fix PR symtab/15597

This patch fixes gdb PR symtab/15597.

The bug is that the .gnu_debugaltlink section includes the build-id of
the alt file, but gdb does not use it.

This patch fixes the problem by changing gdb to do what it ought to
always have done: verify the build id of the file found using the
filename in .gnu_debugaltlink; and if that does not match, try to find
the correct debug file using the build-id and debug-file-directory.

This patch touches BFD.  Previously, gdb had its own code for parsing
.gnu_debugaltlink; I changed it to use the BFD functions after those
were introduced.  However, the BFD functions are incorrect -- they
assume that .gnu_debugaltlink is formatted like .gnu_debuglink.
However, it it is not.  Instead, it consists of a file name followed
by the build-id -- no alignment, and the build-id is not a CRC.

Fixing this properly is a bit of a pain.  But, because
separate_alt_debug_file_exists just has a FIXME for the build-id case,
I did not fix it properly.  Instead I introduced a hack.  This leaves
BFD working just as well as it did before my patch.

I'm willing to do something better here but I could use some guidance
as to what.  It seems that the build-id code in BFD is largely punted
on.

FWIW gdb is the only user of bfd_get_alt_debug_link_info outside of
BFD itself.

I moved the build-id logic out of elfread.c and into a new file.
This seemed cleanest to me.

Writing a test case was a bit of a pain.  I added a couple new
features to the DWARF assembler to handle this.

Built and regtested on x86-64 Fedora 18.

	* bfd-in2.h: Rebuild.
	* opncls.c (bfd_get_alt_debug_link_info): Add buildid_len
	parameter.  Change type of buildid_out.  Update.
	(get_alt_debug_link_info_shim): New function.
	(bfd_follow_gnu_debuglink): Use it.

	* Makefile.in (SFILES): Add build-id.c.
	(HFILES_NO_SRCDIR): Add build-id.h.
	* build-id.c: New file, largely from elfread.c.  Modified
	most functions.
	* build-id.h: New file.
	* dwarf2read.c (dwarf2_get_dwz_file): Update for change to
	bfd_get_alt_debug_link_info.  Verify dwz file's build-id.
	Search for dwz file using build-id.
	* elfread.c (build_id_bfd_get, build_id_verify)
	(build_id_to_debug_filename, find_separate_debug_file): Remove.

	* gdb.dwarf2/dwzbuildid.exp: New file.
	* lib/dwarf.exp (Dwarf::_section): Add "flags" and "type"
	parameters.
	(Dwarf::_defer_output): Change "section" parameter to
	"section_spec"; update.
	(Dwarf::gnu_debugaltlink, Dwarf::_note, Dwarf::build_id): New
	procs.

9 files changed, 488 insertions, 145 deletions

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index de5db77..5f964d1 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,3 +1,16 @@
+2013-10-08  Tom Tromey  <tromey@redhat.com>
+
+	* Makefile.in (SFILES): Add build-id.c.
+	(HFILES_NO_SRCDIR): Add build-id.h.
+	* build-id.c: New file, largely from elfread.c.  Modified
+	most functions.
+	* build-id.h: New file.
+	* dwarf2read.c (dwarf2_get_dwz_file): Update for change to
+	bfd_get_alt_debug_link_info.  Verify dwz file's build-id.
+	Search for dwz file using build-id.
+	* elfread.c (build_id_bfd_get, build_id_verify)
+	(build_id_to_debug_filename, find_separate_debug_file): Remove.
+
 2013-10-08  Joel Brobecker  <brobecker@adacore.com>
 
