arm-pikeos: software single step

On ARM, PikeOS does not support hardware single step, causing various
semi-random errors when trying to next/step over some user code. So
this patch changes this target to use software-single-step instead.

The challenge is that, up to now, the PikeOS target was in all respects
identical to a baremetal target as far as GDB was concerned, meaning
we were using the baremetal osabi for this target too. This is no longer
possible, and we need to introduce a new OSABI variant. Unfortunately,
there isn't anything in the object file that would allow us to
differentiate between the two platforms. So we have to rely on a
heuristic instead, where we look for some known symbols that are
required in a PikeOS application (these symbols are expected to be
defined by the default linker script, and correspond to routines used
to allocate the application stack).

For the long run, the hope is that the stub implementation provided
by PikeOS is enhanced so that it includes vContSupported+ to the
$qSupported query, and then that the reply to the "vCont?" query
only return support for "continue" operations (thus exclusing "step"
operations). We could then use that information to reliably determine
at connection time that the target does not support single-stepping
and therefore automatically turn software single-stepping automatically
based on it.

gdb/ChangeLog:

        * defs.h (enum gdb_osabi): Add GDB_OSABI_PIKEOS.
        * osabi.c (gdb_osabi_names): Add name for GDB_OSABI_PIKEOS.
        * arm-pikeos-tdep.c: New file.
        * configure.tgt: Add arm-pikeos-tdep.o to the case of ARM
        embedded system.
        * Makefile.in (ALL_TARGET_OBS): Add arm-pikeos-tdep.o.

Tested on arm-pikeos and arm-elf using AdaCore's testsuite.
We also evaluated it on armhf-linux as a cross platform.

6 files changed, 106 insertions, 0 deletions

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index fb65595..075dd3d 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,3 +1,13 @@
+2018-11-01  Jerome Guitton  <guitton@adacore.com>
+	    Joel Brobecker  <brobecker@adacore.com>
+
+	* defs.h (enum gdb_osabi): Add GDB_OSABI_PIKEOS.
+	* osabi.c (gdb_osabi_names): Add name for GDB_OSABI_PIKEOS.
+	* arm-pikeos-tdep.c: New file.
+	* configure.tgt: Add arm-pikeos-tdep.o to the case of ARM
+	embedded system.
+	* Makefile.in (ALL_TARGET_OBS): Add arm-pikeos-tdep.o.
+
 2018-11-01  Simon Marchi  <simon.marchi@ericsson.com>
 
