[PINGv3] [PATCH v2] gdb/stack.c: avoid stale pointers when printing frame arguments

[Bruno Larsen <blarsen@redhat.com>]

ping

Cheers!
Bruno Larsen

On 5/24/22 08:25, Bruno Larsen wrote:
> ping
> 
> Cheers!
> Bruno Larsen
> 
> On 5/18/22 08:24, Bruno Larsen wrote:
>> ping
>>
>> Cheers!
>> Bruno Larsen
>>
>> On 5/4/22 16:10, Bruno Larsen wrote:
>>> When a pretty printer calls an inferior function by hand, the whole
>>> framestack cache has to be rebuilt for that function call. However, if
>>> this pretty printer is called when printing the arguments of a frame,
>>> there are many instances where frame_info pointers were kept and end up
>>> stale, or used after free. This could snowball into an infinite
>>> recursion and GDB crash. The problem was documented as PR python/28856.
>>>
>>> To fix this problem, we must ensure that the frame cache is up to date
>>> after printing any arguments, so we cache the frame_id of the pointer
>>> that will become stale, and reset that pointer using frame_find_by_id.
>>>
>>> This commit also adds a testcase that exercises this codepath with 7
>>> different triggers, run, continue, step, backtrace, finish, up and down.
>>> Some of them can seem to be testing the same thing twice, but since this
>>> test relies on stale pointers, there is always a chance that GDB got lucky
>>> when testing, so better to test extra.
>>>
>>> Regression tested on x86_64, using both gcc and clang.
>>>
>>> ---
>>> Changelog:
>>> v2:
>>>   * Addressed Tom Tromey's and Andrew's comments
>>>   * Added the testcase to this patch
>>>   * Slightly changed "finish" test, to deal better with clang
>>>
>>> ---
>>>   gdb/infcall.c                                 |  13 +-
>>>   gdb/infcmd.c                                  |   5 +
>>>   gdb/mi/mi-cmd-stack.c                         |   6 +
>>>   gdb/stack.c                                   |  31 +++-
>>>   .../gdb.python/pretty-print-call-by-hand.c    |  53 +++++++
>>>   .../gdb.python/pretty-print-call-by-hand.exp  | 136 ++++++++++++++++++
>>>   .../gdb.python/pretty-print-call-by-hand.py   |  41 ++++++
>>>   7 files changed, 280 insertions(+), 5 deletions(-)
>>>   create mode 100644 gdb/testsuite/gdb.python/pretty-print-call-by-hand.c
>>>   create mode 100644 gdb/testsuite/gdb.python/pretty-print-call-by-hand.exp
>>>   create mode 100644 gdb/testsuite/gdb.python/pretty-print-call-by-hand.py
>>>
>>> diff --git a/gdb/infcall.c b/gdb/infcall.c
>>> index 5365f97049c..ad31a910da5 100644
>>> --- a/gdb/infcall.c
>>> +++ b/gdb/infcall.c
>>> @@ -738,8 +738,17 @@ call_function_by_hand (struct value *function,
>>>                  type *default_return_type,
>>>                  gdb::array_view<value *> args)
>>>   {
>>> -  return call_function_by_hand_dummy (function, default_return_type,
>>> -                      args, NULL, NULL);
>>> +  scoped_restore_selected_frame restore_frame;
>>> +  struct value *ret = call_function_by_hand_dummy (function,
>>> +                           default_return_type,
>>> +                           args, NULL, NULL);
>>> +  /* Ensure that the frame cache does not contain any frames that were
>>> +     specific to the handmade function call.  If something is leftover
>>> +     while GDB has kept a reference to a frame (for instance, when a
