Fix crash on Python frame filters with unreadable arg

https://bugzilla.redhat.com/show_bug.cgi?id=1126177

ERROR: AddressSanitizer: SEGV on unknown address 0x000000000050 (pc 0x000000992bef sp 0x7ffff9039530 bp 0x7ffff9039540
T0)
    #0 0x992bee in value_type .../gdb/value.c:925
    #1 0x87c951 in py_print_single_arg python/py-framefilter.c:445
    #2 0x87cfae in enumerate_args python/py-framefilter.c:596
    #3 0x87e0b0 in py_print_args python/py-framefilter.c:968

It crashes because frame_arg::val is documented it may contain NULL
(frame_arg::error is then non-NULL) but the code does not handle it.

Another bug is that py_print_single_arg() calls goto out of its TRY_CATCH
which messes up GDB cleanup chain crashing GDB later.

It is probably 7.7 regression (I have not verified it) due to the introduction
of Python frame filters.

gdb/ChangeLog

	PR python/17355
	* python/py-framefilter.c (py_print_single_arg): Handle NULL FA->VAL.
	Fix goto out of TRY_CATCH.

gdb/testsuite/ChangeLog

	PR python/17355
	* gdb.python/amd64-py-framefilter-invalidarg.S: New file.
	* gdb.python/py-framefilter-invalidarg-gdb.py.in: New file.
	* gdb.python/py-framefilter-invalidarg.exp: New file.
	* gdb.python/py-framefilter-invalidarg.py: New file.

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index 8b07584..eb36ec5 100644 (file)
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,3 +1,9 @@
+2014-09-07  Jan Kratochvil  <jan.kratochvil@redhat.com>
+
+       PR python/17355
+       * python/py-framefilter.c (py_print_single_arg): Handle NULL FA->VAL.
+       Fix goto out of TRY_CATCH.
+
 2014-09-06  Doug Evans  <xdje42@gmail.com>
            Tom Tromey  <tromey@redhat.com>
 

diff --git a/gdb/python/py-framefilter.c b/gdb/python/py-framefilter.c
index 9db83c7..d53282f 100644 (file)
--- a/gdb/python/py-framefilter.c
+++ b/gdb/python/py-framefilter.c
@@ -365,9 +365,12 @@ py_print_single_arg (struct ui_out *out,
 {
   struct value *val;
   volatile struct gdb_exception except;
+  enum ext_lang_bt_status retval = EXT_LANG_BT_OK;
 
   if (fa != NULL)
     {
+      if (fa->val == NULL && fa->error == NULL)
+       return EXT_LANG_BT_OK;
       language = language_def (SYMBOL_LANGUAGE (fa->sym));
       val = fa->val;
     }
@@ -433,16 +436,18 @@ py_print_single_arg (struct ui_out *out,
       /* For MI print the type, but only for simple values.  This seems
         weird, but this is how MI choose to format the various output
         types.  */
-      if (args_type == MI_PRINT_SIMPLE_VALUES)
+      if (args_type == MI_PRINT_SIMPLE_VALUES && val != NULL)
        {
          if (py_print_type (out, val) == EXT_LANG_BT_ERROR)
            {
+             retval = EXT_LANG_BT_ERROR;
              do_cleanups (cleanups);
-             goto error;
+             continue;
            }
        }
 
-      annotate_arg_value (value_type (val));
+      if (val != NULL)
+       annotate_arg_value (value_type (val));
 
       /* If the output is to the CLI, and the user option "set print
         frame-arguments" is set to none, just output "...".  */
@@ -454,27 +459,25 @@ py_print_single_arg (struct ui_out *out,
             for the case of MI_PRINT_NO_VALUES.  */
          if (args_type != NO_VALUES)
            {
-             if (py_print_value (out, val, opts, 0, args_type, language)
-                 == EXT_LANG_BT_ERROR)
+             if (val == NULL)
                {
-                 do_cleanups (cleanups);
-                 goto error;
+                 gdb_assert (fa != NULL && fa->error != NULL);
+                 ui_out_field_fmt (out, "value",
+                                   _("<error reading variable: %s>"),
+                                   fa->error);
                }
+             else if (py_print_value (out, val, opts, 0, args_type, language)
+                      == EXT_LANG_BT_ERROR)
+               retval = EXT_LANG_BT_ERROR;
            }
        }
 
       do_cleanups (cleanups);
     }
   if (except.reason < 0)
-    {
-      gdbpy_convert_exception (except);
-      goto error;
-    }
-
-  return EXT_LANG_BT_OK;
+    gdbpy_convert_exception (except);
 
