool bti_fail;	  /* Failed to enable Branch Target Identification.  */
+  bool gcs;		  /* Guarded Control Stack marking.  */
 };