gdb/corefile: write NT_GDB_TDESC based on signalled thread

When creating a core file from within GDB we include a NT_GDB_TDESC
that includes the target description of the architecture in use.

For architectures with dynamic architectures (e.g. AArch64 with
sve/sme) the original architecture, calculated from the original
target description, might not match the per-thread architecture.

In the general case, where each thread has a different architecture,
then we really need a separate NT_GDB_TDESC for each thread, however,
there's currently no way to read in multiple NT_GDB_TDESC.

This commit is a step towards per-thread NT_GDB_TDESC.  In this commit
I have updated the function that writes the NT_GDB_TDESC to accept a
gdbarch (rather than calling target_gdbarch() to find a gdbarch), and
I now pass in the gdbarch of the signalled thread.

In many cases (though NOT all) targets with dynamic architectures
really only use a single architecture, even when there are multiple
threads, so in the common case, this should ensure that GDB emits an
architecture that is more likely to be correct.

Additional work will be needed in order to support corefiles with
truly per-thread architectures, but that will need to be done in the
future.

5 files changed, 23 insertions, 10 deletions

diff --git a/gdb/elf-none-tdep.c b/gdb/elf-none-tdep.c
index ce2db02..cae6600 100644
--- a/gdb/elf-none-tdep.c
+++ b/gdb/elf-none-tdep.c
@@ -110,8 +110,12 @@ elf_none_make_corefile_notes (struct gdbarch *gdbarch, bfd *obfd,
     }
 
 
-  /* Target description.  */
-  gcore_elf_make_tdesc_note (obfd, &note_data, note_size);
+  /* Include the target description when possible.  Some architectures
+     allow for per-thread gdbarch so we should really be emitting a tdesc
+     per-thread, however, we don't currently support reading in a
+     per-thread tdesc, so just emit the tdesc for the signalled thread.  */
+  gdbarch = target_thread_architecture (signalled_thr->ptid);
+  gcore_elf_make_tdesc_note (gdbarch, obfd, &note_data, note_size);
 
   return note_data;
 }
diff --git a/gdb/fbsd-tdep.c b/gdb/fbsd-tdep.c
index dc5020d..d166d78 100644
--- a/gdb/fbsd-tdep.c
+++ b/gdb/fbsd-tdep.c
@@ -773,8 +773,12 @@ fbsd_make_corefile_notes (struct gdbarch *gdbarch, bfd *obfd, int *note_size)
 	return NULL;
     }
 
-  /* Include the target description when possible.  */
-  gcore_elf_make_tdesc_note (obfd, &note_data, note_size);
+  /* Include the target description when possible.  Some architectures
+     allow for per-thread gdbarch so we should really be emitting a tdesc
+     per-thread, however, we don't currently support reading in a
+     per-thread tdesc, so just emit the tdesc for the signalled thread.  */
+  gdbarch = target_thread_architecture (signalled_thr->ptid);
+  gcore_elf_make_tdesc_note (gdbarch, obfd, &note_data, note_size);
 
   return note_data;
 }
diff --git a/gdb/gcore-elf.c b/gdb/gcore-elf.c
index 643426d..0142db8 100644
--- a/gdb/gcore-elf.c
+++ b/gdb/gcore-elf.c
@@ -139,12 +139,12 @@ gcore_elf_build_thread_register_notes
 /* See gcore-elf.h.  */
 
 void
-gcore_elf_make_tdesc_note (bfd *obfd,
+gcore_elf_make_tdesc_note (struct gdbarch *gdbarch, bfd *obfd,
 			   gdb::unique_xmalloc_ptr<char> *note_data,
 			   int *note_size)
 {
   /* Append the target description to the core file.  */
-  const struct target_desc *tdesc = gdbarch_target_desc (target_gdbarch ());
+  const struct target_desc *tdesc = gdbarch_target_desc (gdbarch);
   const char *tdesc_xml
     = tdesc == nullptr ? nullptr : tdesc_get_features_xml (tdesc);
   if (tdesc_xml != nullptr && *tdesc_xml != '\0')
diff --git a/gdb/gcore-elf.h b/gdb/gcore-elf.h
index 2cfeb3e..826e3be 100644
--- a/gdb/gcore-elf.h
+++ b/gdb/gcore-elf.h
@@ -37,11 +37,12 @@ extern void gcore_elf_build_thread_register_notes
    bfd *obfd, gdb::unique_xmalloc_ptr<char> *note_data, int *note_size);
 
 /* Add content to *NOTE_DATA (and update *NOTE_SIZE) to include a note
-   containing the current target's target description.  The core file is
+   containing the target description for GDBARCH.  The core file is
    being written to OBFD.  If something goes wrong then *NOTE_DATA can be
    set to nullptr.  */
 
 extern void gcore_elf_make_tdesc_note
-  (bfd *obfd, gdb::unique_xmalloc_ptr<char> *note_data, int *note_size);
+  (struct gdbarch *gdbarch, bfd *obfd,
+   gdb::unique_xmalloc_ptr<char> *note_data, int *note_size);
 
 #endif /* GCORE_ELF_H */
diff --git a/gdb/linux-tdep.c b/gdb/linux-tdep.c
index 22bb968..98658fa 100644
--- a/gdb/linux-tdep.c
+++ b/gdb/linux-tdep.c
@@ -2128,8 +2128,12 @@ linux_make_corefile_notes (struct gdbarch *gdbarch, bfd *obfd, int *note_size)
   /* File mappings.  */
   linux_make_mappings_corefile_notes (gdbarch, obfd, note_data, note_size);
 
-  /* Target description.  */
-  gcore_elf_make_tdesc_note (obfd, &note_data, note_size);
+  /* Include the target description when possible.  Some architectures
+     allow for per-thread gdbarch so we should really be emitting a tdesc
+     per-thread, however, we don't currently support reading in a
+     per-thread tdesc, so just emit the tdesc for the signalled thread.  */
+  gdbarch = target_thread_architecture (signalled_thr->ptid);
+  gcore_elf_make_tdesc_note (gdbarch, obfd, &note_data, note_size);
 
   return note_data;
 }