- error:
-  return EXT_LANG_BT_ERROR;
+  return retval;
 }
 
 /* Helper function to loop over frame arguments provided by the

diff --git a/gdb/testsuite/ChangeLog b/gdb/testsuite/ChangeLog
index cc95f5e..a83d2b2 100644 (file)
--- a/gdb/testsuite/ChangeLog
+++ b/gdb/testsuite/ChangeLog
@@ -1,3 +1,11 @@
+2014-09-07  Jan Kratochvil  <jan.kratochvil@redhat.com>
+
+       PR python/17355
+       * gdb.python/amd64-py-framefilter-invalidarg.S: New file.
+       * gdb.python/py-framefilter-invalidarg-gdb.py.in: New file.
+       * gdb.python/py-framefilter-invalidarg.exp: New file.
+       * gdb.python/py-framefilter-invalidarg.py: New file.
+
 2014-09-06  Doug Evans  <xdje42@gmail.com>
 
        PR 15276

diff --git a/gdb/testsuite/gdb.python/amd64-py-framefilter-invalidarg.S b/gdb/testsuite/gdb.python/amd64-py-framefilter-invalidarg.S
new file mode 100644 (file)
index 0000000..3ac1b23
--- /dev/null
+++ b/gdb/testsuite/gdb.python/amd64-py-framefilter-invalidarg.S
@@ -0,0 +1,261 @@
+/* This testcase is part of GDB, the GNU debugger.
+
+   Copyright 2014 Free Software Foundation, Inc.
+
+   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/>.  */
+
+/* This file is compiled from a single line
+   int main (int argc, char **argv) { return 0; }
+   using -g -dA -S -O2 and patched as #if-ed below.  */
+
+       .file   "py-framefilter-invalidarg.c"
+       .text
+.Ltext0:
+       .globl  main
+       .type   main, @function
+main:
+.LFB0:
+       .file 1 "py-framefilter-invalidarg.c"
+       # py-framefilter-invalidarg.c:1
+       .loc 1 1 0
+       .cfi_startproc
+# BLOCK 2 seq:0
+# PRED: ENTRY (FALLTHRU)
+       pushq   %rbp
+       .cfi_def_cfa_offset 16
+       .cfi_offset 6, -16
+       movq    %rsp, %rbp
+       .cfi_def_cfa_register 6
+       movl    %edi, -4(%rbp)
+       movq    %rsi, -16(%rbp)
+       # py-framefilter-invalidarg.c:2
+       .loc 1 2 0
+       movl    $0, %eax
+       # py-framefilter-invalidarg.c:3
+       .loc 1 3 0
+       popq    %rbp
+       .cfi_def_cfa 7, 8
+# SUCC: EXIT [100.0%] 
+       ret
+       .cfi_endproc
+.LFE0:
+       .size   main, .-main
+.Letext0:
+       .section        .debug_info,"",@progbits
+.Ldebug_info0:
+       .long   .Le - .Ls       # Length of Compilation Unit Info
+.Ls:
+       .value  0x4     # DWARF version number
+       .long   .Ldebug_abbrev0 # Offset Into Abbrev. Section
+       .byte   0x8     # Pointer Size (in bytes)
+       .uleb128 0x1    # (DIE (0xb) DW_TAG_compile_unit)
+       .long   .LASF3  # DW_AT_producer: "GNU C 4.9.1 20140813 (Red Hat 4.9.1-7) -mtune=generic -march=x86-64 -g"
+       .byte   0x1     # DW_AT_language
+       .long   .LASF4  # DW_AT_name: "py-framefilter-invalidarg.c"
+       .long   .LASF5  # DW_AT_comp_dir: ""
+       .quad   .Ltext0 # DW_AT_low_pc
+       .quad   .Letext0-.Ltext0        # DW_AT_high_pc
+       .long   .Ldebug_line0   # DW_AT_stmt_list
+die2d:
+       .uleb128 0x2    # (DIE (0x2d) DW_TAG_subprogram)
+                       # DW_AT_external
+       .long   .LASF6  # DW_AT_name: "main"
+       .byte   0x1     # DW_AT_decl_file (py-framefilter-invalidarg.c)
+       .byte   0x1     # DW_AT_decl_line
+                       # DW_AT_prototyped
+       .long   die6b-.Ldebug_info0     # DW_AT_type
+       .quad   .LFB0   # DW_AT_low_pc
+       .quad   .LFE0-.LFB0     # DW_AT_high_pc
+       .uleb128 0x1    # DW_AT_frame_base
+       .byte   0x9c    # DW_OP_call_frame_cfa
+                       # DW_AT_GNU_all_call_sites
+die4e:
+       .uleb128 0x3    # (DIE (0x4e) DW_TAG_formal_parameter)
+       .long   .LASF0  # DW_AT_name: "argc"
+       .byte   0x1     # DW_AT_decl_file (py-framefilter-invalidarg.c)
+       .byte   0x1     # DW_AT_decl_line
+       .long   die6b-.Ldebug_info0     # DW_AT_type
+#if 0
+       .uleb128 0x2    # DW_AT_location
+       .byte   0x91    # DW_OP_fbreg
+       .sleb128 -20
+#endif
+#if 0
+       .uleb128 1f - 2f        # DW_AT_location
+2:
+       .byte   0x03    # DW_OP_addr
+       .quad 0
+1:
+#endif
+#if 1
+       .uleb128 1f - 2f        # DW_AT_location
+2:
+       .byte   0x13    # DW_OP_drop
+       .quad 0
+1:
+#endif
+die5c:
+       .uleb128 0x3    # (DIE (0x5c) DW_TAG_formal_parameter)
+       .long   .LASF1  # DW_AT_name: "argv"
+       .byte   0x1     # DW_AT_decl_file (py-framefilter-invalidarg.c)
+       .byte   0x1     # DW_AT_decl_line
+       .long   die72-.Ldebug_info0     # DW_AT_type
+       .uleb128 0x2    # DW_AT_location
+       .byte   0x91    # DW_OP_fbreg
+       .sleb128 -32
+       .byte   0       # end of children of DIE 0x2d
+die6b:
+       .uleb128 0x4    # (DIE (0x6b) DW_TAG_base_type)
+       .byte   0x4     # DW_AT_byte_size
+       .byte   0x5     # DW_AT_encoding
+       .ascii "int\0"  # DW_AT_name
+die72:
+       .uleb128 0x5    # (DIE (0x72) DW_TAG_pointer_type)
+       .byte   0x8     # DW_AT_byte_size
+       .long   die78-.Ldebug_info0     # DW_AT_type
+die78:
+       .uleb128 0x5    # (DIE (0x78) DW_TAG_pointer_type)
+       .byte   0x8     # DW_AT_byte_size
