corefile/bug: Add hook to control the use of target description notes from corefiles

Due to the nature of the AArch64 SVE/SME extensions in GDB, each thread
can potentially have distinct target descriptions/gdbarches.

When loading a gcore-generated core file, at the moment GDB gives priority
to the target description dumped to NT_GDB_TDESC.  Though technically correct
for most targets, it doesn't work correctly for AArch64 with SVE or SME
support.

The correct approach for AArch64/Linux is to either have per-thread target
description notes in the corefiles or to rely on the
gdbarch_core_read_description hook, so it can figure out the proper target
description for a given thread based on the various available register notes.

The former, although more correct, doesn't address the case of existing gdb's
that only output a single target description note.

This patch goes for the latter, and adds a new gdbarch hook to conditionalize
the use of the corefile target description note. The hook is called
use_target_description_from_corefile_notes.

The hook defaults to returning true, meaning targets will use the corefile
target description note.  AArch64 Linux overrides the hook to return false
when it detects any of the SVE or SME register notes in the corefile.

Otherwise it should be fine for AArch64 Linux to use the corefile target
description note.

When we support per-thread target description notes, then we can augment
the AArch64 Linux hook to rely on those notes.

Regression-tested on aarch64-linux Ubuntu 22.04/20.04.

Approved-By: Simon Marchi <simon.marchi@efficios.com>
Reviewed-by: Thiago Jung Bauermann <thiago.bauermann@linaro.org>

gdb/aarch64-linux-tdep.c

@@ -2199,6 +2199,33 @@ aarch64_linux_decode_memtag_section (struct gdbarch *gdbarch,
   return tags;
 }
 
+/* AArch64 Linux implementation of the
+   gdbarch_use_target_description_from_corefile_notes hook.  */
+
+static bool
+aarch64_use_target_description_from_corefile_notes (gdbarch *gdbarch,
+						    bfd *obfd)
+{
+  /* Sanity check.  */
+  gdb_assert (obfd != nullptr);
+
+  /* If the corefile contains any SVE or SME register data, we don't want to
+     use the target description note, as it may be incorrect.
+
+     Currently the target description note contains a potentially incorrect
+     target description if the originating program changed the SVE or SME
+     vector lengths mid-execution.
+
+     Once we support per-thread target description notes in the corefiles, we
+     can always trust those notes whenever they are available.  */
+  if (bfd_get_section_by_name (obfd, ".reg-aarch-sve") != nullptr
+      || bfd_get_section_by_name (obfd, ".reg-aarch-za") != nullptr
+      || bfd_get_section_by_name (obfd, ".reg-aarch-zt") != nullptr)
+    return false;
+
+  return true;
+}
+
 static void
 aarch64_linux_init_abi (struct gdbarch_info info, struct gdbarch *gdbarch)
 {
@@ -2469,6 +2496,12 @@ aarch64_linux_init_abi (struct gdbarch_info info, struct gdbarch *gdbarch)
 					    aarch64_displaced_step_hw_singlestep);
 
   set_gdbarch_gcc_target_options (gdbarch, aarch64_linux_gcc_target_options);
+
+  /* Hook to decide if the target description should be obtained from
+     corefile target description note(s) or inferred from the corefile
+     sections.  */
+  set_gdbarch_use_target_description_from_corefile_notes (gdbarch,
+			    aarch64_use_target_description_from_corefile_notes);
 }
 
 #if GDB_SELF_TEST

gdb/arch-utils.c

@@ -1092,6 +1092,16 @@ default_read_core_file_mappings
 {
 }
 
+/* See arch-utils.h.  */
+bool
+default_use_target_description_from_corefile_notes (struct gdbarch *gdbarch,
+						    struct bfd *corefile_bfd)
+{
+  /* Always trust the corefile target description contained in the target
+     description note.  */
+  return true;
+}
+
 CORE_ADDR
 default_get_return_buf_addr (struct type *val_type, frame_info_ptr cur_frame)
 {

gdb/arch-utils.h

@@ -305,6 +305,12 @@ extern void default_read_core_file_mappings
    read_core_file_mappings_pre_loop_ftype pre_loop_cb,
    read_core_file_mappings_loop_ftype loop_cb);
 
