gdb: work around negative DW_AT_data_member_location GCC 11 bug

g++ 11.1.0 has a bug where it will emit a negative
DW_AT_data_member_location in some cases:

    $ cat test.cpp
    #include <memory>

    int
    main()
    {
      std::unique_ptr<int> ptr;
    }
    $ g++ -g test.cpp
    $ llvm-dwarfdump -F a.out
    ...
    0x00000964:       DW_TAG_member
                        DW_AT_name [DW_FORM_strp]   ("_M_head_impl")
                        DW_AT_decl_file [DW_FORM_data1]     ("/usr/include/c++/11.1.0/tuple")
                        DW_AT_decl_line [DW_FORM_data1]     (125)
                        DW_AT_decl_column [DW_FORM_data1]   (0x27)
                        DW_AT_type [DW_FORM_ref4]   (0x0000067a "default_delete<int>")
                        DW_AT_data_member_location [DW_FORM_sdata]  (-1)
    ...

This leads to a GDB crash (when built with ASan, otherwise probably
garbage results), since it tries to read just before (to the left, in
ASan speak) of the value's buffer:

    ==888645==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x6020000c52af at pc 0x7f711b239f4b bp 0x7fff356bd470 sp 0x7fff356bcc18
    READ of size 1 at 0x6020000c52af thread T0
        #0 0x7f711b239f4a in __interceptor_memcpy /build/gcc/src/gcc/libsanitizer/sanitizer_common/sanitizer_common_interceptors.inc:827
        #1 0x555c4977efa1 in value_contents_copy_raw /home/simark/src/binutils-gdb/gdb/value.c:1347
        #2 0x555c497909cd in value_primitive_field(value*, long, int, type*) /home/simark/src/binutils-gdb/gdb/value.c:3126
        #3 0x555c478f2eaa in cp_print_value_fields(value*, ui_file*, int, value_print_options const*, type**, int) /home/simark/src/binutils-gdb/gdb/cp-valprint.c:333
        #4 0x555c478f63b2 in cp_print_value /home/simark/src/binutils-gdb/gdb/cp-valprint.c:513
        #5 0x555c478f02ca in cp_print_value_fields(value*, ui_file*, int, value_print_options const*, type**, int) /home/simark/src/binutils-gdb/gdb/cp-valprint.c:161
        #6 0x555c478f63b2 in cp_print_value /home/simark/src/binutils-gdb/gdb/cp-valprint.c:513
        #7 0x555c478f02ca in cp_print_value_fields(value*, ui_file*, int, value_print_options const*, type**, int) /home/simark/src/binutils-gdb/gdb/cp-valprint.c:161
        #8 0x555c478f63b2 in cp_print_value /home/simark/src/binutils-gdb/gdb/cp-valprint.c:513
        #9 0x555c478f02ca in cp_print_value_fields(value*, ui_file*, int, value_print_options const*, type**, int) /home/simark/src/binutils-gdb/gdb/cp-valprint.c:161
        #10 0x555c4760d45f in c_value_print_struct /home/simark/src/binutils-gdb/gdb/c-valprint.c:383
        #11 0x555c4760df4c in c_value_print_inner(value*, ui_file*, int, value_print_options const*) /home/simark/src/binutils-gdb/gdb/c-valprint.c:438
        #12 0x555c483ff9a7 in language_defn::value_print_inner(value*, ui_file*, int, value_print_options const*) const /home/simark/src/binutils-gdb/gdb/language.c:632
        #13 0x555c49758b68 in do_val_print /home/simark/src/binutils-gdb/gdb/valprint.c:1048
        #14 0x555c49759b17 in common_val_print(value*, ui_file*, int, value_print_options const*, language_defn const*) /home/simark/src/binutils-gdb/gdb/valprint.c:1151
        #15 0x555c478f2fcb in cp_print_value_fields(value*, ui_file*, int, value_print_options const*, type**, int) /home/simark/src/binutils-gdb/gdb/cp-valprint.c:335
        #16 0x555c478f63b2 in cp_print_value /home/simark/src/binutils-gdb/gdb/cp-valprint.c:513
        #17 0x555c478f02ca in cp_print_value_fields(value*, ui_file*, int, value_print_options const*, type**, int) /home/simark/src/binutils-gdb/gdb/cp-valprint.c:161
