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

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.

Author: brobecke

gdb/ChangeLog

@@ -1,3 +1,8 @@
+2015-11-23  Joel Brobecker  <[email protected]>
+
+	* 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  <[email protected]>
 
 	* amd64-windows-tdep.c (amd64_windows_frame_decode_epilogue):

gdb/stack.c

@@ -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;

gdb/testsuite/ChangeLog

@@ -1,3 +1,9 @@
+2015-11-23  Joel Brobecker  <[email protected]>
+
+	* 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  <[email protected]>
 
 	* testsuite/gdb.ada/var_rec_arr.exp: Add "ptype a1(1)" test.

gdb/testsuite/gdb.base/wrong_frame_bt_full-main.c

@@ -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;
+}

gdb/testsuite/gdb.base/wrong_frame_bt_full-opaque.c

@@ -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.  */
+}

gdb/testsuite/gdb.base/wrong_frame_bt_full.exp

@@ -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\\}\[\r\n\]+.*"
+