[rfc] btrace: control memory access during replay

[Markus Metzger]

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>

2014-04-04  Markus Metzger  <markus.t.metzger@intel.com>

	* record-btrace.c (set_record_btrace_cmdlist)
	(show_record_btrace_cmdlist, cmd_set_record_btrace)
	(cmd_show_record_btrace): New.
	(_initialize_record_btrace): Add commands.
	* NEWS: Announce it.

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

doc/
	* gdb.texinfo: Document it.


---
 gdb/NEWS                          |  4 ++++
 gdb/doc/gdb.texinfo               | 13 ++++++++++++
 gdb/record-btrace.c               | 43 +++++++++++++++++++++++++++++++++++++++
 gdb/testsuite/gdb.btrace/data.exp |  8 ++++++++
 4 files changed, 68 insertions(+)

diff --git a/gdb/NEWS b/gdb/NEWS
index df233fc..dbd6889 100644
--- a/gdb/NEWS
+++ b/gdb/NEWS
@@ -50,6 +50,10 @@ 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 allow-memory-access (on|off)
+show record btrace allow-memory-access
+  Control access to read-write memory 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 b218769..08ebb74 100644
--- a/gdb/doc/gdb.texinfo
+++ b/gdb/doc/gdb.texinfo
@@ -6398,6 +6398,19 @@ results.
 @item show record full memory-query
 Show the current setting of @code{memory-query}.
 
+@kindex set record btrace
+@item set record btrace allow-memory-access
+Control the behavior of the @code{btrace} recording method when
+accessing memory during replay.  If ON, @value{GDBN} will allow
+arbitrary memory accesses.  The accessed memory corresponds to the end
+of the recorded execution trace.  It does not necessarily correspond
+to the current replay position.  If OFF (the default), @value{GDBN}
+will only allow accesses to read-only memory.
+
+@kindex show record btrace
+@item show record btrace allow-memory-access
+Show the current setting of @code{allow-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 bcac165..9cf1ff8 100644
--- a/gdb/record-btrace.c
+++ b/gdb/record-btrace.c
@@ -45,6 +45,10 @@ static struct observer *record_btrace_thread_observer;
 /* Temporarily allow memory accesses.  */
 static int record_btrace_allow_memory_access;
 
+/* 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.  */
 
@@ -1934,6 +1938,22 @@ 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, "");
+}
+
 void _initialize_record_btrace (void);
 
 /* Initialize btrace commands.  */
@@ -1946,6 +1966,29 @@ _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);
+
+  /* Record instructions number limit command.  */
+  add_setshow_boolean_cmd ("allow-memory-access", no_class,
+			   &record_btrace_allow_memory_access, _("\
+Set whether memory accesses are allowed during replay."), _("\
+Show whether memory accesses are allowed during replay."),
+			   _("Default is OFF.\n\n\
+The btrace record target does not trace data.\n\
+The memory therefore corresponds to the end of the trace and not \
+to the current replay position.\n\n\
+When ON, allow accesses to arbitrary memory during replay.\n\
+When OFF, only allow accesses to read-only memory during replay."),
+			   NULL, NULL,
+			   &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..cfb1395 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 allow-memory-access on"
+gdb_test "print glob" "1"
+
+# we can't if we don't explicitly allow it.
+gdb_test_no_output "set record btrace allow-memory-access off"
+gdb_test "print glob" "unavailable\[^\\\r\\\n\]*"
+
 # stop replaying and try again
 gdb_test "record goto end"
 gdb_test "print glob" "1"
-- 
1.8.3.1