>>> +     stacktrace is being printed and a pretty printed called an inferior
>>> +     function), GDB could crash.  */
>>> +  reinit_frame_cache ();
>>> +  return ret;
>>>   }
>>>   /* All this stuff with a dummy frame may seem unnecessarily complicated
>>> diff --git a/gdb/infcmd.c b/gdb/infcmd.c
>>> index e909d4d4c81..380d5f0201b 100644
>>> --- a/gdb/infcmd.c
>>> +++ b/gdb/infcmd.c
>>> @@ -1853,6 +1853,7 @@ finish_command (const char *arg, int from_tty)
>>>        source.  */
>>>     if (from_tty)
>>>       {
>>> +      struct frame_id id = get_frame_id (get_selected_frame (NULL));
>>>         if (execution_direction == EXEC_REVERSE)
>>>       gdb_printf (_("Run back to call of "));
>>>         else
>>> @@ -1866,6 +1867,10 @@ finish_command (const char *arg, int from_tty)
>>>       }
>>>         print_stack_frame (get_selected_frame (NULL), 1, LOCATION, 0);
>>> +      frame = frame_find_by_id (id);
>>> +      if (frame == nullptr)
>>> +      error (_("frames disappeared mid-printing."));
>>> +      frame = get_prev_frame (frame);
>>>       }
>>>     if (execution_direction == EXEC_REVERSE)
>>> diff --git a/gdb/mi/mi-cmd-stack.c b/gdb/mi/mi-cmd-stack.c
>>> index 0fe204dbc66..51c6573e9be 100644
>>> --- a/gdb/mi/mi-cmd-stack.c
>>> +++ b/gdb/mi/mi-cmd-stack.c
>>> @@ -176,10 +176,16 @@ mi_cmd_stack_list_frames (const char *command, char **argv, int argc)
>>>          i++, fi = get_prev_frame (fi))
>>>       {
>>>         QUIT;
>>> +      /* We get the frame ID here because getting it in the for line
>>> +         could result in getting the previous's frame ID instead.  */
>>> +      struct frame_id fid = get_frame_id (fi);
>>>         /* Print the location and the address always, even for level 0.
>>>            If args is 0, don't print the arguments.  */
>>>         print_frame_info (user_frame_print_options,
>>>                   fi, 1, LOC_AND_ADDRESS, 0 /* args */, 0);
>>> +      fi = frame_find_by_id (fid);
>>> +      if (fi == nullptr)
>>> +        error (_("frames disappeared mid-printing."));
>>>       }
>>>       }
>>>   }
>>> diff --git a/gdb/stack.c b/gdb/stack.c
>>> index 71d85985d18..52fc8af3cb9 100644
>>> --- a/gdb/stack.c
>>> +++ b/gdb/stack.c
>>> @@ -364,11 +364,19 @@ print_stack_frame (struct frame_info *frame, int print_level,
>>>     try
>>>       {
>>> +      /* print_frame_info may invalidate FRAME.
>>> +         It's better to use frame_id if we want to set the current sal.  */
>>> +      struct frame_id fid = get_frame_id (frame);
>>>         print_frame_info (user_frame_print_options,
>>>               frame, print_level, print_what, 1 /* print_args */,
>>>               set_current_sal);
>>>         if (set_current_sal)
>>> -    set_current_sal_from_frame (frame);
>>> +        {
>>> +      frame = frame_find_by_id (fid);
>>> +      if (frame == nullptr)
>>> +          error (_("frames disappeared mid-printing."));
>>> +      set_current_sal_from_frame (frame);
>>> +    }
>>>       }
>>>     catch (const gdb_exception_error &e)
>>>       {
>>> @@ -742,6 +750,8 @@ print_frame_args (const frame_print_options &fp_opts,
>>>       = (print_names
>>>          && fp_opts.print_frame_arguments != print_frame_arguments_none);
>>> +  struct frame_id printed_frame = get_frame_id (frame);
>>> +
>>>     /* Temporarily change the selected frame to the given FRAME.
