[PATCH v4 13/16] [gdb/generic] corefile/bug: Fixup (gcore) core file target description reading order

[Luis Machado <luis.machado@arm.com>]

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 target, it doesn't work correctly for AArch64 with SVE or SME
support.

The correct approach for AArch64/Linux is 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.

I think this should work for other architectures as well. If not, we may
need to adjust things so all architectures get the information that they
need for discovering the target description of the core file.

Regression-tested on aarch64-linux Ubuntu 22.04/20.04.
---
 gdb/corelow.c | 24 +++++++++++++++---------
 1 file changed, 15 insertions(+), 9 deletions(-)

diff --git a/gdb/corelow.c b/gdb/corelow.c
index 46bb1077b6d..b650396ae3d 100644
--- a/gdb/corelow.c
+++ b/gdb/corelow.c
@@ -1226,6 +1226,21 @@ core_target::thread_alive (ptid_t ptid)
 const struct target_desc *
 core_target::read_description ()
 {
+  /* 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 != nullptr)
+	return result;
+    }
+
   /* 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;
@@ -1249,15 +1264,6 @@ core_target::read_description ()
 	}
     }
 
-  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)
-	return result;
-    }
-
   return this->beneath ()->read_description ();
 }
 
-- 
2.25.1