+       .long   die7e-.Ldebug_info0     # DW_AT_type
+die7e:
+       .uleb128 0x6    # (DIE (0x7e) DW_TAG_base_type)
+       .byte   0x1     # DW_AT_byte_size
+       .byte   0x6     # DW_AT_encoding
+       .long   .LASF2  # DW_AT_name: "char"
+       .byte   0       # end of children of DIE 0xb
+.Le:
+       .section        .debug_abbrev,"",@progbits
+.Ldebug_abbrev0:
+       .uleb128 0x1    # (abbrev code)
+       .uleb128 0x11   # (TAG: DW_TAG_compile_unit)
+       .byte   0x1     # DW_children_yes
+       .uleb128 0x25   # (DW_AT_producer)
+       .uleb128 0xe    # (DW_FORM_strp)
+       .uleb128 0x13   # (DW_AT_language)
+       .uleb128 0xb    # (DW_FORM_data1)
+       .uleb128 0x3    # (DW_AT_name)
+       .uleb128 0xe    # (DW_FORM_strp)
+       .uleb128 0x1b   # (DW_AT_comp_dir)
+       .uleb128 0xe    # (DW_FORM_strp)
+       .uleb128 0x11   # (DW_AT_low_pc)
+       .uleb128 0x1    # (DW_FORM_addr)
+       .uleb128 0x12   # (DW_AT_high_pc)
+       .uleb128 0x7    # (DW_FORM_data8)
+       .uleb128 0x10   # (DW_AT_stmt_list)
+       .uleb128 0x17   # (DW_FORM_sec_offset)
+       .byte   0
+       .byte   0
+       .uleb128 0x2    # (abbrev code)
+       .uleb128 0x2e   # (TAG: DW_TAG_subprogram)
+       .byte   0x1     # DW_children_yes
+       .uleb128 0x3f   # (DW_AT_external)
+       .uleb128 0x19   # (DW_FORM_flag_present)
+       .uleb128 0x3    # (DW_AT_name)
+       .uleb128 0xe    # (DW_FORM_strp)
+       .uleb128 0x3a   # (DW_AT_decl_file)
+       .uleb128 0xb    # (DW_FORM_data1)
+       .uleb128 0x3b   # (DW_AT_decl_line)
+       .uleb128 0xb    # (DW_FORM_data1)
+       .uleb128 0x27   # (DW_AT_prototyped)
+       .uleb128 0x19   # (DW_FORM_flag_present)
+       .uleb128 0x49   # (DW_AT_type)
+       .uleb128 0x13   # (DW_FORM_ref4)
+       .uleb128 0x11   # (DW_AT_low_pc)
+       .uleb128 0x1    # (DW_FORM_addr)
+       .uleb128 0x12   # (DW_AT_high_pc)
+       .uleb128 0x7    # (DW_FORM_data8)
+       .uleb128 0x40   # (DW_AT_frame_base)
+       .uleb128 0x18   # (DW_FORM_exprloc)
+       .uleb128 0x2117 # (DW_AT_GNU_all_call_sites)
+       .uleb128 0x19   # (DW_FORM_flag_present)
+       .byte   0
+       .byte   0
+       .uleb128 0x3    # (abbrev code)
+       .uleb128 0x5    # (TAG: DW_TAG_formal_parameter)
+       .byte   0       # DW_children_no
+       .uleb128 0x3    # (DW_AT_name)
+       .uleb128 0xe    # (DW_FORM_strp)
+       .uleb128 0x3a   # (DW_AT_decl_file)
+       .uleb128 0xb    # (DW_FORM_data1)
+       .uleb128 0x3b   # (DW_AT_decl_line)
+       .uleb128 0xb    # (DW_FORM_data1)
+       .uleb128 0x49   # (DW_AT_type)
+       .uleb128 0x13   # (DW_FORM_ref4)
+       .uleb128 0x2    # (DW_AT_location)
+       .uleb128 0x18   # (DW_FORM_exprloc)
+       .byte   0
+       .byte   0
+       .uleb128 0x4    # (abbrev code)
+       .uleb128 0x24   # (TAG: DW_TAG_base_type)
+       .byte   0       # DW_children_no
+       .uleb128 0xb    # (DW_AT_byte_size)
+       .uleb128 0xb    # (DW_FORM_data1)
+       .uleb128 0x3e   # (DW_AT_encoding)
+       .uleb128 0xb    # (DW_FORM_data1)
+       .uleb128 0x3    # (DW_AT_name)
+       .uleb128 0x8    # (DW_FORM_string)
+       .byte   0
+       .byte   0
+       .uleb128 0x5    # (abbrev code)
+       .uleb128 0xf    # (TAG: DW_TAG_pointer_type)
+       .byte   0       # DW_children_no
+       .uleb128 0xb    # (DW_AT_byte_size)
+       .uleb128 0xb    # (DW_FORM_data1)
+       .uleb128 0x49   # (DW_AT_type)
+       .uleb128 0x13   # (DW_FORM_ref4)
+       .byte   0
+       .byte   0
+       .uleb128 0x6    # (abbrev code)
+       .uleb128 0x24   # (TAG: DW_TAG_base_type)
+       .byte   0       # DW_children_no
+       .uleb128 0xb    # (DW_AT_byte_size)
+       .uleb128 0xb    # (DW_FORM_data1)
+       .uleb128 0x3e   # (DW_AT_encoding)
+       .uleb128 0xb    # (DW_FORM_data1)
+       .uleb128 0x3    # (DW_AT_name)
+       .uleb128 0xe    # (DW_FORM_strp)
+       .byte   0
+       .byte   0
+       .byte   0
+       .section        .debug_aranges,"",@progbits
+       .long   0x2c    # Length of Address Ranges Info
+       .value  0x2     # DWARF Version
+       .long   .Ldebug_info0   # Offset of Compilation Unit Info
+       .byte   0x8     # Size of Address
+       .byte   0       # Size of Segment Descriptor
+       .value  0       # Pad to 16 byte boundary
+       .value  0
+       .quad   .Ltext0 # Address
+       .quad   .Letext0-.Ltext0        # Length
+       .quad   0
+       .quad   0
+       .section        .debug_line,"",@progbits
+.Ldebug_line0:
+       .section        .debug_str,"MS",@progbits,1
+.LASF1:
+       .string "argv"
+.LASF4:
+       .string "py-framefilter-invalidarg.c"
+.LASF5:
+       .string ""
+.LASF0:
+       .string "argc"
+.LASF3:
+       .string "GNU C 4.9.1 20140813 (Red Hat 4.9.1-7) -mtune=generic -march=x86-64 -g"
+.LASF6:
+       .string "main"
+.LASF2:
+       .string "char"
+       .ident  "GCC: (GNU) 4.9.1 20140813 (Red Hat 4.9.1-7)"
+       .section        .note.GNU-stack,"",@progbits