        #18 0x555c4760d45f in c_value_print_struct /home/simark/src/binutils-gdb/gdb/c-valprint.c:383
        #19 0x555c4760df4c in c_value_print_inner(value*, ui_file*, int, value_print_options const*) /home/simark/src/binutils-gdb/gdb/c-valprint.c:438
        #20 0x555c483ff9a7 in language_defn::value_print_inner(value*, ui_file*, int, value_print_options const*) const /home/simark/src/binutils-gdb/gdb/language.c:632
        #21 0x555c49758b68 in do_val_print /home/simark/src/binutils-gdb/gdb/valprint.c:1048
        #22 0x555c49759b17 in common_val_print(value*, ui_file*, int, value_print_options const*, language_defn const*) /home/simark/src/binutils-gdb/gdb/valprint.c:1151
        #23 0x555c478f2fcb in cp_print_value_fields(value*, ui_file*, int, value_print_options const*, type**, int) /home/simark/src/binutils-gdb/gdb/cp-valprint.c:335
        #24 0x555c4760d45f in c_value_print_struct /home/simark/src/binutils-gdb/gdb/c-valprint.c:383
        #25 0x555c4760df4c in c_value_print_inner(value*, ui_file*, int, value_print_options const*) /home/simark/src/binutils-gdb/gdb/c-valprint.c:438
        #26 0x555c483ff9a7 in language_defn::value_print_inner(value*, ui_file*, int, value_print_options const*) const /home/simark/src/binutils-gdb/gdb/language.c:632
        #27 0x555c49758b68 in do_val_print /home/simark/src/binutils-gdb/gdb/valprint.c:1048
        #28 0x555c49759b17 in common_val_print(value*, ui_file*, int, value_print_options const*, language_defn const*) /home/simark/src/binutils-gdb/gdb/valprint.c:1151
        #29 0x555c4760f04c in c_value_print(value*, ui_file*, value_print_options const*) /home/simark/src/binutils-gdb/gdb/c-valprint.c:587
        #30 0x555c483ff954 in language_defn::value_print(value*, ui_file*, value_print_options const*) const /home/simark/src/binutils-gdb/gdb/language.c:614
        #31 0x555c49759f61 in value_print(value*, ui_file*, value_print_options const*) /home/simark/src/binutils-gdb/gdb/valprint.c:1189
        #32 0x555c48950f70 in print_formatted /home/simark/src/binutils-gdb/gdb/printcmd.c:337
        #33 0x555c48958eda in print_value(value*, value_print_options const&) /home/simark/src/binutils-gdb/gdb/printcmd.c:1258
        #34 0x555c48959891 in print_command_1 /home/simark/src/binutils-gdb/gdb/printcmd.c:1367
        #35 0x555c4895a3df in print_command /home/simark/src/binutils-gdb/gdb/printcmd.c:1458
        #36 0x555c4767f974 in do_simple_func /home/simark/src/binutils-gdb/gdb/cli/cli-decode.c:97
        #37 0x555c47692e25 in cmd_func(cmd_list_element*, char const*, int) /home/simark/src/binutils-gdb/gdb/cli/cli-decode.c:2475
        #38 0x555c4936107e in execute_command(char const*, int) /home/simark/src/binutils-gdb/gdb/top.c:670
        #39 0x555c485f1bff in catch_command_errors /home/simark/src/binutils-gdb/gdb/main.c:523
        #40 0x555c485f249c in execute_cmdargs /home/simark/src/binutils-gdb/gdb/main.c:618
        #41 0x555c485f6677 in captured_main_1 /home/simark/src/binutils-gdb/gdb/main.c:1317
        #42 0x555c485f6c83 in captured_main /home/simark/src/binutils-gdb/gdb/main.c:1338
        #43 0x555c485f6d65 in gdb_main(captured_main_args*) /home/simark/src/binutils-gdb/gdb/main.c:1363
        #44 0x555c46e41ba8 in main /home/simark/src/binutils-gdb/gdb/gdb.c:32
        #45 0x7f71198bcb24 in __libc_start_main (/usr/lib/libc.so.6+0x27b24)
        #46 0x555c46e4197d in _start (/home/simark/build/binutils-gdb-one-target/gdb/gdb+0x77f197d)