 	* ada-lang.c (compare_names_with_case): Renamed from
diff --git a/gdb/Makefile.in b/gdb/Makefile.in
index 6b8927a..2aa8134 100644
--- a/gdb/Makefile.in
+++ b/gdb/Makefile.in
@@ -717,7 +717,7 @@ SFILES = ada-exp.y ada-lang.c ada-typeprint.c ada-valprint.c ada-tasks.c \
 	bfd-target.c \
 	block.c blockframe.c \
 	breakpoint.c break-catch-sig.c break-catch-throw.c \
-	buildsym.c \
+	build-id.c buildsym.c \
 	c-exp.y c-lang.c c-typeprint.c c-valprint.c \
 	charset.c cleanups.c cli-out.c coffread.c coff-pe-read.c \
 	complaints.c completer.c continuations.c corefile.c corelow.c \
@@ -814,7 +814,8 @@ tui/tui-file.h tui/tui-command.h tui/tui-disasm.h tui/tui-wingeneral.h \
 tui/tui-windata.h tui/tui-data.h tui/tui-win.h tui/tui-stack.h \
 tui/tui-winsource.h tui/tui-regs.h tui/tui-io.h tui/tui-layout.h \
 tui/tui-source.h sol2-tdep.h gregset.h sh-tdep.h sh64-tdep.h \
-expression.h score-tdep.h gdb_select.h ser-tcp.h buildsym.h valprint.h \
+expression.h score-tdep.h gdb_select.h ser-tcp.h \
+build-id.h buildsym.h valprint.h \
 typeprint.h mi/mi-getopt.h mi/mi-parse.h mi/mi-console.h \
 mi/mi-out.h mi/mi-main.h mi/mi-common.h mi/mi-cmds.h linux-nat.h \
 complaints.h gdb_proc_service.h gdb_regex.h xtensa-tdep.h inf-loop.h \
@@ -911,7 +912,8 @@ COMMON_OBS = $(DEPFILES) $(CONFIG_OBS) $(YYOBJ) \
 	event-loop.o event-top.o inf-loop.o completer.o \
 	gdbarch.o arch-utils.o gdbtypes.o gdb_bfd.o gdb_obstack.o \
 	osabi.o copying.o \
-	memattr.o mem-break.o target.o parse.o language.o buildsym.o \
+	memattr.o mem-break.o target.o parse.o language.o \
+	build-id.o buildsym.o \
 	findcmd.o \
 	std-regs.o \
 	signals.o \
diff --git a/gdb/build-id.c b/gdb/build-id.c
new file mode 100644
index 0000000..330ed18
--- /dev/null
+++ b/gdb/build-id.c
@@ -0,0 +1,168 @@
+/* build-id-related functions.
+
+   Copyright (C) 1991-2013 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/>.  */
+
+#include "defs.h"
+#include "bfd.h"
+#include "elf-bfd.h"
+#include "gdb_bfd.h"
+#include "build-id.h"
+#include "gdb_string.h"
+#include "gdb_vecs.h"
+#include "symfile.h"
+#include "objfiles.h"
+#include "filenames.h"
+
+/* Locate NT_GNU_BUILD_ID from ABFD and return its content.  */
+
+static const struct elf_build_id *
+build_id_bfd_get (bfd *abfd)
+{
+  if (!bfd_check_format (abfd, bfd_object)
+      || bfd_get_flavour (abfd) != bfd_target_elf_flavour
+      /* Although this is ELF_specific, it is safe to do in generic
+	 code because it does not rely on any ELF-specific symbols at
+	 link time, and if the ELF code is not available in BFD, then
+	 ABFD will not have the ELF flavour.  */
+      || elf_tdata (abfd)->build_id == NULL)
+    return NULL;
+
+  return elf_tdata (abfd)->build_id;
+}
+
+/* See build-id.h.  */
+
+int
+build_id_verify (bfd *abfd, size_t check_len, const bfd_byte *check)
+{
+  const struct elf_build_id *found;
+  int retval = 0;
+
+  found = build_id_bfd_get (abfd);
+
+  if (found == NULL)
+    warning (_("File \"%s\" has no build-id, file skipped"),
+	     bfd_get_filename (abfd));
+  else if (found->size != check_len
+           || memcmp (found->data, check, found->size) != 0)
+    warning (_("File \"%s\" has a different build-id, file skipped"),
+	     bfd_get_filename (abfd));
+  else
+    retval = 1;
+
+  return retval;
+}
+
+/* See build-id.h.  */
+
+bfd *
+build_id_to_debug_bfd (size_t build_id_len, const bfd_byte *build_id)
+{
+  char *link, *debugdir;
+  VEC (char_ptr) *debugdir_vec;
+  struct cleanup *back_to;
+  int ix;
+  bfd *abfd = NULL;
+
+  /* DEBUG_FILE_DIRECTORY/.build-id/ab/cdef */
+  link = alloca (strlen (debug_file_directory) + (sizeof "/.build-id/" - 1) + 1