+/* Default implementation of gdbarch
+   use_target_description_from_corefile_notes.  */
+extern bool default_use_target_description_from_corefile_notes
+  (struct gdbarch *gdbarch,
+  struct bfd *corefile_bfd);
+
 /* Default implementation of gdbarch default_get_return_buf_addr method.  */
 extern CORE_ADDR default_get_return_buf_addr (struct type *val_typegdbarch,
 					      frame_info_ptr cur_frame);

gdb/corelow.c

@@ -1229,35 +1229,47 @@ core_target::thread_alive (ptid_t ptid)
 const struct target_desc *
 core_target::read_description ()
 {
-  /* If the core file contains a target description note then we will use
-     that in preference to anything else.  */
-  bfd_size_type tdesc_note_size = 0;
-  struct bfd_section *tdesc_note_section
-    = bfd_get_section_by_name (core_bfd, ".gdb-tdesc");
-  if (tdesc_note_section != nullptr)
-    tdesc_note_size = bfd_section_size (tdesc_note_section);
-  if (tdesc_note_size > 0)
+  /* First check whether the target wants us to use the corefile target
+     description notes.  */
+  if (gdbarch_use_target_description_from_corefile_notes (m_core_gdbarch,
+							  core_bfd))
     {
-      gdb::char_vector contents (tdesc_note_size + 1);
-      if (bfd_get_section_contents (core_bfd, tdesc_note_section,
-				    contents.data (), (file_ptr) 0,
-				    tdesc_note_size))
+      /* If the core file contains a target description note then go ahead and
+	 use that.  */
+      bfd_size_type tdesc_note_size = 0;
+      struct bfd_section *tdesc_note_section
+	= bfd_get_section_by_name (core_bfd, ".gdb-tdesc");
+      if (tdesc_note_section != nullptr)
+	tdesc_note_size = bfd_section_size (tdesc_note_section);
+      if (tdesc_note_size > 0)
 	{
-	  /* Ensure we have a null terminator.  */
-	  contents[tdesc_note_size] = '\0';
-	  const struct target_desc *result
-	    = string_read_description_xml (contents.data ());
-	  if (result != nullptr)
-	    return result;
+	  gdb::char_vector contents (tdesc_note_size + 1);
+	  if (bfd_get_section_contents (core_bfd, tdesc_note_section,
+					contents.data (), (file_ptr) 0,
+					tdesc_note_size))
+	    {
+	      /* Ensure we have a null terminator.  */
+	      contents[tdesc_note_size] = '\0';
+	      const struct target_desc *result
+		= string_read_description_xml (contents.data ());
+	      if (result != nullptr)
+		return result;
+	    }
 	}
     }
 
+  /* If the architecture provides a corefile target description hook, use
+     it now.  Even if the core file contains a target description in a note
+     section, it is not useful for targets that can potentially have distinct
+     descriptions for each thread.  One example is AArch64's SVE/SME
+     extensions that allow per-thread vector length changes, resulting in
+     registers with different sizes.  */
   if (m_core_gdbarch && gdbarch_core_read_description_p (m_core_gdbarch))
     {
       const struct target_desc *result;
 
       result = gdbarch_core_read_description (m_core_gdbarch, this, core_bfd);
-      if (result != NULL)
+      if (result != nullptr)
 	return result;
     }
 

gdb/gdbarch-gen.h

@@ -1717,3 +1717,13 @@ extern void set_gdbarch_get_pc_address_flags (struct gdbarch *gdbarch, gdbarch_g
 typedef void (gdbarch_read_core_file_mappings_ftype) (struct gdbarch *gdbarch, struct bfd *cbfd, read_core_file_mappings_pre_loop_ftype pre_loop_cb, read_core_file_mappings_loop_ftype loop_cb);
 extern void gdbarch_read_core_file_mappings (struct gdbarch *gdbarch, struct bfd *cbfd, read_core_file_mappings_pre_loop_ftype pre_loop_cb, read_core_file_mappings_loop_ftype loop_cb);
 extern void set_gdbarch_read_core_file_mappings (struct gdbarch *gdbarch, gdbarch_read_core_file_mappings_ftype *read_core_file_mappings);
+
+/* Return true if the target description for all threads should be read from the
+   target description core file note(s).  Return false if the target description
+   for all threads should be inferred from the core file contents/sections.
+
+   The corefile's bfd is passed through COREFILE_BFD. */
+
+typedef bool (gdbarch_use_target_description_from_corefile_notes_ftype) (struct gdbarch *gdbarch, struct bfd *corefile_bfd);
+extern bool gdbarch_use_target_description_from_corefile_notes (struct gdbarch *gdbarch, struct bfd *corefile_bfd);
+extern void set_gdbarch_use_target_description_from_corefile_notes (struct gdbarch *gdbarch, gdbarch_use_target_description_from_corefile_notes_ftype *use_target_description_from_corefile_notes);