diff --git a/gdb/testsuite/gdb.python/py-framefilter-invalidarg-gdb.py.in b/gdb/testsuite/gdb.python/py-framefilter-invalidarg-gdb.py.in
new file mode 100644 (file)
index 0000000..1fa6ffc
--- /dev/null
+++ b/gdb/testsuite/gdb.python/py-framefilter-invalidarg-gdb.py.in
@@ -0,0 +1,48 @@
+# Copyright (C) 2014 Free Software Foundation, Inc.
+
+# 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/>.
+
+# This file is part of the GDB testsuite.  It tests Python-based
+# frame-filters.
+import gdb
+import itertools
+from gdb.FrameDecorator import FrameDecorator
+
+
+class FrameObjFile ():
+
+    def __init__ (self):
+        self.name = "Filter1"
+        self.priority = 1
+        self.enabled = False
+        gdb.current_progspace().frame_filters ["Progspace" + self.name] = self
+        gdb.current_objfile().frame_filters ["ObjectFile" + self.name] = self
+
+    def filter (self, frame_iter):
+        return frame_iter
+
+class FrameObjFile2 ():
+
+    def __init__ (self):
+        self.name = "Filter2"
+        self.priority = 100
+        self.enabled = True
+        gdb.current_progspace().frame_filters ["Progspace" + self.name] = self
+        gdb.current_objfile().frame_filters ["ObjectFile" + self.name] = self
+
+    def filter (self, frame_iter):
+        return frame_iter
+
+FrameObjFile()
+FrameObjFile2()

