[PATCH v2] ld: add --package-metadata

[luca.boccassi@gmail.com]

From: Luca Boccassi <bluca@debian.org>

Generate a .note.package FDO package metadata ELF note, following
the spec: https://systemd.io/ELF_PACKAGE_METADATA/

If the jansson library is available at build time (and not explicitly
disabled), link ld to it, and use it to validate that the input is
correct JSON, to avoid writing garbage to the file.
If no configure option is passed and jansson (or pkg-config) is not
found, then it is just skipped (with a non-fatal warning). The
configure option --enable-jansson can be used to explicitly enable it
(error out when not found) or to explicitly disable it (don't even
look for it). This allows bootstrappers (or others who are not
interested) to seamlessly skip it without issues."
---
v2: fix style issues
    fix ASAN issue: note storage is rounded up, but that size
    was used to read the json input, which is fixed
    add entry to NEWS
    add note about json validation to docs

 bfd/elf-bfd.h                        |   8 ++
 bfd/elf.c                            |   8 +-
 ld/Makefile.am                       |   6 +-
 ld/NEWS                              |   6 ++
 ld/configure.ac                      |  35 +++++++
 ld/emultempl/elf.em                  |   9 ++
 ld/ld.texi                           |  12 +++
 ld/ldelf.c                           | 136 ++++++++++++++++++++++++++-
 ld/ldelf.h                           |   2 +
 ld/lexsup.c                          |   2 +
 ld/testsuite/ld-elf/package-note.exp |  45 +++++++++
 ld/testsuite/ld-elf/package-note.rd  |   6 ++
 12 files changed, 268 insertions(+), 7 deletions(-)
 create mode 100644 ld/testsuite/ld-elf/package-note.exp
 create mode 100644 ld/testsuite/ld-elf/package-note.rd

diff --git a/bfd/elf-bfd.h b/bfd/elf-bfd.h
index c7c0a793b15..65bd1128263 100644
--- a/bfd/elf-bfd.h
+++ b/bfd/elf-bfd.h
@@ -1918,6 +1918,14 @@ struct output_elf_obj_tdata
     asection *sec;
   } build_id;
 
+  /* FDO_PACKAGING_METADATA note type info.  */
+  struct
+  {
+    bool (*after_write_object_contents) (bfd *);
+    const char *json;
+    asection *sec;
+  } package_metadata;
+
   /* Records the result of `get_program_header_size'.  */
   bfd_size_type program_header_size;
 
diff --git a/bfd/elf.c b/bfd/elf.c
index c493aa5b172..fcc7d14a1d6 100644
--- a/bfd/elf.c
+++ b/bfd/elf.c
@@ -6779,8 +6779,12 @@ _bfd_elf_write_object_contents (bfd *abfd)
     return false;
 
   /* This is last since write_shdrs_and_ehdr can touch i_shdrp[0].  */
-  if (t->o->build_id.after_write_object_contents != NULL)
-    return (*t->o->build_id.after_write_object_contents) (abfd);
+  if (t->o->build_id.after_write_object_contents != NULL
+       && !(*t->o->build_id.after_write_object_contents) (abfd))
+    return false;
+  if (t->o->package_metadata.after_write_object_contents != NULL
+       && !(*t->o->package_metadata.after_write_object_contents) (abfd))
+    return false;
 
   return true;
 }
diff --git a/ld/Makefile.am b/ld/Makefile.am
index e53bef13bb8..a3cd7891840 100644
--- a/ld/Makefile.am
+++ b/ld/Makefile.am
@@ -45,7 +45,7 @@ ELF_CLFAGS=-DELF_LIST_OPTIONS=@elf_list_options@ \
 	   -DELF_PLT_UNWIND_LIST_OPTIONS=@elf_plt_unwind_list_options@
 WARN_CFLAGS = @WARN_CFLAGS@
 NO_WERROR = @NO_WERROR@
-AM_CFLAGS = $(WARN_CFLAGS) $(ELF_CLFAGS)
+AM_CFLAGS = $(WARN_CFLAGS) $(ELF_CLFAGS) $(JANSSON_CFLAGS)
 