    0x6020000c52af is located 1 bytes to the left of 8-byte region [0x6020000c52b0,0x6020000c52b8)
    allocated by thread T0 here:
        #0 0x7f711b2b7459 in __interceptor_calloc /build/gcc/src/gcc/libsanitizer/asan/asan_malloc_linux.cpp:154
        #1 0x555c470acdc9 in xcalloc /home/simark/src/binutils-gdb/gdb/alloc.c:100
        #2 0x555c49b775cd in xzalloc(unsigned long) /home/simark/src/binutils-gdb/gdbsupport/common-utils.cc:29
        #3 0x555c4977bdeb in allocate_value_contents /home/simark/src/binutils-gdb/gdb/value.c:1029
        #4 0x555c4977be25 in allocate_value(type*) /home/simark/src/binutils-gdb/gdb/value.c:1040
        #5 0x555c4979030d in value_primitive_field(value*, long, int, type*) /home/simark/src/binutils-gdb/gdb/value.c:3092
        #6 0x555c478f6280 in cp_print_value /home/simark/src/binutils-gdb/gdb/cp-valprint.c:501
        #7 0x555c478f02ca in cp_print_value_fields(value*, ui_file*, int, value_print_options const*, type**, int) /home/simark/src/binutils-gdb/gdb/cp-valprint.c:161
        #8 0x555c478f63b2 in cp_print_value /home/simark/src/binutils-gdb/gdb/cp-valprint.c:513
        #9 0x555c478f02ca in cp_print_value_fields(value*, ui_file*, int, value_print_options const*, type**, int) /home/simark/src/binutils-gdb/gdb/cp-valprint.c:161
        #10 0x555c478f63b2 in cp_print_value /home/simark/src/binutils-gdb/gdb/cp-valprint.c:513
        #11 0x555c478f02ca in cp_print_value_fields(value*, ui_file*, int, value_print_options const*, type**, int) /home/simark/src/binutils-gdb/gdb/cp-valprint.c:161
        #12 0x555c4760d45f in c_value_print_struct /home/simark/src/binutils-gdb/gdb/c-valprint.c:383
        #13 0x555c4760df4c in c_value_print_inner(value*, ui_file*, int, value_print_options const*) /home/simark/src/binutils-gdb/gdb/c-valprint.c:438
        #14 0x555c483ff9a7 in language_defn::value_print_inner(value*, ui_file*, int, value_print_options const*) const /home/simark/src/binutils-gdb/gdb/language.c:632
        #15 0x555c49758b68 in do_val_print /home/simark/src/binutils-gdb/gdb/valprint.c:1048
        #16 0x555c49759b17 in common_val_print(value*, ui_file*, int, value_print_options const*, language_defn const*) /home/simark/src/binutils-gdb/gdb/valprint.c:1151
        #17 0x555c478f2fcb in cp_print_value_fields(value*, ui_file*, int, value_print_options const*, type**, int) /home/simark/src/binutils-gdb/gdb/cp-valprint.c:335
        #18 0x555c478f63b2 in cp_print_value /home/simark/src/binutils-gdb/gdb/cp-valprint.c:513
        #19 0x555c478f02ca in cp_print_value_fields(value*, ui_file*, int, value_print_options const*, type**, int) /home/simark/src/binutils-gdb/gdb/cp-valprint.c:161
        #20 0x555c4760d45f in c_value_print_struct /home/simark/src/binutils-gdb/gdb/c-valprint.c:383
        #21 0x555c4760df4c in c_value_print_inner(value*, ui_file*, int, value_print_options const*) /home/simark/src/binutils-gdb/gdb/c-valprint.c:438
        #22 0x555c483ff9a7 in language_defn::value_print_inner(value*, ui_file*, int, value_print_options const*) const /home/simark/src/binutils-gdb/gdb/language.c:632
        #23 0x555c49758b68 in do_val_print /home/simark/src/binutils-gdb/gdb/valprint.c:1048
        #24 0x555c49759b17 in common_val_print(value*, ui_file*, int, value_print_options const*, language_defn const*) /home/simark/src/binutils-gdb/gdb/valprint.c:1151
        #25 0x555c478f2fcb in cp_print_value_fields(value*, ui_file*, int, value_print_options const*, type**, int) /home/simark/src/binutils-gdb/gdb/cp-valprint.c:335
        #26 0x555c4760d45f in c_value_print_struct /home/simark/src/binutils-gdb/gdb/c-valprint.c:383
        #27 0x555c4760df4c in c_value_print_inner(value*, ui_file*, int, value_print_options const*) /home/simark/src/binutils-gdb/gdb/c-valprint.c:438
        #28 0x555c483ff9a7 in language_defn::value_print_inner(value*, ui_file*, int, value_print_options const*) const /home/simark/src/binutils-gdb/gdb/language.c:632
        #29 0x555c49758b68 in do_val_print /home/simark/src/binutils-gdb/gdb/valprint.c:1048

Since there are some binaries with this in the wild, I think it would be
useful for GDB to work around this.  I did the obvious simple thing, if
the DW_AT_data_member_location's value is -1, replace it with 0.  I
added a producer check to only apply this fixup for GCC 11.  The idea is
that if some other compiler ever uses a DW_AT_data_member_location value
of -1 by mistake, we don't know (before analyzing the bug at least) if
they did mean 0 or some other value.  So I wouldn't want to apply the
fixup in that case.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=28063
Change-Id: Ieef3459b0b9bbce8bdad838ba83b4b64e7269d42