+		 + 2 * build_id_len + (sizeof ".debug" - 1) + 1);
+
+  /* Keep backward compatibility so that DEBUG_FILE_DIRECTORY being "" will
+     cause "/.build-id/..." lookups.  */
+
+  debugdir_vec = dirnames_to_char_ptr_vec (debug_file_directory);
+  back_to = make_cleanup_free_char_ptr_vec (debugdir_vec);
+
+  for (ix = 0; VEC_iterate (char_ptr, debugdir_vec, ix, debugdir); ++ix)
+    {
+      size_t debugdir_len = strlen (debugdir);
+      const gdb_byte *data = build_id;
+      size_t size = build_id_len;
+      char *s;
+      char *filename = NULL;
+
+      memcpy (link, debugdir, debugdir_len);
+      s = &link[debugdir_len];
+      s += sprintf (s, "/.build-id/");
+      if (size > 0)
+	{
+	  size--;
+	  s += sprintf (s, "%02x", (unsigned) *data++);
+	}
+      if (size > 0)
+	*s++ = '/';
+      while (size-- > 0)
+	s += sprintf (s, "%02x", (unsigned) *data++);
+      strcpy (s, ".debug");
+
+      /* lrealpath() is expensive even for the usually non-existent files.  */
+      if (access (link, F_OK) == 0)
+	filename = lrealpath (link);
+
+      if (filename == NULL)
+	continue;
+
+      /* We expect to be silent on the non-existing files.  */
+      abfd = gdb_bfd_open_maybe_remote (filename);
+      if (abfd == NULL)
+	continue;
+
+      if (build_id_verify (abfd, build_id_len, build_id))
+	break;
+
+      gdb_bfd_unref (abfd);
+      abfd = NULL;
+    }
+
+  do_cleanups (back_to);
+  return abfd;
+}
+
+/* See build-id.h.  */
+
+char *
+find_separate_debug_file_by_buildid (struct objfile *objfile)
+{
+  const struct elf_build_id *build_id;
+
+  build_id = build_id_bfd_get (objfile->obfd);
+  if (build_id != NULL)
+    {
+      bfd *abfd;
+
+      abfd = build_id_to_debug_bfd (build_id->size, build_id->data);
+      /* Prevent looping on a stripped .debug file.  */
+      if (abfd != NULL
+	  && filename_cmp (bfd_get_filename (abfd),
+			   objfile_name (objfile)) == 0)
+        {
+	  warning (_("\"%s\": separate debug info file has no debug info"),
+		   bfd_get_filename (abfd));
+	  gdb_bfd_unref (abfd);
+	}
+      else if (abfd != NULL)
+	{
+	  char *result = xstrdup (bfd_get_filename (abfd));
+
+	  gdb_bfd_unref (abfd);
+	  return result;
+	}
+    }
+  return NULL;
+}
diff --git a/gdb/build-id.h b/gdb/build-id.h
new file mode 100644
index 0000000..b3ea4fb
--- /dev/null
+++ b/gdb/build-id.h
@@ -0,0 +1,44 @@
+/* build-id-related functions.
+
+   Copyright (C) 1991-2013 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/>.  */
+
+#ifndef BUILD_ID_H
+#define BUILD_ID_H
+
+/* Return true if ABFD has NT_GNU_BUILD_ID matching the CHECK value.
+   Otherwise, issue a warning and return false.  */
+
+extern int build_id_verify (bfd *abfd,
+			    size_t check_len, const bfd_byte *check);
+
+
+/* Find and open a BFD given a build-id.  If no BFD can be found,
+   return NULL.  The returned reference to the BFD must be released by
+   the caller.  */
+
+extern bfd *build_id_to_debug_bfd (size_t build_id_len,
+				   const bfd_byte *build_id);
+
+/* Find the separate debug file for OBJFILE, by using the build-id
+   associated with OBJFILE's BFD.  If successful, returns a malloc'd
+   file name for the separate debug file.  The caller must free this.
+   Otherwise, returns NULL.  */
+
+extern char *find_separate_debug_file_by_buildid (struct objfile *objfile);
+
+#endif /* BUILD_ID_H */
diff --git a/gdb/dwarf2read.c b/gdb/dwarf2read.c
index 5e2ace7..4cb66db 100644
--- a/gdb/dwarf2read.c
+++ b/gdb/dwarf2read.c
@@ -69,6 +69,7 @@
 #include "f-lang.h"
 #include "source.h"
 #include "filestuff.h"
+#include "build-id.h"
 
