[PING 2][PATCH v3] Fix gdb crash when trying to print the address of a synthetic C++ reference

Martin Galvan
2016-04-15T20:24:23Z

After compiling a program which uses C++ references some optimizations may convert
the references into synthetic "pointers". Trying to print the address of one
of such synthetic references causes gdb to crash with the following error:

(gdb) print &ref
/build/buildd/gdb-7.7.1/gdb/dwarf2loc.c:1624: internal-error: Should not be able to create a lazy value with an enclosing type
A problem internal to GDB has been detected,
further debugging may prove unreliable.

Apparently, what was causing it was that value_addr returns a copy of the value
that represents the reference with its type set to T* instead of T&. However,
its enclosing_type is left untouched, which fails a check done in read_pieced_value.
We only see the crash happen for references that are synthetic because they're treated
as pieced values, thus the call to read_pieced_value.

This was previously discussed here:

https://sourceware.org/ml/gdb-patches/2015-06/msg00278.html

though the proposed patch was somewhat different.

In the previous thread (i.e. sans 'PING') I mentioned I'd try to use readjust_indirect_value_type,
however I desisted because it seems a bit weird to pass arg2 as both a const and non-const
parameter.

On a related note, from what we discussed with Pedro it seems that in general
there are all sorts of breakage when working with synthetic references.
I reported this here:

https://sourceware.org/bugzilla/show_bug.cgi?id=19893

I'll be working on fixing those other issues, but for now I think this patch can go in.
I'll add new test cases as I go along.

I have write access and copyright assignment; let me know if this is ok to commit
or if you have any feedback.

gdb/ChangeLog:
2016-04-08 Martin Galvan <martin.galvan@tallertechnologies.com>

* valops.c (value_addr): For C++ references, set the copied value's
enclosing_type as well.

gdb/testsuite/ChangeLog:
2016-04-08 Martin Galvan <martin.galvan@tallertechnologies.com>

* gdb.dwarf2/implref.exp: New file.

---
gdb/testsuite/gdb.dwarf2/implref.exp | 99 ++++++++++++++++++++++++++++++++++++
gdb/valops.c | 16 ++++--
2 files changed, 111 insertions(+), 4 deletions(-)
create mode 100644 gdb/testsuite/gdb.dwarf2/implref.exp

diff --git a/gdb/testsuite/gdb.dwarf2/implref.exp b/gdb/testsuite/gdb.dwarf2/implref.exp
new file mode 100644
index 0000000..8599715
--- /dev/null
+++ b/gdb/testsuite/gdb.dwarf2/implref.exp
@@ -0,0 +1,99 @@
+# Copyright 2016 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/>.
+
+# Test C++ references marked with DW_OP_GNU_implicit_pointer.
+
+# TODO: Add more test statements after fixing bug #19893:
+# https://sourceware.org/bugzilla/show_bug.cgi?id=19893.
+
+load_lib dwarf.exp
+
+# This test can only be run on targets which support DWARF-2 and use gas.
+if {![dwarf2_support]} {
+ return 0
+}
+
+# We'll place the output of Dwarf::assemble in implref.S.
+standard_testfile main.c .S
+
+# ${testfile} is now "implref". srcfile2 is "implref.S".
+set executable ${testfile}
+set asm_file [standard_output_file ${srcfile2}]
+
+# Create the DWARF. We need a regular variable and a reference to it that'll
+# be marked with DW_OP_GNU_implicit_pointer.
+Dwarf::assemble $asm_file {
+ global srcdir subdir srcfile
+
+ cu { version 3 addr_size 4 } {
+ DW_TAG_compile_unit {
+ {DW_AT_producer "GNU C++ 4.8.4"}
+ {DW_AT_language @DW_LANG_C_plus_plus}
+ {DW_AT_name "implref.S"}
+ } {
+ declare_labels int_label const_label variable_label ref_label
+
+ int_label: DW_TAG_base_type {
+ {DW_AT_byte_size 4 DW_FORM_udata}
+ {DW_AT_encoding @DW_ATE_signed}
+ {DW_AT_name "int"}
+ }
+
+ ref_label: DW_TAG_reference_type {
+ {DW_AT_byte_size 4 DW_FORM_udata}
+ {DW_AT_type :${int_label}}
+ }
+
+ const_label: DW_TAG_const_type {
+ {DW_AT_type :${ref_label}}
+ }
+
+ DW_TAG_subprogram {
+ {MACRO_AT_func { "main" "${srcdir}/${subdir}/${srcfile}" }}
+ {DW_AT_type :${int_label}}
+ {DW_AT_external 1 DW_FORM_flag}
+ } {
+ variable_label: DW_TAG_variable {
+ {DW_AT_name "var"}
+ {DW_AT_type :${int_label}}
+ {DW_AT_const_value 42 DW_FORM_udata}
+ }
+
+ DW_TAG_variable {
+ {DW_AT_name "ref"}
+ {DW_AT_type :${const_label}}
+ {DW_AT_location {DW_OP_GNU_implicit_pointer ${variable_label} 0} SPECIAL_expr}
+ }
+ }
+ }
+ }
+}
+
+if [prepare_for_testing ${testfile}.exp ${executable} "${asm_file} ${srcfile}" {}] {
+ return -1
+}
+
+# DW_OP_GNU_implicit_pointer implementation requires a valid frame.
+if ![runto_main] {
+ return -1
+}
+
+gdb_test "print ref" " = \\(int &\\) <synthetic pointer>" "print ref"
+gdb_test "print &ref" " = \\(int \\*\\) <synthetic pointer>" "print &ref"
+
+# gdb assumes C++ references are implemented as pointers, and print &(&ref)
+# shows us the underlying pointer's address.
+# Since in this case there's no physical pointer, gdb should tell us so.
+gdb_test "print &\(&ref\)" "Attempt to take address of value not located in memory." "print &(&ref)"
diff --git a/gdb/valops.c b/gdb/valops.c
index 5a244a9..40559c2 100644
--- a/gdb/valops.c
+++ b/gdb/valops.c
@@ -1463,11 +1463,19 @@ value_addr (struct value *arg1)
if (TYPE_CODE (type) == TYPE_CODE_REF)
{
/* Copy the value, but change the type from (T&) to (T*). We
- keep the same location information, which is efficient, and
- allows &(&X) to get the location containing the reference. */
+ keep the same location information, which is efficient, and
+ allows &(&X) to get the location containing the reference.
+ Do the same to its enclosing type for consistency. */
+ struct type *type_ptr = lookup_pointer_type (TYPE_TARGET_TYPE (type));
+
+ struct type *enclosing_type = check_typedef (value_enclosing_type (arg1));
+ struct type *enclosing_type_ptr =
+ lookup_pointer_type (TYPE_TARGET_TYPE (enclosing_type));
+
arg2 = value_copy (arg1);
- deprecated_set_value_type (arg2,
- lookup_pointer_type (TYPE_TARGET_TYPE (type)));
+ deprecated_set_value_type (arg2, type_ptr);
+ set_value_enclosing_type (arg2, enclosing_type_ptr);
+
return arg2;
}
if (TYPE_CODE (type) == TYPE_CODE_FUNC)
--
2.8.0