gdb: write target description into core file

When a core file is created from within GDB add the target description
into a note within the core file.

When loading a core file, if the target description note is present
then load the target description from the core file.

The benefit of this is that we can be sure that, when analysing the
core file within GDB, that we are using the exact same target
description as was in use at the time the core file was created.

GDB already supports a mechanism for figuring out the target
description from a given corefile; gdbarch_core_read_description.
This new mechanism (GDB adding the target description) is not going to
replace the old mechanism.  Core files generated outside of GDB will
not include a target description, and so GDB still needs to be able to
figure out a target description for these files.

My primary motivation for adding this feature is that, in a future
commit, I will be adding support for bare metal core dumps on some
targets.  For RISC-V specifically, I want to be able to dump all the
available control status registers.  As different targets will present
different sets of register in their target description, including
registers that are possibly not otherwise known to GDB I wanted a way
to capture these registers in the core dump.

I therefore need a mechanism to write out an arbitrary set of
registers, and to then derive a target description from this arbitrary
set when later loading the core file.  The obvious approach (I think)
is to just reuse the target description.

Once I'd decided to add support for writing out the target description
I could either choose to make this RISC-V only, or make it generic.  I
figure that having the target description in the core file doesn't
hurt, and _might_ be helpful.  So that's how I got here, general
support for including the target description in GDB generated core
files.

In previous versions of this patch I added the target description from
generic code (in gcore.c).  However, doing this creates a dependency
between GDB's common code and bfd ELF support.  As ELF support in gdb
is optional (for example the target x86_64-apple-darwin20.3.0 does not
include ELF support) then having gcore.c require ELF support would
break the GDB build in some cases.

Instead, in this version of the patch, writing the target description
note is done from each specific targets make notes function.  Each of
these now calls a common function in gcore-elf.c (which is only linked
in when bfd has ELF support).  And so only targets that are ELF based
will call the new function and we can therefore avoid an unconditional
dependency on ELF support.

gdb/ChangeLog:

	* corelow.c: Add 'xml-tdesc.h' include.
	(core_target::read_description): Load the target description from
	the core file when possible.
	* fbsd-tdep.c (fbsd_make_corefile_notes): Add target description
	note.
	* gcore-elf.c: Add 'gdbsupport/tdesc.h' include.
	(gcore_elf_make_tdesc_note): New function.
	* gcore-elf.h (gcore_elf_make_tdesc_note): Declare.
	* linux-tdep.c (linux_make_corefile_notes): Add target description
	note.

6 files changed, 82 insertions, 0 deletions

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index 6d2d390..dbcc524 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,3 +1,17 @@
+2021-03-05  Craig Blackmore  <craig.blackmore@embecosm.com>
+	    Andrew Burgess  <andrew.burgess@embecosm.com>
+
+	* corelow.c: Add 'xml-tdesc.h' include.
+	(core_target::read_description): Load the target description from
+	the core file when possible.
+	* fbsd-tdep.c (fbsd_make_corefile_notes): Add target description
+	note.
+	* gcore-elf.c: Add 'gdbsupport/tdesc.h' include.
+	(gcore_elf_make_tdesc_note): New function.
+	* gcore-elf.h (gcore_elf_make_tdesc_note): Declare.
+	* linux-tdep.c (linux_make_corefile_notes): Add target description
+	note.
+
 2021-03-05  Andrew Burgess  <andrew.burgess@embecosm.com>
 
 	* Makefile.in (SFILES): Add gcore-elf.c.
diff --git a/gdb/corelow.c b/gdb/corelow.c
index a63eab4..a2d2d20 100644
--- a/gdb/corelow.c
+++ b/gdb/corelow.c
@@ -49,6 +49,7 @@
 #include <unordered_map>
 #include <unordered_set>
 #include "gdbcmd.h"
+#include "xml-tdesc.h"
 
 #ifndef O_LARGEFILE
 #define O_LARGEFILE 0
@@ -1000,6 +1001,29 @@ 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)
+    {
+      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 (m_core_gdbarch && gdbarch_core_read_description_p (m_core_gdbarch))
     {
       const struct target_desc *result;
diff --git a/gdb/fbsd-tdep.c b/gdb/fbsd-tdep.c
index dc4278c..170e8f5 100644
--- a/gdb/fbsd-tdep.c
+++ b/gdb/fbsd-tdep.c
@@ -712,6 +712,9 @@ 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);
+
   return note_data;
 }
 
diff --git a/gdb/gcore-elf.c b/gdb/gcore-elf.c
index b0fb808..d9c216b 100644
--- a/gdb/gcore-elf.c
+++ b/gdb/gcore-elf.c
@@ -24,6 +24,7 @@
 #include "gdbthread.h"
 #include "inferior.h"
 #include "regset.h"
+#include "gdbsupport/tdesc.h"
 
 /* Structure for passing information from GCORE_COLLECT_THREAD_REGISTERS
    via an iterator to GCORE_COLLECT_REGSET_SECTION_CB. */
@@ -134,3 +135,32 @@ gcore_elf_build_thread_register_notes
   gcore_elf_collect_thread_registers (regcache, info->ptid, obfd,
 				      note_data, note_size, stop_signal);
 }
+
+/* See gcore-elf.h.  */
+
+void
+gcore_elf_make_tdesc_note (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 char *tdesc_xml
+    = tdesc == nullptr ? nullptr : tdesc_get_features_xml (tdesc);
+  if (tdesc_xml != nullptr && *tdesc_xml != '\0')
+    {
+      /* Skip the leading '@'.  */
+      if (*tdesc_xml == '@')
+	++tdesc_xml;
+
+      /* Include the null terminator in the length.  */
+      size_t tdesc_len = strlen (tdesc_xml) + 1;
+
+      /* Now add the target description into the core file.  */
+      note_data->reset (elfcore_write_register_note (obfd,
+						     note_data->release (),
+						     note_size,
+						     ".gdb-tdesc", tdesc_xml,
+						     tdesc_len));
+    }
+}
diff --git a/gdb/gcore-elf.h b/gdb/gcore-elf.h
index d667686..b750165 100644
--- a/gdb/gcore-elf.h
+++ b/gdb/gcore-elf.h
@@ -36,4 +36,12 @@ extern void gcore_elf_build_thread_register_notes
   (struct gdbarch *gdbarch, struct thread_info *info, gdb_signal stop_signal,
    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 targtet's target description.  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);
+
 #endif /* GCORE_ELF_H */
diff --git a/gdb/linux-tdep.c b/gdb/linux-tdep.c
index 5bfd82d..ab3402a 100644
--- a/gdb/linux-tdep.c
+++ b/gdb/linux-tdep.c
@@ -1935,6 +1935,9 @@ 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);
+
   return note_data;
 }