gdb/gdbarch.c

@@ -256,6 +256,7 @@ struct gdbarch
   gdbarch_type_align_ftype *type_align = default_type_align;
   gdbarch_get_pc_address_flags_ftype *get_pc_address_flags = default_get_pc_address_flags;
   gdbarch_read_core_file_mappings_ftype *read_core_file_mappings = default_read_core_file_mappings;
+  gdbarch_use_target_description_from_corefile_notes_ftype *use_target_description_from_corefile_notes = default_use_target_description_from_corefile_notes;
 };
 
 /* Create a new ``struct gdbarch'' based on information provided by
@@ -523,6 +524,7 @@ verify_gdbarch (struct gdbarch *gdbarch)
   /* Skip verify of type_align, invalid_p == 0 */
   /* Skip verify of get_pc_address_flags, invalid_p == 0 */
   /* Skip verify of read_core_file_mappings, invalid_p == 0 */
+  /* Skip verify of use_target_description_from_corefile_notes, invalid_p == 0 */
   if (!log.empty ())
     internal_error (_("verify_gdbarch: the following are invalid ...%s"),
 		    log.c_str ());
@@ -1373,6 +1375,9 @@ gdbarch_dump (struct gdbarch *gdbarch, struct ui_file *file)
   gdb_printf (file,
 	      "gdbarch_dump: read_core_file_mappings = <%s>\n",
 	      host_address_to_string (gdbarch->read_core_file_mappings));
+  gdb_printf (file,
+	      "gdbarch_dump: use_target_description_from_corefile_notes = <%s>\n",
+	      host_address_to_string (gdbarch->use_target_description_from_corefile_notes));
   if (gdbarch->dump_tdep != NULL)
     gdbarch->dump_tdep (gdbarch, file);
 }
@@ -5409,3 +5414,20 @@ set_gdbarch_read_core_file_mappings (struct gdbarch *gdbarch,
 {
   gdbarch->read_core_file_mappings = read_core_file_mappings;
 }
+
+bool
+gdbarch_use_target_description_from_corefile_notes (struct gdbarch *gdbarch, struct bfd *corefile_bfd)
+{
+  gdb_assert (gdbarch != NULL);
+  gdb_assert (gdbarch->use_target_description_from_corefile_notes != NULL);
+  if (gdbarch_debug >= 2)
+    gdb_printf (gdb_stdlog, "gdbarch_use_target_description_from_corefile_notes called\n");
+  return gdbarch->use_target_description_from_corefile_notes (gdbarch, corefile_bfd);
+}
+
+void
+set_gdbarch_use_target_description_from_corefile_notes (struct gdbarch *gdbarch,
+							gdbarch_use_target_description_from_corefile_notes_ftype use_target_description_from_corefile_notes)
+{
+  gdbarch->use_target_description_from_corefile_notes = use_target_description_from_corefile_notes;
+}

gdb/gdbarch_components.py

@@ -2732,3 +2732,18 @@ Read core file mappings
     predefault="default_read_core_file_mappings",
     invalid=False,
 )
+
+Method(
+    comment="""
+Return true if the target description for all threads should be read from the
+target description core file note(s).  Return false if the target description
+for all threads should be inferred from the core file contents/sections.
+
+The corefile's bfd is passed through COREFILE_BFD.
+""",
+    type="bool",
+    name="use_target_description_from_corefile_notes",
+    params=[("struct bfd *", "corefile_bfd")],
+    predefault="default_use_target_description_from_corefile_notes",
+    invalid=False,
+)