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error/internal-error printing local variable during "bt full".

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One of our users reported an internal error using the "bt full"
command. In their situation, reproducing involved the following
scenario:

    (gdb) frame 1
    (gdb) bt full
    #0  0xf7783430 in __kernel_vsyscall ()
    No symbol table info available.
    #1  0xf5550aeb in waitpid () at ../sysdeps/unix/syscall-template.S:81
    No locals.
    [...]
    #6  0x0fe83139 in xxxx (arg=...)
    [...some locals printed, and then...]
    <S17b> =
    [...]/dwarf2loc.c:364: internal-error: dwarf_expr_frame_base: Assertion
    `framefunc != NULL' failed.

As shown above, the error happens while GDB is trying to print the value
of <S17b>, which is a local string internally generated by the compiler.
For that, it finds that the array lives in memory, and therefore tries
to create a struct value for it via:

        case DWARF_VALUE_MEMORY:
          {
            CORE_ADDR address = dwarf_expr_fetch_address (ctx, 0);
            [...]
            retval = value_at_lazy (type, address + byte_offset);

Unfortunately for us, TYPE happens to be an array whose bounds
are dynamic. More precisely, the bounds of our arrays are described
in the debugging info as being...

 <4><2c1985e>: Abbrev Number: 33 (DW_TAG_subrange_type)
    <2c1985f>   DW_AT_type        : <0x2c1989c>
    <2c19863>   DW_AT_lower_bound : <0x2c19835>
    <2c19867>   DW_AT_upper_bound : <0x2c19841>

... which are references to a pair of local variables. For instance,
the lower bound is a reference to the following DIE

 <3><2c19835>: Abbrev Number: 32 (DW_TAG_variable)
    <2c19836>   DW_AT_name        : [...]
    <2c1983a>   DW_AT_type        : <0x2c198b4>
    <2c1983e>   DW_AT_artificial  : 1
    <2c1983e>   DW_AT_location    : 2 byte block: 91 58         (DW_OP_fbreg: -40)

As a result of the above, value_at_lazy indirectly triggers
a resolution of TYPE (via value_from_contents_and_address),
which means a resolution of TYPE's bounds, and as seen in
the DW_AT_location attribute above for our bounds, computing
the bound's location requires the frame (its location expression
uses DW_OP_fbreg).

Unfortunately for us, value_at_lazy does not get passed a frame,
we've lost the relevant frame when we try to resolve the array's
bounds. Instead, resolve_dynamic_range gets calls dwarf2_evaluate_property
with NULL as the frame:

    static struct type *
    resolve_dynamic_range (struct type *dyn_range_type,
                           struct property_addr_info *addr_stack)
    {
      [...]
      if (dwarf2_evaluate_property (prop, NULL, addr_stack, &value))
                                          ^^^^

... which then handles this by using the selected frame instead:

    if (frame == NULL && has_stack_frames ())
      frame = get_selected_frame (NULL);

In our case, the selected frame happens to be frame #1, which is
a frame where we have a minimal amount of debugging info, and in
particular, no debug info for the function itself. And because of that,
when we try to determine the frame's base...

    static void
    dwarf_expr_frame_base (void *baton, const gdb_byte **start,
                           size_t * length)
    {
      struct dwarf_expr_baton *debaton = (struct dwarf_expr_baton *) baton;
      const struct block *bl = get_frame_block (debaton->frame, NULL);
      [...]
      framefunc = block_linkage_function (bl);

... framefunc ends up being NULL, which triggers the assert
in that same function:

      gdb_assert (framefunc != NULL);

This patches avoids the issue by temporarily setting the selected_frame
before printing the locals of each frames.

This patch also adds a small testcase, which reproduces the same
issue, but with a slightly different outcome:

    (gdb) bt full
    #0  0x000000000040049a in opaque_routine ()
    No symbol table info available.
    #1  0x0000000000400532 in main () at wrong_frame_bt_full-main.c:20
            my_table_size = 3
            my_table = <error reading variable my_table (frame address is not available.)>

With this patch, the output becomes:

    (gdb) bt full
    [...]
            my_table = {0, 1, 2}

gdb/ChangeLog:

        * stack.c (print_frame_local_vars): Temporarily set the selected
        frame to FRAME while printing the frame's local variables.

gdb/testsuite/ChangeLog:

        * gdb.base/wrong_frame_bt_full-main.c: New file.
        * gdb.base/wrong_frame_bt_full-opaque.c: New file.
        * gdb.base/wrong_frame_bt_full.exp: New file.
This commit is contained in:
Joel Brobecker 2015-11-23 10:02:50 -08:00
parent 80d82c1964
commit 16c3b12f19
6 changed files with 144 additions and 3 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