 #include <fcntl.h>
 #include "gdb_string.h"
@@ -2364,14 +2365,15 @@ dwarf2_get_dwz_file (void)
   struct cleanup *cleanup;
   const char *filename;
   struct dwz_file *result;
-  unsigned long buildid;
+  size_t buildid_len;
+  bfd_byte *buildid;
 
   if (dwarf2_per_objfile->dwz_file != NULL)
     return dwarf2_per_objfile->dwz_file;
 
   bfd_set_error (bfd_error_no_error);
   data = bfd_get_alt_debug_link_info (dwarf2_per_objfile->objfile->obfd,
-				      &buildid);
+				      &buildid_len, &buildid);
   if (data == NULL)
     {
       if (bfd_get_error () == bfd_error_no_error)
@@ -2380,6 +2382,7 @@ dwarf2_get_dwz_file (void)
 	     bfd_errmsg (bfd_get_error ()));
     }
   cleanup = make_cleanup (xfree, data);
+  make_cleanup (xfree, buildid);
 
   filename = (const char *) data;
   if (!IS_ABSOLUTE_PATH (filename))
@@ -2396,20 +2399,25 @@ dwarf2_get_dwz_file (void)
       filename = rel;
     }
 
-  /* The format is just a NUL-terminated file name, followed by the
-     build-id.  For now, though, we ignore the build-id.  */
+  /* First try the file name given in the section.  If that doesn't
+     work, try to use the build-id instead.  */
   dwz_bfd = gdb_bfd_open (filename, gnutarget, -1);
-  if (dwz_bfd == NULL)
-    error (_("could not read '%s': %s"), filename,
-	   bfd_errmsg (bfd_get_error ()));
-
-  if (!bfd_check_format (dwz_bfd, bfd_object))
+  if (dwz_bfd != NULL)
     {
-      gdb_bfd_unref (dwz_bfd);
-      error (_("file '%s' was not usable: %s"), filename,
-	     bfd_errmsg (bfd_get_error ()));
+      if (!build_id_verify (dwz_bfd, buildid_len, buildid))
+	{
+	  gdb_bfd_unref (dwz_bfd);
+	  dwz_bfd = NULL;
+	}
     }
 
+  if (dwz_bfd == NULL)
+    dwz_bfd = build_id_to_debug_bfd (buildid_len, buildid);
+
+  if (dwz_bfd == NULL)
+    error (_("could not find '.gnu_debugaltlink' file for %s"),
+	   objfile_name (dwarf2_per_objfile->objfile));
+
   result = OBSTACK_ZALLOC (&dwarf2_per_objfile->objfile->objfile_obstack,
 			   struct dwz_file);
   result->dwz_bfd = dwz_bfd;
diff --git a/gdb/elfread.c b/gdb/elfread.c
index 4a3d713..0095ad9 100644
--- a/gdb/elfread.c
+++ b/gdb/elfread.c
@@ -45,6 +45,7 @@
 #include "regcache.h"
 #include "bcache.h"
 #include "gdb_bfd.h"
+#include "build-id.h"
 
 extern void _initialize_elfread (void);
 
@@ -1087,131 +1088,6 @@ elf_gnu_ifunc_resolver_return_stop (struct breakpoint *b)
   update_breakpoint_locations (b, sals, sals_end);
 }
 