diff --git a/gdb/testsuite/gdb.python/py-framefilter-invalidarg.exp b/gdb/testsuite/gdb.python/py-framefilter-invalidarg.exp
new file mode 100644 (file)
index 0000000..f70d16e
--- /dev/null
+++ b/gdb/testsuite/gdb.python/py-framefilter-invalidarg.exp
@@ -0,0 +1,67 @@
+# Copyright (C) 2014 Free Software Foundation, Inc.
+
+# 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/>.
+
+load_lib gdb-python.exp
+
+standard_testfile amd64-py-framefilter-invalidarg.S
+
+if { ![istarget x86_64-*-* ] || ![is_lp64_target] } {
+    verbose "Skipping py-framefilter-invalidarg."
+    return
+}
+
+# We cannot use prepare_for_testing as we have to set the safe-patch
+# to check objfile and progspace printers.
+if {[build_executable $testfile.exp $testfile $srcfile {}] == -1} {
+    return -1
+}
+
+# Start with a fresh gdb.
+gdb_exit
+gdb_start
+
+# Skip all tests if Python scripting is not enabled.
+if { [skip_python_tests] } { continue }
+
+# Make the -gdb.py script available to gdb, it is automagically loaded by gdb.
+# Care is taken to put it in the same directory as the binary so that
+# gdb will find it.
+set remote_obj_python_file \
+    [remote_download \
+        host ${srcdir}/${subdir}/${testfile}-gdb.py.in \
+        [standard_output_file ${testfile}-gdb.py]]
+
+gdb_reinitialize_dir $srcdir/$subdir
+gdb_test_no_output "set auto-load safe-path ${remote_obj_python_file}" \
+    "set auto-load safe-path"
+gdb_load ${binfile}
+# Verify gdb loaded the script.
+gdb_test "info auto-load python-scripts" "Yes.*/${testfile}-gdb.py.*" \
+    "Test auto-load had loaded python scripts"
+
+if ![runto_main] then {
+    perror "couldn't run to breakpoint"
+    return
+}
+gdb_test_no_output "set python print-stack full" \
+    "Set python print-stack to full"
+
+# Load global frame-filters
+set remote_python_file [gdb_remote_download host \
+                           ${srcdir}/${subdir}/${testfile}.py]
+gdb_test_no_output "python exec (open ('${remote_python_file}').read ())" \
+    "Load python file"
+
+gdb_test "bt" " in niam \\(argc=<error reading variable: dwarf expression stack underflow>, argv=0x\[0-9a-f\]+\\) at py-framefilter-invalidarg.c:\[0-9\]+" "bt full with filters"