5
gdb/ChangeLog
View File

@ -1,3 +1,8 @@
2015-11-23 Joel Brobecker <brobecker@adacore.com>
* stack.c (print_frame_local_vars): Temporarily set the selected
frame to FRAME while printing the frame's local variables.
2015-11-23 Joel Brobecker <brobecker@adacore.com>
* amd64-windows-tdep.c (amd64_windows_frame_decode_epilogue):

25
gdb/stack.c
View File

@ -2082,6 +2082,7 @@ print_frame_local_vars (struct frame_info *frame, int num_tabs,
struct print_variable_and_value_data cb_data;
const struct block *block;
CORE_ADDR pc;
struct gdb_exception except = exception_none;
if (!get_frame_pc_if_available (frame, &pc))
{
@ -2102,9 +2103,27 @@ print_frame_local_vars (struct frame_info *frame, int num_tabs,
cb_data.stream = stream;
cb_data.values_printed = 0;
iterate_over_block_local_vars (block,
do_print_variable_and_value,
&cb_data);
/* 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. */
select_frame (frame);
TRY
{
iterate_over_block_local_vars (block,
do_print_variable_and_value,
&cb_data);
}
CATCH (ex, RETURN_MASK_ALL)
{
except = ex;
}
END_CATCH
/* Restore the selected frame, and then rethrow if there was a problem. */
select_frame (frame_find_by_id (cb_data.frame_id));
if (except.reason < 0)
throw_exception (except);
/* do_print_variable_and_value invalidates FRAME. */
frame = NULL;

6
gdb/testsuite/ChangeLog
View File

@ -1,3 +1,9 @@
2015-11-23 Joel Brobecker <brobecker@adacore.com>
* gdb.base/wrong_frame_bt_full-main.c: New file.
* gdb.base/wrong_frame_bt_full-opaque.c: New file.
* gdb.base/wrong_frame_bt_full.exp: New file.
2015-11-23 Joel Brobecker <brobecker@adacore.com>
* testsuite/gdb.ada/var_rec_arr.exp: Add "ptype a1(1)" test.

34
gdb/testsuite/gdb.base/wrong_frame_bt_full-main.c Normal file
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@ -0,0 +1,34 @@
/* Copyright (C) 2015 Free Software Foundation, Inc.
This file is part of GDB.
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/>. */
extern void opaque_routine (void);
int dyn_arr_size = 4;
int
main (void)
{
int i;
int my_table_size = dyn_arr_size - 1;
int my_table [my_table_size];
for (i = 0; i < my_table_size; i++)
my_table[i] = i;
opaque_routine ();
return 0;
}

22
gdb/testsuite/gdb.base/wrong_frame_bt_full-opaque.c Normal file
View File

@ -0,0 +1,22 @@
/* Copyright (C) 2015 Free Software Foundation, Inc.
This file is part of GDB.
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/>. */
void
opaque_routine (void)
{
/* Do nothing. */
}

55
gdb/testsuite/gdb.base/wrong_frame_bt_full.exp Normal file
View File

@ -0,0 +1,55 @@
# Copyright (C) 2015 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/>.
# Build wrong_frame_bt_full-main using two C files:
# - wrong_frame_bt_full-opaque.c, which needs to be built without
# debugging info;
# - wrong_frame_bt_full-main.c, which needs to be built with
# debugging info.
# This is why we use gdb_compile instead of relying on he usual call
# to prepare_for_testing.
set main_testfile wrong_frame_bt_full-main
set opaque_testfile wrong_frame_bt_full-opaque
if {[gdb_compile "${srcdir}/${subdir}/$opaque_testfile.c" \
$opaque_testfile.o \
object {}] != ""} {
untested "failed to compile $opaque_testfile.c"
return -1
}
if {[gdb_compile \
[list ${srcdir}/${subdir}/$main_testfile.c $opaque_testfile.o] \
[standard_output_file ${main_testfile}] \
executable {debug}] != ""} {
untested "failed to build $main_testfile"
return -1
}
clean_restart ${main_testfile}
if ![runto opaque_routine] {
untested "could not run to opaque_routine"
return -1
}
# Make sure that "bt full" command is capable of displaying MY_TABLE
# correctly when frame #0 (the frame which does not have any debugging
# info) is the selected frame.
gdb_test "bt full" \
".*\[\r\n\]+ *my_table = \\{0, 1, 2\\}\[\r\n\]+.*"