gdb, gdbarch: Introduce gdbarch method to get the shadow stack pointer.

This patch is required by the following commit.

Reviewed-by: Thiago Jung Bauermann <thiago.bauermann@linaro.org>

gdb/arch-utils.c

@@ -1218,6 +1218,16 @@ default_gdbarch_return_value
 				readbuf, writebuf);
 }
 
+/* See arch-utils.h.  */
+
+std::optional<CORE_ADDR>
+default_get_shadow_stack_pointer (gdbarch *gdbarch, regcache *regcache,
+				  bool &shadow_stack_enabled)
+{
+  shadow_stack_enabled = false;
+  return {};
+}
+
 obstack *gdbarch_obstack (gdbarch *arch)
 {
   return &arch->obstack;

gdb/arch-utils.h

@@ -414,4 +414,9 @@ extern enum return_value_convention default_gdbarch_return_value
       struct regcache *regcache, struct value **read_value,
       const gdb_byte *writebuf);
 
+/* Default implementation of gdbarch default_get_shadow_stack_pointer
+   method.  */
+extern std::optional<CORE_ADDR> default_get_shadow_stack_pointer
+  (gdbarch *gdbarch, regcache *regcache, bool &shadow_stack_enabled);
+
 #endif /* GDB_ARCH_UTILS_H */

gdb/gdbarch-gen.c

@@ -263,6 +263,7 @@ struct gdbarch
   gdbarch_use_target_description_from_corefile_notes_ftype *use_target_description_from_corefile_notes = default_use_target_description_from_corefile_notes;
   gdbarch_core_parse_exec_context_ftype *core_parse_exec_context = default_core_parse_exec_context;
   gdbarch_shadow_stack_push_ftype *shadow_stack_push = nullptr;
+  gdbarch_get_shadow_stack_pointer_ftype *get_shadow_stack_pointer = default_get_shadow_stack_pointer;
 };
 
 /* Create a new ``struct gdbarch'' based on information provided by
@@ -537,6 +538,7 @@ verify_gdbarch (struct gdbarch *gdbarch)
   /* Skip verify of use_target_description_from_corefile_notes, invalid_p == 0.  */
   /* Skip verify of core_parse_exec_context, invalid_p == 0.  */
   /* Skip verify of shadow_stack_push, has predicate.  */
+  /* Skip verify of get_shadow_stack_pointer, invalid_p == 0.  */
   if (!log.empty ())
     internal_error (_("verify_gdbarch: the following are invalid ...%s"),
 		    log.c_str ());
@@ -1414,6 +1416,9 @@ gdbarch_dump (struct gdbarch *gdbarch, struct ui_file *file)
   gdb_printf (file,
 	      "gdbarch_dump: shadow_stack_push = <%s>\n",
 	      host_address_to_string (gdbarch->shadow_stack_push));
+  gdb_printf (file,
+	      "gdbarch_dump: get_shadow_stack_pointer = <%s>\n",
+	      host_address_to_string (gdbarch->get_shadow_stack_pointer));
   if (gdbarch->dump_tdep != NULL)
     gdbarch->dump_tdep (gdbarch, file);
 }
@@ -5583,3 +5588,20 @@ set_gdbarch_shadow_stack_push (struct gdbarch *gdbarch,
 {
   gdbarch->shadow_stack_push = shadow_stack_push;
 }
+
+std::optional<CORE_ADDR>
+gdbarch_get_shadow_stack_pointer (struct gdbarch *gdbarch, regcache *regcache, bool &shadow_stack_enabled)
+{
+  gdb_assert (gdbarch != NULL);
+  gdb_assert (gdbarch->get_shadow_stack_pointer != NULL);
+  if (gdbarch_debug >= 2)
+    gdb_printf (gdb_stdlog, "gdbarch_get_shadow_stack_pointer called\n");
+  return gdbarch->get_shadow_stack_pointer (gdbarch, regcache, shadow_stack_enabled);
+}
+
+void
+set_gdbarch_get_shadow_stack_pointer (struct gdbarch *gdbarch,
+				      gdbarch_get_shadow_stack_pointer_ftype get_shadow_stack_pointer)
+{
+  gdbarch->get_shadow_stack_pointer = get_shadow_stack_pointer;
+}

gdb/gdbarch-gen.h

@@ -1806,6 +1806,8 @@ extern void set_gdbarch_core_parse_exec_context (struct gdbarch *gdbarch, gdbarc
    technologies.  For example, the Intel Control-Flow Enforcement Technology
    (Intel CET) on x86 provides a shadow stack and indirect branch tracking.
    To enable inferior calls the function shadow_stack_push has to be provided.
+   The method get_shadow_stack_pointer has to be provided to enable displaced
+   stepping.
 
    Push the address NEW_ADDR on the shadow stack and update the shadow stack
    pointer. */
@@ -1815,3 +1817,11 @@ extern bool gdbarch_shadow_stack_push_p (struct gdbarch *gdbarch);
 typedef void (gdbarch_shadow_stack_push_ftype) (struct gdbarch *gdbarch, CORE_ADDR new_addr, regcache *regcache);
 extern void gdbarch_shadow_stack_push (struct gdbarch *gdbarch, CORE_ADDR new_addr, regcache *regcache);
 extern void set_gdbarch_shadow_stack_push (struct gdbarch *gdbarch, gdbarch_shadow_stack_push_ftype *shadow_stack_push);
+
+/* If possible, return the shadow stack pointer.  On some architectures, the
+   shadow stack pointer is available even if the feature is disabled.  To
+   return the shadow stack enablement state configure SHADOW_STACK_ENABLED. */
+
+typedef std::optional<CORE_ADDR> (gdbarch_get_shadow_stack_pointer_ftype) (struct gdbarch *gdbarch, regcache *regcache, bool &shadow_stack_enabled);
+extern std::optional<CORE_ADDR> gdbarch_get_shadow_stack_pointer (struct gdbarch *gdbarch, regcache *regcache, bool &shadow_stack_enabled);
+extern void set_gdbarch_get_shadow_stack_pointer (struct gdbarch *gdbarch, gdbarch_get_shadow_stack_pointer_ftype *get_shadow_stack_pointer);

gdb/gdbarch_components.py

@@ -2855,6 +2855,8 @@ Some targets support special hardware-assisted control-flow protection
 technologies.  For example, the Intel Control-Flow Enforcement Technology
 (Intel CET) on x86 provides a shadow stack and indirect branch tracking.
 To enable inferior calls the function shadow_stack_push has to be provided.
+The method get_shadow_stack_pointer has to be provided to enable displaced
+stepping.
 
 Push the address NEW_ADDR on the shadow stack and update the shadow stack
 pointer.
@@ -2864,3 +2866,16 @@ pointer.
     params=[("CORE_ADDR", "new_addr"), ("regcache *", "regcache")],
     predicate=True,
 )
+
+Method(
+    comment="""
+If possible, return the shadow stack pointer.  On some architectures, the
+shadow stack pointer is available even if the feature is disabled.  To
+return the shadow stack enablement state configure SHADOW_STACK_ENABLED.
+""",
+    type="std::optional<CORE_ADDR>",
+    name="get_shadow_stack_pointer",
+    params=[("regcache *", "regcache"), ("bool &", "shadow_stack_enabled")],
+    predefault="default_get_shadow_stack_pointer",
+    invalid=False,
+)