-/* Locate NT_GNU_BUILD_ID from ABFD and return its content.  */
-
-static const struct elf_build_id *
-build_id_bfd_get (bfd *abfd)
-{
-  if (!bfd_check_format (abfd, bfd_object)
-      || bfd_get_flavour (abfd) != bfd_target_elf_flavour
-      || elf_tdata (abfd)->build_id == NULL)
-    return NULL;
-
-  return elf_tdata (abfd)->build_id;
-}
-
-/* Return if FILENAME has NT_GNU_BUILD_ID matching the CHECK value.  */
-
-static int
-build_id_verify (const char *filename, const struct elf_build_id *check)
-{
-  bfd *abfd;
-  const struct elf_build_id *found;
-  int retval = 0;
-
-  /* We expect to be silent on the non-existing files.  */
-  abfd = gdb_bfd_open_maybe_remote (filename);
-  if (abfd == NULL)
-    return 0;
-
-  found = build_id_bfd_get (abfd);
-
-  if (found == NULL)
-    warning (_("File \"%s\" has no build-id, file skipped"), filename);
-  else if (found->size != check->size
-           || memcmp (found->data, check->data, found->size) != 0)
-    warning (_("File \"%s\" has a different build-id, file skipped"),
-	     filename);
-  else
-    retval = 1;
-
-  gdb_bfd_unref (abfd);
-
-  return retval;
-}
-
-static char *
-build_id_to_debug_filename (const struct elf_build_id *build_id)
-{
-  char *link, *debugdir, *retval = NULL;
-  VEC (char_ptr) *debugdir_vec;
-  struct cleanup *back_to;
-  int ix;
-
-  /* DEBUG_FILE_DIRECTORY/.build-id/ab/cdef */
-  link = alloca (strlen (debug_file_directory) + (sizeof "/.build-id/" - 1) + 1
-		 + 2 * build_id->size + (sizeof ".debug" - 1) + 1);
-
-  /* Keep backward compatibility so that DEBUG_FILE_DIRECTORY being "" will
-     cause "/.build-id/..." lookups.  */
-
-  debugdir_vec = dirnames_to_char_ptr_vec (debug_file_directory);
-  back_to = make_cleanup_free_char_ptr_vec (debugdir_vec);
-
-  for (ix = 0; VEC_iterate (char_ptr, debugdir_vec, ix, debugdir); ++ix)
-    {
-      size_t debugdir_len = strlen (debugdir);
-      const gdb_byte *data = build_id->data;
-      size_t size = build_id->size;
-      char *s;
-
-      memcpy (link, debugdir, debugdir_len);
-      s = &link[debugdir_len];
-      s += sprintf (s, "/.build-id/");
-      if (size > 0)
-	{
-	  size--;
-	  s += sprintf (s, "%02x", (unsigned) *data++);
-	}
-      if (size > 0)
-	*s++ = '/';
-      while (size-- > 0)
-	s += sprintf (s, "%02x", (unsigned) *data++);
-      strcpy (s, ".debug");
-
-      /* lrealpath() is expensive even for the usually non-existent files.  */
-      if (access (link, F_OK) == 0)
-	retval = lrealpath (link);
-
-      if (retval != NULL && !build_id_verify (retval, build_id))
-	{
-	  xfree (retval);
-	  retval = NULL;
-	}
-
-      if (retval != NULL)
-	break;
-    }
-
-  do_cleanups (back_to);
-  return retval;
-}
-
-static char *
-find_separate_debug_file_by_buildid (struct objfile *objfile)
-{
-  const struct elf_build_id *build_id;
-
-  build_id = build_id_bfd_get (objfile->obfd);
-  if (build_id != NULL)
-    {
-      char *build_id_name;
-
-      build_id_name = build_id_to_debug_filename (build_id);
-      /* Prevent looping on a stripped .debug file.  */
-      if (build_id_name != NULL
-	  && filename_cmp (build_id_name, objfile_name (objfile)) == 0)
-        {
-	  warning (_("\"%s\": separate debug info file has no debug info"),
-		   build_id_name);
-	  xfree (build_id_name);
-	}
-      else if (build_id_name != NULL)
-        return build_id_name;
-    }
-  return NULL;
-}
-
 /* Scan and build partial symbols for a symbol file.
    We have been initialized by a call to elf_symfile_init, which
    currently does nothing.
diff --git a/gdb/testsuite/ChangeLog b/gdb/testsuite/ChangeLog
index 4c15ae8..3e5e970 100644
--- a/gdb/testsuite/ChangeLog
+++ b/gdb/testsuite/ChangeLog
@@ -1,3 +1,13 @@
+2013-10-08  Tom Tromey  <tromey@redhat.com>
+
+	* gdb.dwarf2/dwzbuildid.exp: New file.
+	* lib/dwarf.exp (Dwarf::_section): Add "flags" and "type"
+	parameters.
+	(Dwarf::_defer_output): Change "section" parameter to
+	"section_spec"; update.
+	(Dwarf::gnu_debugaltlink, Dwarf::_note, Dwarf::build_id): New
+	procs.
+
 2013-10-08  Joel Brobecker  <brobecker@adacore.com>
 