>>>        This allows routines that rely on the selected frame instead
>>>        of being given a frame as parameter to use the correct frame.  */
>>> @@ -902,6 +912,9 @@ print_frame_args (const frame_print_options &fp_opts,
>>>           }
>>>         first = 0;
>>> +      frame = frame_find_by_id (printed_frame);
>>> +      if (frame == nullptr)
>>> +        error (_("frames disappeared mid-printing."));
>>>       }
>>>       }
>>> @@ -1109,12 +1122,21 @@ print_frame_info (const frame_print_options &fp_opts,
>>>        to get the line containing FRAME->pc.  */
>>>     symtab_and_line sal = find_frame_sal (frame);
>>> +  /* print_frame can invalidate frame, so cache the frame_id to rebuild
>>> +     the whole stack later, if needed.  */
>>> +  struct frame_id frame_id = get_frame_id (frame);
>>> +
>>>     location_print = (print_what == LOCATION
>>>               || print_what == SRC_AND_LOC
>>>               || print_what == LOC_AND_ADDRESS
>>>               || print_what == SHORT_LOCATION);
>>>     if (location_print || !sal.symtab)
>>> -    print_frame (fp_opts, frame, print_level, print_what, print_args, sal);
>>> +    {
>>> +      print_frame (fp_opts, frame, print_level, print_what, print_args, sal);
>>> +      frame = frame_find_by_id (frame_id);
>>> +      if (frame == nullptr)
>>> +      error (_("frames disappeared mid-printing."));
>>> +    }
>>>     source_print = (print_what == SRC_LINE || print_what == SRC_AND_LOC);
>>> @@ -2060,6 +2082,7 @@ backtrace_command_1 (const frame_print_options &fp_opts,
>>>         for (fi = trailing; fi && count--; fi = get_prev_frame (fi))
>>>       {
>>>         QUIT;
>>> +      struct frame_id frame_id = get_frame_id (fi);
>>>         /* Don't use print_stack_frame; if an error() occurs it probably
>>>            means further attempts to backtrace would fail (on the other
>>> @@ -2067,9 +2090,11 @@ backtrace_command_1 (const frame_print_options &fp_opts,
>>>            the frame->prev field gets set to NULL in that case).  */
>>>         print_frame_info (fp_opts, fi, 1, LOCATION, 1, 0);
>>> +      fi = frame_find_by_id (frame_id);
>>> +      if (fi == nullptr)
>>> +        error (_("frames disappeared mid-printing."));
>>>         if ((flags & PRINT_LOCALS) != 0)
>>>           {
>>> -          struct frame_id frame_id = get_frame_id (fi);
>>>             print_frame_local_vars (fi, false, NULL, NULL, 1, gdb_stdout);
>>> diff --git a/gdb/testsuite/gdb.python/pretty-print-call-by-hand.c b/gdb/testsuite/gdb.python/pretty-print-call-by-hand.c
>>> new file mode 100644
>>> index 00000000000..3be5675b096
>>> --- /dev/null
>>> +++ b/gdb/testsuite/gdb.python/pretty-print-call-by-hand.c
>>> @@ -0,0 +1,53 @@
>>> +/* This testcase is part of GDB, the GNU debugger.
>>> +
>>> +   Copyright 2022 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/>.  */
>>> +
>>> +struct mytype
>>> +{
>>> +  char *x;
>>> +};
>>> +
>>> +void
>>> +rec (int i)
>>> +{
>>> +  if (i <= 0)
>>> +    return;
>>> +  rec (i-1);
>>> +}
>>> +
>>> +int
>>> +f ()
>>> +{
>>> +  rec (100);
>>> +  return 2;
>>> +}
>>> +
>>> +void
>>> +g (struct mytype mt, int depth)
>>> +{
>>> +  if (depth <= 0)
>>> +    return; /* TAG: final frame */
>>> +  g (mt, depth - 1); /* TAG: first frame */
>>> +}
>>> +
>>> +int
>>> +main ()
>>> +{
>>> +  struct mytype mt;
>>> +  mt.x = "hello world";
>>> +  g (mt, 10); /* TAG: outside the frame */
>>> +  return 0;
>>> +}
>>> diff --git a/gdb/testsuite/gdb.python/pretty-print-call-by-hand.exp b/gdb/testsuite/gdb.python/pretty-print-call-by-hand.exp
>>> new file mode 100644
>>> index 00000000000..0aeb2218f91
>>> --- /dev/null
>>> +++ b/gdb/testsuite/gdb.python/pretty-print-call-by-hand.exp
>>> @@ -0,0 +1,136 @@
>>> +# Copyright (C) 2022 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 a pretty printer that
>>> +# calls an inferior function by hand, triggering a Use-after-Free bug
>>> +# (PR gdb/28856).
