[PATCH v3 1/3] btrace: control memory access during replay

[Markus Metzger <markus dot t dot metzger at intel dot com>]

The btrace record target does not trace data.  We therefore do not allow
accessing read-write memory during replay.

In some cases, this might be useful to advanced users, though, who we assume
to know what they are doing.

Add a set|show command pair to turn this memory access restriction off.

CC: Eli Zaretskii <eliz@gnu.org>

2014-05-21  Markus Metzger  <markus.t.metzger@intel.com>

	* record-btrace.c (record_btrace_allow_memory_access): Remove.
	(replay_memory_access_read_only, replay_memory_access_read_write)
	(replay_memory_access_types, replay_memory_access)
	(set_record_btrace_cmdlist, show_record_btrace_cmdlist)
	(cmd_set_record_btrace, cmd_show_record_btrace)
	(cmd_show_replay_memory_access): New.
	(record_btrace_xfer_partial, record_btrace_insert_breakpoint)
	(record_btrace_remove_breakpoint): Replace
	record_btrace_allow_memory_access with replay_memory_access.
	(_initialize_record_btrace): Add commands.
	* NEWS: Announce it.

testsuite/
	* gdb.btrace/data.exp: Test it.

doc/
	* gdb.texinfo (Process Record and Replay): Document it.


---
 gdb/NEWS                          |  4 ++
 gdb/doc/gdb.texinfo               | 22 ++++++++++
 gdb/record-btrace.c               | 88 ++++++++++++++++++++++++++++++++++-----
 gdb/testsuite/gdb.btrace/data.exp | 10 ++++-
 4 files changed, 112 insertions(+), 12 deletions(-)

diff --git a/gdb/NEWS b/gdb/NEWS
index b23c8a0..62e0adc 100644
--- a/gdb/NEWS
+++ b/gdb/NEWS
@@ -61,6 +61,10 @@ maint ada show ignore-descriptive-types
   the user manual for more details on descriptive types and the intended
   usage of this option.
 
+set record btrace replay-memory-access (read-only|read-write)
+show record btrace replay-memory-access
+  Control what memory accesses are allowed during replay.
+
 * New features in the GDB remote stub, GDBserver
 
   ** New option --debug-format=option1[,option2,...] allows one to add
diff --git a/gdb/doc/gdb.texinfo b/gdb/doc/gdb.texinfo
index 6092ff4..b9e7964 100644
--- a/gdb/doc/gdb.texinfo
+++ b/gdb/doc/gdb.texinfo
@@ -6400,6 +6400,28 @@ results.
 @item show record full memory-query
 Show the current setting of @code{memory-query}.
 
+@kindex set record btrace
+The @code{btrace} record target does not trace data.  As a
+convenience, when replaying, @value{GDBN} reads read-only memory off
+the live program directly, assuming that the addresses of the
+read-only areas don't change.  This for example makes it possible to
+disassemble code while replaying, but not to print variables.
+In some cases, being able to inspect variables might be useful.
+You can use the following command for that:
+
+@item set record btrace replay-memory-access
+Control the behavior of the @code{btrace} recording method when
+accessing memory during replay.  If @code{read-only} (the default),
+@value{GDBN} will only allow accesses to read-only memory.
+If @code{read-write}, @value{GDBN} will allow accesses to read-only
+and to read-write memory.  Beware that the accessed memory corresponds
+to the live target and not necessarily to the current replay
+position.
+
+@kindex show record btrace
+@item show record btrace replay-memory-access
+Show the current setting of @code{replay-memory-access}.
+
 @kindex info record
 @item info record
 Show various statistics about the recording depending on the recording
diff --git a/gdb/record-btrace.c b/gdb/record-btrace.c
index d768225..f43436d 100644
--- a/gdb/record-btrace.c
+++ b/gdb/record-btrace.c
@@ -42,8 +42,22 @@ static struct target_ops record_btrace_ops;
 /* A new thread observer enabling branch tracing for the new thread.  */
 static struct observer *record_btrace_thread_observer;
 
-/* Temporarily allow memory accesses.  */
-static int record_btrace_allow_memory_access;
+/* Memory access types used in set/show record btrace replay-memory-access.  */
+static const char replay_memory_access_read_only[] = "read-only";
+static const char replay_memory_access_read_write[] = "read-write";
+static const char *const replay_memory_access_types[] =
+{
+  replay_memory_access_read_only,
+  replay_memory_access_read_write,
+  NULL
+};
+
+/* The currently allowed replay memory access type.  */
+static const char *replay_memory_access = replay_memory_access_read_only;
+
+/* Command lists for "set/show record btrace".  */
+static struct cmd_list_element *set_record_btrace_cmdlist;
+static struct cmd_list_element *show_record_btrace_cmdlist;
 
 /* Print a record-btrace debug message.  Use do ... while (0) to avoid
    ambiguities when used in if statements.  */
@@ -815,7 +829,8 @@ record_btrace_xfer_partial (struct target_ops *ops, enum target_object object,
   struct target_ops *t;
 
   /* Filter out requests that don't make sense during replay.  */
-  if (!record_btrace_allow_memory_access && record_btrace_is_replaying (ops))
+  if (replay_memory_access == replay_memory_access_read_only
+      && record_btrace_is_replaying (ops))
     {
       switch (object)
 	{
@@ -869,18 +884,19 @@ record_btrace_insert_breakpoint (struct target_ops *ops,
 				 struct bp_target_info *bp_tgt)
 {
   volatile struct gdb_exception except;
-  int old, ret;
+  const char *old;
+  int ret;
 
