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.

1 files changed, 32 insertions, 9 deletions

diff --git a/bfd/opncls.c b/bfd/opncls.c
index cd9c826..f29b2c8 100644
--- a/bfd/opncls.c
+++ b/bfd/opncls.c
@@ -1194,18 +1194,21 @@ FUNCTION
 	bfd_get_alt_debug_link_info
 
 SYNOPSIS
-	char *bfd_get_alt_debug_link_info (bfd *abfd, unsigned long *crc32_out);
+	char *bfd_get_alt_debug_link_info (bfd * abfd, size_t *buildid_len,
+			                   bfd_byte **buildid_out);
 
 DESCRIPTION
 	Fetch the filename and BuildID value for any alternate debuginfo
 	associated with @var{abfd}.  Return NULL if no such info found,
-	otherwise return filename and update @var{buildid_out}.  The
-	returned filename is allocated with @code{malloc}; freeing it
-	is the responsibility of the caller.
+	otherwise return filename and update @var{buildid_len} and
+	@var{buildid_out}.  The returned filename and build_id are
+	allocated with @code{malloc}; freeing them is the
+	responsibility of the caller.
 */
 
 char *
-bfd_get_alt_debug_link_info (bfd * abfd, unsigned long * buildid_out)
+bfd_get_alt_debug_link_info (bfd * abfd, size_t *buildid_len,
+			     bfd_byte **buildid_out)
 {
   asection *sect;
   bfd_byte *contents;
@@ -1213,6 +1216,7 @@ bfd_get_alt_debug_link_info (bfd * abfd, unsigned long * buildid_out)
   char *name;
 
   BFD_ASSERT (abfd);
+  BFD_ASSERT (buildid_len);
   BFD_ASSERT (buildid_out);
 
   sect = bfd_get_section_by_name (abfd, GNU_DEBUGALTLINK);
@@ -1227,12 +1231,13 @@ bfd_get_alt_debug_link_info (bfd * abfd, unsigned long * buildid_out)
       return NULL;
     }
 
-  /* BuildID value is stored after the filename, aligned up to 4 bytes.  */
+  /* BuildID value is stored after the filename.  */
   name = (char *) contents;
   buildid_offset = strlen (name) + 1;
-  buildid_offset = (buildid_offset + 3) & ~3;
 
-  * buildid_out = bfd_get_32 (abfd, contents + buildid_offset);
+  *buildid_len = bfd_get_section_size (sect) - buildid_offset;
+  *buildid_out = bfd_malloc (*buildid_len);
+  memcpy (*buildid_out, contents + buildid_offset, *buildid_len);
 
   return name;
 }
@@ -1466,6 +1471,24 @@ bfd_follow_gnu_debuglink (bfd *abfd, const char *dir)
 				   separate_debug_file_exists);
 }
 
+/* Helper for bfd_follow_gnu_debugaltlink.  It just pretends to return
+   a CRC.  .gnu_debugaltlink supplies a build-id, which is different,
+   but this is ok because separate_alt_debug_file_exists ignores the
+   CRC anyway.  */
+
+static char *
+get_alt_debug_link_info_shim (bfd * abfd, unsigned long *crc32_out)
+{
+  size_t len;
+  bfd_byte *buildid = NULL;
+  char *result = bfd_get_alt_debug_link_info (abfd, &len, &buildid);
+
+  *crc32_out = 0;
+  free (buildid);
+
+  return result;
+}
+
 /*
 FUNCTION
 	bfd_follow_gnu_debugaltlink
@@ -1496,7 +1519,7 @@ char *
 bfd_follow_gnu_debugaltlink (bfd *abfd, const char *dir)
 {
   return find_separate_debug_file (abfd, dir,
-				   bfd_get_alt_debug_link_info,
+				   get_alt_debug_link_info_shim,
 				   separate_alt_debug_file_exists);
 }