>>> +
>>> +load_lib gdb-python.exp
>>> +
>>> +standard_testfile
>>> +
>>> +# gdb needs to be started here for skip_python_tests to work.
>>> +# prepare_for_testing could be used instead, but it could compile the program
>>> +# unnecessarily, so starting GDB like this is preferable.
>>> +gdb_start
>>> +
>>> +# Skip all tests if Python scripting is not enabled.
>>> +if { [skip_python_tests] } { continue }
>>> +
>>> +if { [prepare_for_testing "failed to prepare" $testfile $srcfile debug] } {
>>> +    untested "failed to compile"
>>> +    return -1
>>> +}
>>> +
>>> +# This proc restarts GDB, makes the inferior reach the desired spot - marked
>>> +# by a comment in the .c file - and turns on the pretty printer for testing.
>>> +# Starting with a new GDB is important because the test may crash GDB.  The
>>> +# return values are here to avoid us trying to test the pretty printer if
>>> +# there was a problem getting to main.
>>> +proc start_test { breakpoint_comment } {
>>> +    global srcdir subdir testfile binfile
>>> +
>>> +    # Start with a fresh gdb.
>>> +    # This is important because the test can crash GDB.
>>> +
>>> +    clean_restart ${binfile}
>>> +
>>> +    if { ![runto_main] } then {
>>> +    untested "couldn't run to breakpoint"
>>> +    return -1
>>> +    }
>>> +
>>> +    # Let GDB get to the return line.
>>> +    gdb_breakpoint [gdb_get_line_number ${breakpoint_comment} ${testfile}.c ]
>>> +    gdb_continue_to_breakpoint ${breakpoint_comment} ".*"
>>> +
>>> +    gdb_test_no_output "set print pretty on" "starting to pretty print"
>>> +
>>> +    set remote_python_file [gdb_remote_download host ${srcdir}/${subdir}/${testfile}.py]
>>> +    gdb_test_no_output "source ${remote_python_file}" "load python file"
>>> +
>>> +    return 0
>>> +}
>>> +
>>> +# Start by testing the "run" command, it can't leverage start_test
>>> +with_test_prefix "run to frame" {
>>> +    if { ![runto_main] } then {
>>> +    untested "couldn't run to main"
>>> +    }
>>> +
>>> +    gdb_test_no_output "set print pretty on" "starting to pretty print"
>>> +
>>> +    set remote_python_file [gdb_remote_download host ${srcdir}/${subdir}/${testfile}.py]
>>> +    gdb_test_no_output "source ${remote_python_file}" "load python file"
>>> +
>>> +    gdb_breakpoint [gdb_get_line_number "TAG: final frame" ${testfile}.c]
>>> +    gdb_continue_to_breakpoint "TAG: final frame" ".*"
>>> +}
>>> +
>>> +# Testing the backtrace command.
>>> +with_test_prefix "frame print" {
>>> +    if { [start_test "TAG: final frame"] == 0 } {
>>> +    gdb_test "backtrace -frame-arguments all" [multi_line \
>>> +    "#0 .*g \\(mt=mytype is .*\\, depth=0\\).*"\
>>> +    "#1 .*g \\(mt=mytype is .*\\, depth=1\\).*"\
>>> +    "#2 .*g \\(mt=mytype is .*\\, depth=2\\).*"\
>>> +    "#3 .*g \\(mt=mytype is .*\\, depth=3\\).*"\
>>> +    "#4 .*g \\(mt=mytype is .*\\, depth=4\\).*"\
>>> +    "#5 .*g \\(mt=mytype is .*\\, depth=5\\).*"\
>>> +    "#6 .*g \\(mt=mytype is .*\\, depth=6\\).*"\
>>> +    "#7 .*g \\(mt=mytype is .*\\, depth=7\\).*"\
>>> +    "#8 .*g \\(mt=mytype is .*\\, depth=8\\).*"\
>>> +    "#9 .*g \\(mt=mytype is .*\\, depth=9\\).*"\
>>> +    "#10 .*g \\(mt=mytype is .*\\, depth=10\\).*"\
>>> +    "#11 .*main \\(\\).*"] \
>>> +    "backtrace test"
>>> +    }
>>> +}
>>> +# Testing the down command.