Author: simark

gdb/dwarf2/cu.c

@@ -32,6 +32,7 @@ dwarf2_cu::dwarf2_cu (dwarf2_per_cu_data *per_cu,
     checked_producer (false),
     producer_is_gxx_lt_4_6 (false),
     producer_is_gcc_lt_4_3 (false),
+    producer_is_gcc_11 (false),
     producer_is_icc (false),
     producer_is_icc_lt_14 (false),
     producer_is_codewarrior (false),

gdb/dwarf2/cu.h

@@ -253,6 +253,7 @@ struct dwarf2_cu
   bool checked_producer : 1;
   bool producer_is_gxx_lt_4_6 : 1;
   bool producer_is_gcc_lt_4_3 : 1;
+  bool producer_is_gcc_11 : 1;
   bool producer_is_icc : 1;
   bool producer_is_icc_lt_14 : 1;
   bool producer_is_codewarrior : 1;

gdb/dwarf2/read.c

@@ -14320,6 +14320,7 @@ check_producer (struct dwarf2_cu *cu)
     {
       cu->producer_is_gxx_lt_4_6 = major < 4 || (major == 4 && minor < 6);
       cu->producer_is_gcc_lt_4_3 = major < 4 || (major == 4 && minor < 3);
+      cu->producer_is_gcc_11 = major == 11;
     }
   else if (producer_is_icc (cu->producer, &major, &minor))
     {
@@ -14464,6 +14465,19 @@ handle_member_location (struct die_info *die, struct dwarf2_cu *cu,
       if (attr->form_is_constant ())
 	{
 	  LONGEST offset = attr->constant_value (0);
+
+	  /* Work around this GCC 11 bug, where it would erroneously use -1
+	     data member locations, instead of 0:
+
+	       Negative DW_AT_data_member_location
+	       https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101378
+	     */
+	  if (offset == -1 && cu->producer_is_gcc_11)
+	    {
+	      complaint (_("DW_AT_data_member_location value of -1, assuming 0"));
+	      offset = 0;
+	    }
+
 	  field->set_loc_bitpos (offset * bits_per_byte);
 	}
       else if (attr->form_is_section_offset ())

gdb/testsuite/gdb.dwarf2/negative-data-member-location.c

@@ -0,0 +1,31 @@
+/* Copyright (C) 2021-2022 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/>.  */
+
+struct the_struct
+{
+  /* Define the field as an array of char instead of an int to support targets
+     that have sizeof(int) != 4.  */
+  char x[4];
+};
+
+struct the_struct s = { { 0x11, 0x22, 0x22, 0x11 } };
+
+int
+main (void)
+{
+  return 0;
+}

gdb/testsuite/gdb.dwarf2/negative-data-member-location.exp

@@ -0,0 +1,77 @@
+# Copyright 2021-2022 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/>.
+
+# Test our workaround for a GCC 11 bug, where it sometimes puts a -1 value for
+# DW_AT_data_member_location:
+#
+#   Negative DW_AT_data_member_location
+#   https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101378
+
+load_lib dwarf.exp
+
+# This test can only be run on targets which support DWARF-2 and use gas.
+if { ![dwarf2_support] } {
+    return 0
+}
+
+standard_testfile .c -dw.S
+
+set asm_file [standard_output_file ${srcfile2}]
+
+Dwarf::assemble ${asm_file} {
+    cu {} {
+	DW_TAG_compile_unit {
+	    {DW_AT_language @DW_LANG_C99}
+	    {DW_AT_producer "GNU C++17 11.1.0 -mtune=generic -march=x86-64 -g3 -O0"}
+	    {name ${::srcfile}}
+	} {
+	    declare_labels int_label struct_label
+
+	    int_label: DW_TAG_base_type {
+		{DW_AT_byte_size 4 DW_FORM_udata}
+		{DW_AT_encoding @DW_ATE_signed}
+		{DW_AT_name "int"}
+	    }
+
+	    struct_label: DW_TAG_structure_type {
+		{DW_AT_name "the_struct"}
+		{DW_AT_byte_size 4 DW_FORM_udata}
+	    } {
+		DW_TAG_member {
+		    {DW_AT_name "field"}
+		    {DW_AT_type :$int_label}
+		    {DW_AT_data_member_location -1 DW_FORM_sdata}
+		}
+	    }
+
+	    DW_TAG_variable {
+		{DW_AT_name "s"}
+		{DW_AT_type :$struct_label}
+		{DW_AT_location {DW_OP_addr [gdb_target_symbol "s"]} SPECIAL_expr}
+	    }
+	}
+    }
+}
+
+if { [prepare_for_testing "failed to prepare" ${testfile} [list $srcfile $asm_file] {nodebug}] } {
+    return
+}
+
+if { ![runto_main] } {
+    return
+}
+
+gdb_test "print /x s" " = {field = 0x11222211}"
+gdb_test "print /x s.field" " = 0x11222211"