 	* gdb.ada/mi_catch_ex.exp: Make "mi_execute_to" test names unique.
diff --git a/gdb/testsuite/gdb.dwarf2/dwzbuildid.exp b/gdb/testsuite/gdb.dwarf2/dwzbuildid.exp
new file mode 100644
index 0000000..e6ae881
--- /dev/null
+++ b/gdb/testsuite/gdb.dwarf2/dwzbuildid.exp
@@ -0,0 +1,170 @@
+# Copyright 2013 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
+}
+
+# No remote host testing either.
+if {[is_remote host]} {
+    return 0
+}
+
+
+# Lots of source files since we test a few cases and make new files
+# for each.
+# The tests are:
+#     ok - the main file refers to a dwz and the buildids match
+#     mismatch - the buildids do not match
+#     fallback - the buildids do not match but a match is found via buildid
+standard_testfile main.c \
+    dwzbuildid-ok-base.S dwzbuildid-ok-sep.S \
+    dwzbuildid-mismatch-base.S dwzbuildid-mismatch-sep.S \
+    dwzbuildid-fallback-base.S dwzbuildid-fallback-sep.S \
+    dwzbuildid-fallback-ok.S
+    
+# Write some assembly that just has a .gnu_debugaltlink section.
+proc write_just_debugaltlink {filename dwzname buildid} {
+    set asm_file [standard_output_file $filename]
+
+    Dwarf::assemble $asm_file {
+	upvar dwzname dwzname
+	upvar buildid buildid
+
+	gnu_debugaltlink $dwzname $buildid
+
+	# Only the DWARF reader checks .gnu_debugaltlink, so make sure
+	# there is a bit of DWARF in here.
+	cu {} {
+	    compile_unit {{language @DW_LANG_C}} {
+	    }
+	}
+    }
+}
+
+# Write some DWARF that also sets the buildid.
+proc write_dwarf_file {filename buildid {value 99}} {
+    set asm_file [standard_output_file $filename]
+
+    Dwarf::assemble $asm_file {
+	declare_labels partial_label double_label int_label int_label2
+
+	upvar buildid buildid
+	upvar value value
+
+	build_id $buildid
+
+	cu {} {
+	    compile_unit {{language @DW_LANG_C}} {
+		int_label2: base_type {
+		    {name int}
+		    {byte_size 4 sdata}
+		    {encoding @DW_ATE_signed}
+		}
+
+		constant {
+		    {name the_int}
+		    {type :$int_label2}
+		    {const_value $value data1}
+		}
+	    }
+	}
+    }
+}
+
+if  { [gdb_compile ${srcdir}/${subdir}/${srcfile} ${binfile}1.o \
+	   object {nodebug}] != "" } {
+    return -1
+}
+
+# The values don't really matter, just whether they are equal.
+set ok_prefix 01
+set ok_suffix 0203040506
+set ok_suffix2 02030405ff
+set ok_buildid ${ok_prefix}${ok_suffix}
+set ok_buildid2 ${ok_prefix}${ok_suffix2}
+set bad_buildid ffffffffffff
+
+set outdir [standard_output_file {}]
+set basedir $outdir/.build-id
+file mkdir $basedir $basedir/$ok_prefix
+
+# Test where the separate debuginfo's buildid matches.
+write_just_debugaltlink $srcfile2 ${binfile}3.o $ok_buildid
+write_dwarf_file $srcfile3 $ok_buildid
+
+# Test where the separate debuginfo's buildid does not match.
+write_just_debugaltlink $srcfile4 ${binfile}5.o $ok_buildid
+write_dwarf_file $srcfile5 $bad_buildid
+
+# Test where the separate debuginfo's buildid does not match, but then
+# we find a match in the .build-id directory.
+write_just_debugaltlink $srcfile6 ${binfile}7.o $ok_buildid2
+# Use 77 as the value so that if we load the bad debuginfo, we will
+# see the wrong result.
+write_dwarf_file $srcfile7 $bad_buildid 77
+write_dwarf_file $srcfile8 $ok_buildid2
+
+# Compile everything.
+for {set i 2} {$i <= 8} {incr i} {
+    if {[gdb_compile [standard_output_file [set srcfile$i]] \
+	     ${binfile}$i.o object nodebug] != ""} {
+	return -1
+    }
+}
+
+# Copy a file into the .build-id place for the "fallback" test.
+file copy -force -- ${binfile}8.o $basedir/$ok_prefix/$ok_suffix2.debug
+
+# Link the executables.
+if {[gdb_compile [list ${binfile}1.o ${binfile}2.o] ${binfile}-ok \
+	 executable {}] != ""} {
+    return -1
+}
+
+if {[gdb_compile [list ${binfile}1.o ${binfile}4.o] ${binfile}-mismatch \
+	 executable {}] != ""} {
+    return -1
+}
+
+if {[gdb_compile [list ${binfile}1.o ${binfile}6.o] ${binfile}-fallback \
+	 executable {}] != ""} {
+    return -1
+}
+
+
+foreach testname {ok mismatch fallback} {
+    with_test_prefix $testname {
+	gdb_exit
+	gdb_start
+	gdb_reinitialize_dir $srcdir/$subdir
+
+	gdb_test_no_output "set debug-file-directory $outdir"
+
+	gdb_load ${binfile}-${testname}
+
+	if {[runto_main]} {
+	    if {$testname == "mismatch"} {
+		gdb_test "print the_int" \
+		    "No symbol table is loaded.*"
+	    } else {
+		gdb_test "print the_int" " = 99"
+	    }
+	}
+    }
+}
diff --git a/gdb/testsuite/lib/dwarf.exp b/gdb/testsuite/lib/dwarf.exp
index 5264def..c28b986 100644
--- a/gdb/testsuite/lib/dwarf.exp
+++ b/gdb/testsuite/lib/dwarf.exp
@@ -523,20 +523,27 @@ namespace eval Dwarf {
 	}
     }
 
