[PATCH 7/9] gdb: Provide gdbarch hook to distinguish shadow stack backtrace elements.

[Christina Schimpe <christina.schimpe@intel.com>]

On x86 with CET there can be elements on the shadow stack which are not
return addresses.  In this case, we just want to print the element itself
in the shadow stack backtrace, but no further information.

Provide a gdbarch hook to distinguish between return and non-return
addresses and use it to print the shadow stack backtrace as described
above.
---
 gdb/doc/gdb.texinfo       | 19 ++++++++++++
 gdb/gdbarch-gen.c         | 32 ++++++++++++++++++++
 gdb/gdbarch-gen.h         | 15 +++++++++
 gdb/gdbarch.h             |  1 +
 gdb/gdbarch_components.py | 17 +++++++++++
 gdb/shadow-stack.c        | 64 +++++++++++++++++----------------------
 gdb/shadow-stack.h        | 37 ++++++++++++++++++++++
 7 files changed, 148 insertions(+), 37 deletions(-)

diff --git a/gdb/doc/gdb.texinfo b/gdb/doc/gdb.texinfo
index ebda4546b58..a0fde385a8e 100644
--- a/gdb/doc/gdb.texinfo
+++ b/gdb/doc/gdb.texinfo
@@ -8887,6 +8887,25 @@ This is how a shadow stack backtrace looks like on amd64:
 @end group
 @end smallexample
 
+There can be elements on the shadow stack which are not return addresses,
+for example on x86 with the Intel Control-Flow Enforcement Technology
+(@xref{CET}).  In case of signals, the old shadow stack pointer is pushed
+in a special format with bit 63 set.  For such shadow stack elements, the
+shadow stack frame just contains the level and the address on the shadow
+stack, as shown in the following example by frame 1:
+
+@smallexample
+@group
+(gdb) bt shadow 4
+#0  0x00007ffff7c54d90 in __restore_rt from /lib64/libc.so.6
+#1  0x80007ffff79fffd8
+#2  0x00007ffff7c54ce6 in __GI_raise at ../sysdeps/posix/raise.c:27
+#3  0x000000000040115d in main at /tmp/amd64-shadow-stack-signal.c:32
+(More shadow stack frames follow...)
+@end group
+@end smallexample
+
+
 @node Selection
 @section Selecting a Frame
 
diff --git a/gdb/gdbarch-gen.c b/gdb/gdbarch-gen.c
index 640576f4b85..21def8fd3e7 100644
--- a/gdb/gdbarch-gen.c
+++ b/gdb/gdbarch-gen.c
@@ -266,6 +266,7 @@ struct gdbarch
   gdbarch_top_addr_empty_shadow_stack_ftype *top_addr_empty_shadow_stack = nullptr;
   gdbarch_address_in_shadow_stack_memory_range_ftype *address_in_shadow_stack_memory_range = nullptr;
   int shadow_stack_element_size_aligned = 8;
+  gdbarch_is_no_return_shadow_stack_address_ftype *is_no_return_shadow_stack_address = nullptr;
 };
 
 /* Create a new ``struct gdbarch'' based on information provided by
@@ -543,6 +544,7 @@ verify_gdbarch (struct gdbarch *gdbarch)
   /* Skip verify of top_addr_empty_shadow_stack, has predicate.  */
   /* Skip verify of address_in_shadow_stack_memory_range, has predicate.  */
   /* Skip verify of shadow_stack_element_size_aligned, invalid_p == 0.  */
+  /* Skip verify of is_no_return_shadow_stack_address, has predicate.  */
   if (!log.empty ())
     internal_error (_("verify_gdbarch: the following are invalid ...%s"),
 		    log.c_str ());
@@ -1432,6 +1434,12 @@ gdbarch_dump (struct gdbarch *gdbarch, struct ui_file *file)
   gdb_printf (file,
 	      "gdbarch_dump: shadow_stack_element_size_aligned = %s\n",
 	      plongest (gdbarch->shadow_stack_element_size_aligned));
+  gdb_printf (file,
+	      "gdbarch_dump: gdbarch_is_no_return_shadow_stack_address_p() = %d\n",
+	      gdbarch_is_no_return_shadow_stack_address_p (gdbarch));
+  gdb_printf (file,
+	      "gdbarch_dump: is_no_return_shadow_stack_address = <%s>\n",
+	      host_address_to_string (gdbarch->is_no_return_shadow_stack_address));
   if (gdbarch->dump_tdep != NULL)
     gdbarch->dump_tdep (gdbarch, file);
 }
@@ -5659,3 +5667,27 @@ set_gdbarch_shadow_stack_element_size_aligned (struct gdbarch *gdbarch,
 {
   gdbarch->shadow_stack_element_size_aligned = shadow_stack_element_size_aligned;
 }
+
+bool
+gdbarch_is_no_return_shadow_stack_address_p (struct gdbarch *gdbarch)
+{
+  gdb_assert (gdbarch != NULL);
+  return gdbarch->is_no_return_shadow_stack_address != NULL;
+}
+
+bool
+gdbarch_is_no_return_shadow_stack_address (struct gdbarch *gdbarch, const shadow_stack_frame_info &frame)
+{
+  gdb_assert (gdbarch != NULL);
+  gdb_assert (gdbarch->is_no_return_shadow_stack_address != NULL);
+  if (gdbarch_debug >= 2)
+    gdb_printf (gdb_stdlog, "gdbarch_is_no_return_shadow_stack_address called\n");
+  return gdbarch->is_no_return_shadow_stack_address (gdbarch, frame);
+}
+
+void
+set_gdbarch_is_no_return_shadow_stack_address (struct gdbarch *gdbarch,
+					       gdbarch_is_no_return_shadow_stack_address_ftype is_no_return_shadow_stack_address)
+{
+  gdbarch->is_no_return_shadow_stack_address = is_no_return_shadow_stack_address;
+}
diff --git a/gdb/gdbarch-gen.h b/gdb/gdbarch-gen.h
index 34a32c346e8..0b92a448c78 100644
--- a/gdb/gdbarch-gen.h
+++ b/gdb/gdbarch-gen.h
@@ -1856,3 +1856,18 @@ extern void set_gdbarch_address_in_shadow_stack_memory_range (struct gdbarch *gd
 