>>> +with_test_prefix "frame movement down" {
>>> +    if { [start_test "TAG: first frame"] == 0 } {
>>> +    gdb_test "up" [multi_line "#1 .*in main \\(\\) at .*" ".*outside the frame.*"]
>>> +    gdb_test "down" [multi_line "#0\\s+g \\(mt=mytype is .*\\, depth=10\\).*" ".*first frame.*"]
>>> +    }
>>> +}
>>> +
>>> +# Testing the up command.
>>> +with_test_prefix "frame movement up" {
>>> +    if { [start_test "TAG: final frame"] == 0 } {
>>> +    gdb_test "up" [multi_line "#1 .*in g \\(mt=mytype is .*\\, depth=1\\).*" ".*first frame.*"]
>>> +    }
>>> +}
>>> +
>>> +# Testing the finish command.
>>> +with_test_prefix "frame exit through finish" {
>>> +    if { [start_test "TAG: final frame"] == 0 } {
>>> +    gdb_test "finish" [multi_line ".*.*g \\(mt=mytype is .*\\, depth=0\\).*" ".*g \\(mt=mytype is .*\\, depth=1\\).*" ".*"]
>>> +    }
>>> +}
>>> +
>>> +# Testing the step command.
>>> +with_test_prefix "frame enter through step" {
>>> +    if { [start_test "TAG: outside the frame"] == 0 } {
>>> +    gdb_test "step" [multi_line "g \\(mt=mytype is .*\\, depth=10\\).*" "41.*if \\(depth \\<= 0\\)"]
>>> +    }
>>> +}
>>> +
>>> +# Testing the continue command.
>>> +with_test_prefix "frame enter through continue" {
>>> +    if { [start_test "TAG: outside the frame"] == 0 } {
>>> +    gdb_breakpoint [gdb_get_line_number "TAG: first frame" ${testfile}.c ]
>>> +    gdb_continue_to_breakpoint "TAG: first frame" ".*TAG: first frame.*"
>>> +    }
>>> +}
>>> diff --git a/gdb/testsuite/gdb.python/pretty-print-call-by-hand.py b/gdb/testsuite/gdb.python/pretty-print-call-by-hand.py
>>> new file mode 100644
>>> index 00000000000..f8f5df678f8
>>> --- /dev/null
>>> +++ b/gdb/testsuite/gdb.python/pretty-print-call-by-hand.py
>>> @@ -0,0 +1,41 @@
>>> +# Copyright (C) 2022 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/>.
>>> +
>>> +
>>> +class MytypePrinter:
>>> +    """pretty print my type"""
>>> +
>>> +    def __init__(self, val):
>>> +        self.val = val
>>> +
>>> +    def to_string(self):
>>> +        calls = gdb.parse_and_eval('f()')
>>> +        return "mytype is %s" % self.val['x']
>>> +
>>> +def ec_lookup_function(val):
>>> +    typ = val.type
>>> +    if typ.code == gdb.TYPE_CODE_REF:
>>> +        typ = typ.target()
>>> +    if str(typ) == 'struct mytype':
>>> +        return MytypePrinter(val)
>>> +    return None
>>> +
>>> +def disable_lookup_function():
>>> +    ec_lookup_function.enabled = False
>>> +
>>> +def enable_lookup_function():
>>> +    ec_lookup_function.enabled = True
>>> +
>>> +gdb.pretty_printers.append(ec_lookup_function)