-    proc _section {name} {
-	_emit "        .section $name"
+    proc _section {name {flags ""} {type ""}} {
+	if {$flags == "" && $type == ""} {
+	    _emit "        .section $name"
+	} elseif {$type == ""} {
+	    _emit "        .section $name, \"$flags\""
+	} else {
+	    _emit "        .section $name, \"$flags\", %$type"
+	}
     }
 
-    proc _defer_output {section body} {
+    # SECTION_SPEC is a list of arguments to _section.
+    proc _defer_output {section_spec body} {
 	variable _defer
 	variable _deferred_output
 
 	set old_defer $_defer
-	set _defer $section
+	set _defer [lindex $section_spec 0]
 
 	if {![info exists _deferred_output($_defer)]} {
 	    set _deferred_output($_defer) ""
-	    _section $section
+	    eval _section $section_spec
 	}
 
 	uplevel $body
@@ -948,6 +955,51 @@ namespace eval Dwarf {
 	unset the_array(_)
     }
 
+    # Emit a .gnu_debugaltlink section with the given file name and
+    # build-id.  The buildid should be represented as a hexadecimal
+    # string, like "ffeeddcc".
+    proc gnu_debugaltlink {filename buildid} {
+	_defer_output .gnu_debugaltlink {
+	    _op .ascii [_quote $filename]
+	    foreach {a b} [split $buildid {}] {
+		_op .byte 0x$a$b
+	    }
+	}
+    }
+
+    proc _note {type name hexdata} {
+	set namelen [expr [string length $name] + 1]
+
+	# Name size.
+	_op .4byte $namelen
+	# Data size.
+	_op .4byte [expr [string length $hexdata] / 2]
+	# Type.
+	_op .4byte $type
+	# The name.
+	_op .ascii [_quote $name]
+	# Alignment.
+	set align 2
+	set total [expr {($namelen + (1 << $align) - 1) & (-1 << $align)}]
+	for {set i $namelen} {$i < $total} {incr i} {
+	    _op .byte 0
+	}
+	# The data.
+	foreach {a b} [split $hexdata {}] {
+	    _op .byte 0x$a$b
+	}
+    }
+
+    # Emit a note section holding the given build-id.
+    proc build_id {buildid} {
+	_defer_output {.note.gnu.build-id a note} {
+	    # From elf/common.h.
+	    set NT_GNU_BUILD_ID 3
+
+	    _note $NT_GNU_BUILD_ID GNU $buildid
+	}
+    }
+
     # The top-level interface to the DWARF assembler.
     # FILENAME is the name of the file where the generated assembly
     # code is written.