 extern int gdbarch_shadow_stack_element_size_aligned (struct gdbarch *gdbarch);
 extern void set_gdbarch_shadow_stack_element_size_aligned (struct gdbarch *gdbarch, int shadow_stack_element_size_aligned);
+
+/* There can be elements on the shadow stack which are not return addresses.
+   This happens for example on x86 with CET in case of signals.
+   If an architecture implements the command 'backtrace shadow' and the
+   shadow stack can contain elements which are not return addresses, this
+   function has to be provided.
+
+   Return true, if FRAME does not contain a return address in FRAME->VALUE
+   but another valid value for the architecture's shadow stack. */
+
+extern bool gdbarch_is_no_return_shadow_stack_address_p (struct gdbarch *gdbarch);
+
+typedef bool (gdbarch_is_no_return_shadow_stack_address_ftype) (struct gdbarch *gdbarch, const shadow_stack_frame_info &frame);
+extern bool gdbarch_is_no_return_shadow_stack_address (struct gdbarch *gdbarch, const shadow_stack_frame_info &frame);
+extern void set_gdbarch_is_no_return_shadow_stack_address (struct gdbarch *gdbarch, gdbarch_is_no_return_shadow_stack_address_ftype *is_no_return_shadow_stack_address);
diff --git a/gdb/gdbarch.h b/gdb/gdbarch.h
index 20497612811..57b8f021e93 100644
--- a/gdb/gdbarch.h
+++ b/gdb/gdbarch.h
@@ -31,6 +31,7 @@
 #include "gdbsupport/gdb-checked-static-cast.h"
 #include "registry.h"
 #include "solib.h"
+#include "shadow-stack.h"
 
 struct floatformat;
 struct ui_file;
diff --git a/gdb/gdbarch_components.py b/gdb/gdbarch_components.py
index 5223553941f..d925f5a7e87 100644
--- a/gdb/gdbarch_components.py
+++ b/gdb/gdbarch_components.py
@@ -2938,3 +2938,20 @@ this value.
     predefault="8",
     invalid=False,
 )
+
+Method(
+    comment="""
+There can be elements on the shadow stack which are not return addresses.
+This happens for example on x86 with CET in case of signals.
+If an architecture implements the command 'backtrace shadow' and the
+shadow stack can contain elements which are not return addresses, this
+function has to be provided.
+
+Return true, if FRAME does not contain a return address in FRAME->VALUE
+but another valid value for the architecture's shadow stack.
+""",
+    type="bool",
+    name="is_no_return_shadow_stack_address",
+    params=[("const shadow_stack_frame_info &", "frame")],
+    predicate=True,
+)
diff --git a/gdb/shadow-stack.c b/gdb/shadow-stack.c
index 759ef6989ea..42032f51781 100644
--- a/gdb/shadow-stack.c
+++ b/gdb/shadow-stack.c
@@ -222,43 +222,6 @@ pc_in_middle_of_statement (CORE_ADDR pc, symtab_and_line sal)
   return pc > sal.pc && pc <= sal.end;
 }
 