diff --git a/gdb/testsuite/gdb.python/py-framefilter-invalidarg.py b/gdb/testsuite/gdb.python/py-framefilter-invalidarg.py
new file mode 100644 (file)
index 0000000..d5f92cb
--- /dev/null
+++ b/gdb/testsuite/gdb.python/py-framefilter-invalidarg.py
@@ -0,0 +1,59 @@
+# Copyright (C) 2014 Free Software Foundation, Inc.
+
+# 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/>.
+
+# This file is part of the GDB testsuite.  It tests Python-based
+# frame-filters.
+import gdb
+import itertools
+from gdb.FrameDecorator import FrameDecorator
+import copy
+
+class Reverse_Function (FrameDecorator):
+
+    def __init__(self, fobj):
+        super(Reverse_Function, self).__init__(fobj)
+        self.fobj = fobj
+
+    def function (self):
+        fname = str (self.fobj.function())
+        if (fname == None or fname == ""):
+            return None
+        if fname == 'end_func':
+            extra = self.fobj.inferior_frame().read_var('str').string()
+        else:
+            extra = ''
+        fname = fname[::-1] + extra
+        return fname
+
+class FrameFilter ():
+
+    def __init__ (self):
+        self.name = "Reverse"
+        self.priority = 100
+        self.enabled = True
+        gdb.frame_filters [self.name] = self
+
+    def filter (self, frame_iter):
+        # Python 3.x moved the itertools.imap functionality to map(),
+        # so check if it is available.
+        if hasattr(itertools, "imap"):
+            frame_iter = itertools.imap (Reverse_Function,
+                                         frame_iter)
+        else:
+            frame_iter = map(Reverse_Function, frame_iter)
+
+        return frame_iter
+
+FrameFilter()