dwarf2read.c: Check type of string valued attributes prior to decoding.

This change introduces a new function, dwarf2_string_attr(), which is
a wrapper for dwarf2_attr().  dwarf2read.c has been updated to
call dwarf2_string_attr in most instances where a string-valued
attribute is decoded to produce a string value.  In most cases, it
simplifies the code; in some instances, the complexity of the code
remains unchanged.

I performed this change by looking for instances where the
result of DW_STRING was used in an assignment.  Many of these
had a pattern which (roughly) looks something like this:

  struct attribute *attr = NULL;

  attr = dwarf2_attr (die, name, cu);
  if (attr != NULL && DW_STRING (attr))
    {
      const char *str;
      ...
      str = DW_STRING (attr);
      ... /* Use str in some fashion.  */
    }

Code of this form is transformed to look like this instead:

  const char *str;

  str = dwarf2_string_attr (die, name, cu)
  if (str != NULL)
    {
       ...
       /* Use str in some fashion.  */
       ...
    }

In addition to invoking dwarf2_attr() and DW_STRING(),
dwarf2_string_attr() checks to make sure that the attribute's
`form' field matches one of DW_FORM_strp, DW_FORM_string, or
DW_FORM_GNU_strp_alt.  If it does not match one of these forms,
it will return a NULL value in addition to calling complaint().

An earlier version of this patch did this type checking for one
particular instance where a string attribute was being decoded.
The situation that I was attempting to handle in that earlier patch is
this:

The Texas Instruments compiler uses the encoding for
DW_AT_MIPS_linkage_name for other purposes.  TI uses the encoding,
0x2007, for TI_AT_TI_end_line which, unlike DW_AT_MIPS_linkage_name,
does not have a string-typed value.  In this instance, GDB was attempting
to use an integer value as a string pointer, with predictable results.
(GDB would die with a segmentation fault.)

I've added a test which reproduces the problem that I was orignally
wanting to fix.  It uses DW_AT_MIPS_linkage name with an associate
value which is a string, and again, where the value is a small
integer.

My test case causes GDB to segfault in an unpatched GDB.  There
will be two PASSes in a patched GDB.

Unpatched GDB:

(gdb) ptype f
ERROR: Process no longer exists
UNRESOLVED: gdb.dwarf2/dw2-bad-mips-linkage-name.exp: ptype f
ERROR: Couldn't send ptype g to GDB.
UNRESOLVED: gdb.dwarf2/dw2-bad-mips-linkage-name.exp: ptype g

Patched GDB:

(gdb) ptype f
type = bool ()
(gdb) PASS: gdb.dwarf2/dw2-bad-mips-linkage-name.exp: ptype f
ptype g
type = bool ()
(gdb) PASS: gdb.dwarf2/dw2-bad-mips-linkage-name.exp: ptype g

I see no regressions on an x86_64 native target.

gdb/ChangeLog:

	* dwarf2read.c (dwarf2_string_attr): New function.
	(lookup_dwo_unit, process_psymtab_comp_unit_reader)
	(dwarf2_compute_name, dwarf2_physname, find_file_and_directory)
	(read_call_site_scope, namespace_name, guess_full_die_structure_name)
	(anonymous_struct_prefix, prepare_one_comp_unit): Use
	dwarf2_string_attr in place of dwarf2_attr and DW_STRING.

gdb/testsuite/ChangeLog:

	* gdb.dwarf2/dw2-bad-mips-linkage-name.c: New file.
	* gdb.dwarf2/dw2-bad-mips-linkage-name.exp: New file.

2 files changed, 113 insertions, 0 deletions

diff --git a/gdb/testsuite/gdb.dwarf2/dw2-bad-mips-linkage-name.c b/gdb/testsuite/gdb.dwarf2/dw2-bad-mips-linkage-name.c
new file mode 100644
index 0000000..ffbebf6
--- /dev/null
+++ b/gdb/testsuite/gdb.dwarf2/dw2-bad-mips-linkage-name.c
@@ -0,0 +1,41 @@
+/* This testcase is part of GDB, the GNU debugger.
+
+   Copyright 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/>.  */
+
+/* Dummy main function.  */
+
+int
+main (void)
+{
+  asm ("main_label: .globl main_label");
+  return 0;
+}
+
+/* dummy f function, DWARF will describe arguments and type differently.  */
+int
+f (char *x)
+{
+  asm (".global f_end_lbl\nf_end_lbl:");
+  return 0;
+}
+
+/* dummy g function, DWARF will describe arguments and type differently.  */
+int
+g (char *x)
+{
+  asm (".global g_end_lbl\ng_end_lbl:");
+  return 0;
+}
diff --git a/gdb/testsuite/gdb.dwarf2/dw2-bad-mips-linkage-name.exp b/gdb/testsuite/gdb.dwarf2/dw2-bad-mips-linkage-name.exp
new file mode 100644
index 0000000..77f6175
--- /dev/null
+++ b/gdb/testsuite/gdb.dwarf2/dw2-bad-mips-linkage-name.exp
@@ -0,0 +1,72 @@
+# Copyright 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/>.
+
+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 dw2-bad-mips-linkage-name.c dw2-bad-mips-linkage-name.S
+
+# Set up the DWARF for the test.
+
+set asm_file [standard_output_file $srcfile2]
+Dwarf::assemble $asm_file {
+    cu {} {
+	DW_TAG_compile_unit {
+                {DW_AT_language @DW_LANG_C}
+                {DW_AT_name     dw2-bad-mips-linkage-name.c}
+                {DW_AT_comp_dir /tmp}
+
+        } {
+	    declare_labels b_l
+
+	    b_l: DW_TAG_base_type {
+		{DW_AT_byte_size 1 DW_FORM_sdata}
+		{DW_AT_encoding  @DW_ATE_signed}
+		{DW_AT_name      bool}
+	    }
+            DW_TAG_subprogram {
+                {name f}
+                {low_pc f addr}
+                {high_pc f_end_lbl addr}
+                {type :$b_l}
+		{DW_AT_MIPS_linkage_name _Z1fv}
+	    }
+            DW_TAG_subprogram {
+                {name g}
+                {low_pc g addr}
+                {high_pc g_end_lbl addr}
+                {type :$b_l}
+		{DW_AT_MIPS_linkage_name 42 DW_FORM_data1}
+	    }
+	}
+    }
+}
+
+if { [prepare_for_testing ${testfile}.exp ${testfile} \
+	  [list $srcfile $asm_file] {nodebug}] } {
+    return -1
+}
+
+# A successful run will have two PASSes.  A GDB that's lacking
+# attribute type checking will segfault at some point.  It doesn't
+# much matter what we test here, so long as we do something to make
+# sure that the DWARF is read.
+
+gdb_test "ptype f" " = bool \\(\\)"
+gdb_test "ptype g" " = bool \\(\\)"