[PATCH v2 13/17] [gdb/generic] corefile/bug: Use thread-specific gdbarch when dumping register state to core files

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

When we have a core file generated by gdb (via the gcore command), gdb dumps
the target description to a note.  During loading of that core file, gdb will
first try to load that saved target description.

This works fine for almost all architectures. But AArch64 has a few
dynamically-generated target descriptions/gdbarch depending on the vector
length that was in use at the time the core file was generated.

The target description gdb dumps to the core file note is the one generated
at the time of attachment/startup.  If, for example, the SVE vector length
changed during execution, this would not reflect on the core file, as gdb
would still dump the initial target description.

Another issue is that the gdbarch potentially doesn't match the thread's
real gdbarch, and so things like the register cache may have different formats
and sizes.

To address this, fetch the thread's architecture before dumping its register
state.  That way we will always use the correct target description/gdbarch.
---
 gdb/linux-tdep.c | 18 +++++++++++++++++-
 1 file changed, 17 insertions(+), 1 deletion(-)

diff --git a/gdb/linux-tdep.c b/gdb/linux-tdep.c
index b5eee5e108c..7d0976932c6 100644
--- a/gdb/linux-tdep.c
+++ b/gdb/linux-tdep.c
@@ -2099,12 +2099,28 @@ linux_make_corefile_notes (struct gdbarch *gdbarch, bfd *obfd, int *note_size)
 					  stop_signal);
 
   if (signalled_thr != nullptr)
-    linux_corefile_thread (signalled_thr, &thread_args);
+    {
+      /* On some architectures, like AArch64, each thread can have a distinct
+	 gdbarch (due to scalable extensions), and using the inferior gdbarch
+	 is incorrect.
+
+	 Fetch each thread's gdbarch and pass it down to the lower layers so
+	 we can dump the right set of registers.  */
+      thread_args.gdbarch = target_thread_architecture (signalled_thr->ptid);
+      linux_corefile_thread (signalled_thr, &thread_args);
+    }
   for (thread_info *thr : current_inferior ()->non_exited_threads ())
     {
       if (thr == signalled_thr)
 	continue;
 
+      /* On some architectures, like AArch64, each thread can have a distinct
+	 gdbarch (due to scalable extensions), and using the inferior gdbarch
+	 is incorrect.
+
+	 Fetch each thread's gdbarch and pass it down to the lower layers so
+	 we can dump the right set of registers.  */
+      thread_args.gdbarch = target_thread_architecture (thr->ptid);
       linux_corefile_thread (thr, &thread_args);
     }
 
-- 
2.25.1