   /* Inserting breakpoints requires accessing memory.  Allow it for the
      duration of this function.  */
-  old = record_btrace_allow_memory_access;
-  record_btrace_allow_memory_access = 1;
+  old = replay_memory_access;
+  replay_memory_access = replay_memory_access_read_write;
 
   ret = 0;
   TRY_CATCH (except, RETURN_MASK_ALL)
     ret = ops->beneath->to_insert_breakpoint (ops->beneath, gdbarch, bp_tgt);
 
-  record_btrace_allow_memory_access = old;
+  replay_memory_access = old;
 
   if (except.reason < 0)
     throw_exception (except);
@@ -896,18 +912,19 @@ record_btrace_remove_breakpoint (struct target_ops *ops,
 				 struct bp_target_info *bp_tgt)
 {
   volatile struct gdb_exception except;
-  int old, ret;
+  const char *old;
+  int ret;
 
   /* Removing breakpoints requires accessing memory.  Allow it for the
      duration of this function.  */
-  old = record_btrace_allow_memory_access;
-  record_btrace_allow_memory_access = 1;
+  old = replay_memory_access;
+  replay_memory_access = replay_memory_access_read_write;
 
   ret = 0;
   TRY_CATCH (except, RETURN_MASK_ALL)
     ret = ops->beneath->to_remove_breakpoint (ops->beneath, gdbarch, bp_tgt);
 
-  record_btrace_allow_memory_access = old;
+  replay_memory_access = old;
 
   if (except.reason < 0)
     throw_exception (except);
@@ -1938,6 +1955,32 @@ cmd_record_btrace_start (char *args, int from_tty)
   execute_command ("target record-btrace", from_tty);
 }
 
+/* The "set record btrace" command.  */
+
+static void
+cmd_set_record_btrace (char *args, int from_tty)
+{
+  cmd_show_list (set_record_btrace_cmdlist, from_tty, "");
+}
+
+/* The "show record btrace" command.  */
+
+static void
+cmd_show_record_btrace (char *args, int from_tty)
+{
+  cmd_show_list (show_record_btrace_cmdlist, from_tty, "");
+}
+
+/* The "show record btrace replay-memory-access" command.  */
+
+static void
+cmd_show_replay_memory_access (struct ui_file *file, int from_tty,
+			       struct cmd_list_element *c, const char *value)
+{
+  fprintf_filtered (gdb_stdout, _("Replay memory access is %s.\n"),
+		    replay_memory_access);
+}
+
 void _initialize_record_btrace (void);
 
 /* Initialize btrace commands.  */
@@ -1950,6 +1993,29 @@ _initialize_record_btrace (void)
 	   &record_cmdlist);
   add_alias_cmd ("b", "btrace", class_obscure, 1, &record_cmdlist);
 
+  add_prefix_cmd ("btrace", class_support, cmd_set_record_btrace,
+		  _("Set record options"), &set_record_btrace_cmdlist,
+		  "set record btrace ", 0, &set_record_cmdlist);
+
+  add_prefix_cmd ("btrace", class_support, cmd_show_record_btrace,
+		  _("Show record options"), &show_record_btrace_cmdlist,
+		  "show record btrace ", 0, &show_record_cmdlist);
+
+  add_setshow_enum_cmd ("replay-memory-access", no_class,
+			replay_memory_access_types, &replay_memory_access, _("\
+Set what memory accesses are allowed during replay."), _("\
+Show what memory accesses are allowed during replay."),
+			   _("Default is READ-ONLY.\n\n\
+The btrace record target does not trace data.\n\
+The memory therefore corresponds to the live target and not \
+to the current replay position.\n\n\
+When READ-ONLY, allow accesses to read-only memory during replay.\n\
+When READ-WRITE, allow accesses to read-only and read-write memory during \
+replay."),
+			   NULL, cmd_show_replay_memory_access,
+			   &set_record_btrace_cmdlist,
+			   &show_record_btrace_cmdlist);
+
   init_record_btrace_ops ();
   add_target (&record_btrace_ops);
 
diff --git a/gdb/testsuite/gdb.btrace/data.exp b/gdb/testsuite/gdb.btrace/data.exp
index 64c5443..91d8d8b 100644
--- a/gdb/testsuite/gdb.btrace/data.exp
+++ b/gdb/testsuite/gdb.btrace/data.exp
@@ -40,6 +40,14 @@ gdb_test "reverse-step" ".*test\.4.*"
 gdb_test "print glob" "unavailable\[^\\\r\\\n\]*"
 gdb_test "print loc" "unavailable\[^\\\r\\\n\]*"
 
+# we can read memory if we explicitly allow it.
+gdb_test_no_output "set record btrace replay-memory-access read-write"
+gdb_test "print glob" "1"
+
+# we can't if we don't explicitly allow it.
+gdb_test_no_output "set record btrace replay-memory-access read-only"
+gdb_test "print glob" "unavailable\[^\\\r\\\n\]*"
+
 # stop replaying and try again
-gdb_test "record goto end"
+gdb_test "record goto end" ".*main\.3.*"
 gdb_test "print glob" "1"
-- 
1.8.3.1