-enum class ssp_unwind_stop_reason
-{
-  /* No particular reason; either we haven't tried unwinding yet, or we
-     didn't fail.  */
-  no_error = 0,
-
-  /* We could not read the memory of the shadow stack element.  */
-  memory_read_error
-};
-
-/* Information of a shadow stack frame belonging to a shadow stack element
-   at shadow stack pointer SSP.  */
-
-class shadow_stack_frame_info
-{
-public:
-  /* If possible, unwind the previous shadow stack frame info.  RANGE is
-     the shadow stack memory range [start_address, end_address) belonging
-     to this frame's shadow stack pointer.  If we cannot unwind the
-     previous frame info, set the unwind_stop_reason attribute.  If we
-     reached the bottom of the shadow stack just don't return a value.  */
-  std::optional<shadow_stack_frame_info> unwind_prev_shadow_stack_frame_info
-    (gdbarch *gdbarch, std::pair<CORE_ADDR, CORE_ADDR> range);
-
-  /* The shadow stack pointer.  */
-  CORE_ADDR ssp;
-  /* The value of the shadow stack at SSP.  */
-  CORE_ADDR value;
-  /* The level of the element on the shadow stack.  */
-  unsigned long level;
-  /* If unwinding of the previous frame info fails assign this value to a
-     matching condition ssp_unwind_stop_reason
-     > ssp_unwind_stop_reason::no_error.  */
-  ssp_unwind_stop_reason unwind_stop_reason
-    = ssp_unwind_stop_reason::no_error;
-};
-
 static
 gdb::unique_xmalloc_ptr<char> find_pc_funname (CORE_ADDR pc)
 {
@@ -285,6 +248,33 @@ do_print_shadow_stack_frame_info
    const shadow_stack_print_options &print_options,
    const shadow_stack_frame_info &frame, print_what print_what)
 {
+  if (gdbarch_is_no_return_shadow_stack_address_p (gdbarch)
+      && gdbarch_is_no_return_shadow_stack_address (gdbarch, frame))
+    {
+      /* It is possible, for the x86 architecture for instance, that an
+	 element on the shadow stack is not a return address.  We still
+	 want to print the address in that case but no further
+	 information.  */
+      ui_out_emit_tuple tuple_emitter (uiout, "shadow-stack-frame");
+      uiout->text ("#");
+      uiout->field_fmt_signed (2, ui_left, "level", frame.level);
+
+      /* On x86 there can be a shadow stack token at bit 63.  For x32, the
+	 address size is only 32 bit.  Thus, we still must use
+	 gdbarch_shadow_stack_element_size_aligned (and not
+	 gdbarch_addr_bit) to determine the width of the address to be
+	 printed.  */
+      const int element_size
+	 = gdbarch_shadow_stack_element_size_aligned (gdbarch);
+
+      uiout->field_string
+	("addr", hex_string_custom (frame.value, element_size * 2),
+	 address_style.style ());
+      uiout->text ("\n");
+      gdb_flush (gdb_stdout);
+      return;
+    }
+
   if (print_options.print_frame_info != print_frame_info_auto)
     {
       /* Use the specific frame information desired by the user.  */
diff --git a/gdb/shadow-stack.h b/gdb/shadow-stack.h
index 2d33efafd22..f801d077820 100644
--- a/gdb/shadow-stack.h
+++ b/gdb/shadow-stack.h
@@ -55,4 +55,41 @@ std::array<gdb::option::option_def_group, 1>
   make_backtrace_shadow_options_def_group
     (shadow_stack_print_options *print_options);
 
+enum class ssp_unwind_stop_reason
+{
+  /* No particular reason; either we haven't tried unwinding yet, or we
+     didn't fail.  */
+  no_error = 0,
+
+  /* We could not read the memory of the shadow stack element.  */
+  memory_read_error
+};
+
+/* Information of a shadow stack frame belonging to a shadow stack element
+   at shadow stack pointer SSP.  */
+
+class shadow_stack_frame_info
+{
+public:
+  /* If possible, unwind the previous shadow stack frame info.  RANGE is
+     the shadow stack memory range [start_address, end_address) belonging
+     to this frame's shadow stack pointer.  If we cannot unwind the
+     previous frame info, set the unwind_stop_reason attribute.  If we
+     reached the bottom of the shadow stack just don't return a value.  */
+  std::optional<shadow_stack_frame_info> unwind_prev_shadow_stack_frame_info
+    (gdbarch *gdbarch, std::pair<CORE_ADDR, CORE_ADDR> range);
+
+  /* The shadow stack pointer.  */
+  CORE_ADDR ssp;
+  /* The value of the shadow stack at SSP.  */
+  CORE_ADDR value;
+  /* The level of the element on the shadow stack.  */
+  unsigned long level;
+  /* If unwinding of the previous frame info fails assign this value to a
+     matching condition ssp_unwind_stop_reason
+     > ssp_unwind_stop_reason::no_error.  */
+  ssp_unwind_stop_reason unwind_stop_reason
+    = ssp_unwind_stop_reason::no_error;
+};
+
 #endif /* GDB_SHADOW_STACK_H */
-- 
2.34.1

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