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

[Bruno Larsen <blarsen@redhat.com>]

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)