 	* common/pathstuff.c (get_standard_temp_dir): New.
diff --git a/gdb/Makefile.in b/gdb/Makefile.in
index 73e15fc..5c04a59 100644
--- a/gdb/Makefile.in
+++ b/gdb/Makefile.in
@@ -670,6 +670,7 @@ ALL_TARGET_OBS = \
 	arm-linux-tdep.o \
 	arm-nbsd-tdep.o \
 	arm-obsd-tdep.o \
+	arm-pikeos-tdep.o \
 	arm-symbian-tdep.o \
 	arm-tdep.o \
 	arm-wince-tdep.o \
diff --git a/gdb/arm-pikeos-tdep.c b/gdb/arm-pikeos-tdep.c
new file mode 100644
index 0000000..4662a28
--- /dev/null
+++ b/gdb/arm-pikeos-tdep.c
@@ -0,0 +1,92 @@
+/* Copyright (C) 2016-2018 Free Software Foundation, Inc.
+
+   This file is part of GDB.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+#include "defs.h"
+#include "objfiles.h"
+#include "arm-tdep.h"
+#include "osabi.h"
+
+/* The gdbarch_register_osabi handler for ARM PikeOS; it performs
+   the gdbarch initialization for that platform.  */
+
+static void
+arm_pikeos_init_abi (struct gdbarch_info info, struct gdbarch *gdbarch)
+{
+  /* Single stepping.  */
+  set_gdbarch_software_single_step (gdbarch, arm_software_single_step);
+}
+
+/* The ARM PikeOS OSABI sniffer (see gdbarch_register_osabi_sniffer).
+   Returns GDB_OSABI_PIKEOS if the given BFD is a PikeOS binary,
+   GDB_OSABI_UNKNOWN otherwise.  */
+
+static enum gdb_osabi
+arm_pikeos_osabi_sniffer (bfd *abfd)
+{
+  long number_of_symbols;
+  long i;
+  int pikeos_stack_found = 0;
+  int pikeos_stack_size_found = 0;
+
+  /* The BFD target of PikeOS is really just standard elf, so we
+     cannot use it to detect this variant.  The only common thing that
+     may be found in PikeOS modules are symbols _vm_stack/__p4_stack and
+     _vm_stack_size/__p4_stack_end. They are used to specify the stack
+     location and size; and defined by the default linker script.
+
+     OS ABI sniffers are called before the minimal symtabs are
+     created. So inspect the symbol table using BFD.  */
+
+  long storage_needed = bfd_get_symtab_upper_bound (abfd);
+  if (storage_needed <= 0)
+    return GDB_OSABI_UNKNOWN;
+
+  gdb::unique_xmalloc_ptr<asymbol *> symbol_table
+    ((asymbol **) xmalloc (storage_needed));
+  number_of_symbols = bfd_canonicalize_symtab (abfd, symbol_table.get ());
+
+  if (number_of_symbols <= 0)
+    return GDB_OSABI_UNKNOWN;
+
+  for (i = 0; i < number_of_symbols; i++)
+    {
+      const char *name = bfd_asymbol_name (symbol_table.get ()[i]);
+
+      if (strcmp (name, "_vm_stack") == 0
+	  || strcmp (name, "__p4_stack") == 0)
+	pikeos_stack_found = 1;
+
+      if (strcmp (name, "_vm_stack_size") == 0
+	  || strcmp (name, "__p4_stack_end") == 0)
+	pikeos_stack_size_found = 1;
+    }
+
+  if (pikeos_stack_found && pikeos_stack_size_found)
+    return GDB_OSABI_PIKEOS;
+  else
+    return GDB_OSABI_UNKNOWN;
+}
+
+void
+_initialize_arm_pikeos_tdep (void)
+{
+  /* Register the sniffer for the PikeOS targets.  */
+  gdbarch_register_osabi_sniffer (bfd_arch_arm, bfd_target_elf_flavour,
+                                  arm_pikeos_osabi_sniffer);
+  gdbarch_register_osabi (bfd_arch_arm, 0, GDB_OSABI_PIKEOS,
+                          arm_pikeos_init_abi);
+}
diff --git a/gdb/configure.tgt b/gdb/configure.tgt
index 5e9bc36..e6ba333 100644
--- a/gdb/configure.tgt
+++ b/gdb/configure.tgt
@@ -180,6 +180,7 @@ arm*-*-symbianelf*)
 	;;
 arm*-*-*)
 	# Target: ARM embedded system
+	gdb_target_obs="arm-pikeos-tdep.o"
 	gdb_sim=../sim/arm/libsim.a
 	;;
 
diff --git a/gdb/defs.h b/gdb/defs.h
index 6e3f4df..9e87971 100644
--- a/gdb/defs.h
+++ b/gdb/defs.h
@@ -498,6 +498,7 @@ enum gdb_osabi
   GDB_OSABI_LYNXOS178,
   GDB_OSABI_NEWLIB,
   GDB_OSABI_SDE,
+  GDB_OSABI_PIKEOS,
 
   GDB_OSABI_INVALID		/* keep this last */
 };
diff --git a/gdb/osabi.c b/gdb/osabi.c
index 7d0540b..68f4665 100644
--- a/gdb/osabi.c
+++ b/gdb/osabi.c
@@ -80,6 +80,7 @@ static const struct osabi_names gdb_osabi_names[] =
   { "LynxOS178", NULL },
   { "Newlib", NULL },
   { "SDE", NULL },
+  { "PikeOS", NULL },
 
   { "<invalid>", NULL }
 };