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, 57 insertions, 5 deletions

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.