 # We put the scripts in the directory $(scriptdir)/ldscripts.
 # We can't put the scripts in $(datadir) because the SEARCH_DIR
@@ -964,8 +964,8 @@ ld_new_SOURCES = ldgram.y ldlex-wrapper.c lexsup.c ldlang.c mri.c ldctor.c ldmai
 	ldwrite.c ldexp.c ldemul.c ldver.c ldmisc.c ldfile.c ldcref.c plugin.c \
 	ldbuildid.c
 ld_new_DEPENDENCIES = $(EMULATION_OFILES) $(EMUL_EXTRA_OFILES) \
-		      $(BFDLIB) $(LIBCTF) $(LIBIBERTY) $(LIBINTL_DEP)
-ld_new_LDADD = $(EMULATION_OFILES) $(EMUL_EXTRA_OFILES) $(BFDLIB) $(LIBCTF) $(LIBIBERTY) $(LIBINTL) $(ZLIB)
+		      $(BFDLIB) $(LIBCTF) $(LIBIBERTY) $(LIBINTL_DEP) $(JANSSON_LIBS)
+ld_new_LDADD = $(EMULATION_OFILES) $(EMUL_EXTRA_OFILES) $(BFDLIB) $(LIBCTF) $(LIBIBERTY) $(LIBINTL) $(ZLIB) $(JANSSON_LIBS)
 
 # Dependency tracking for the generated emulation files.
 EXTRA_ld_new_SOURCES += $(ALL_EMULATION_SOURCES) $(ALL_64_EMULATION_SOURCES)
diff --git a/ld/NEWS b/ld/NEWS
index 514d1d9f207..d2ade345b5d 100644
--- a/ld/NEWS
+++ b/ld/NEWS
@@ -36,6 +36,12 @@
 * Remove (rudimentary) support for the x86-64 sub-architectures Intel L1OM and
   Intel K1OM.
 
+* The ELF linker now supports a new --package-metadata option that allows
+  embedding a JSON payload in accordance to the Package Metadata specification.
+  If libjansson is present at build time, the linker will use it to validate
+  the input. This can be disabled with --disable-jansson.
+  For more details, see: https://systemd.io/ELF_PACKAGE_METADATA/
+
 Changes in 2.38:
 
 * Add -z pack-relative-relocs/-z no pack-relative-relocs to x86 ELF
diff --git a/ld/configure.ac b/ld/configure.ac
index 01121480c6d..a5e7afa89e8 100644
--- a/ld/configure.ac
+++ b/ld/configure.ac
@@ -289,6 +289,41 @@ fi
 AM_CONDITIONAL(ENABLE_LIBCTF, test "${enable_libctf}" = yes)
 AC_SUBST(enable_libctf)
 
+# If pkg-config and libjansson are available, use them, skip if not.
+# Allow explicitly enabling (error if not found)/disabling (skip even if found).
+AC_ARG_ENABLE([jansson],
+  [AS_HELP_STRING([--enable-jansson],
+    [enable jansson [default=auto]])],
+  [enable_jansson=$enableval],
+  [enable_jansson="auto"])
+
+if test "x$enable_jansson" != "xno"; then
+  PKG_PROG_PKG_CONFIG
+  AS_IF([test -n "$PKG_CONFIG"],
+    [
+      PKG_CHECK_MODULES(JANSSON, [jansson],
+        [
+          AC_DEFINE(HAVE_JANSSON, 1, [The jansson library is to be used])
+          AC_SUBST([JANSSON_CFLAGS])
+          AC_SUBST([JANSSON_LIBS])
+        ],
+        [
+          if test "x$enable_jansson" = "xyes"; then
+            AC_MSG_ERROR([Cannot find jansson library])
+          else
+            AC_MSG_WARN([Cannot find jansson library])
+          fi
+        ])
+    ],
+    [
+      if test "x$enable_jansson" = "xyes"; then
+        AC_MSG_ERROR([Cannot find pkg-config])
+      else
+        AC_MSG_WARN([Cannot find pkg-config])
+      fi
+    ])
+fi
+
 AM_BINUTILS_WARNINGS
 
 AM_LC_MESSAGES
diff --git a/ld/emultempl/elf.em b/ld/emultempl/elf.em
index c027559908c..c52484f35d3 100644
--- a/ld/emultempl/elf.em
+++ b/ld/emultempl/elf.em
@@ -572,6 +572,7 @@ enum elf_options
   OPTION_EXCLUDE_LIBS,
   OPTION_HASH_STYLE,
   OPTION_BUILD_ID,
+  OPTION_PACKAGE_METADATA,
   OPTION_AUDIT,
   OPTION_COMPRESS_DEBUG
 };
@@ -602,6 +603,7 @@ EOF
 fi
 fragment <<EOF
     {"build-id", optional_argument, NULL, OPTION_BUILD_ID},
+    {"package-metadata", optional_argument, NULL, OPTION_PACKAGE_METADATA},
     {"compress-debug-sections", required_argument, NULL, OPTION_COMPRESS_DEBUG},
 EOF
 if test x"$GENERATE_SHLIB_SCRIPT" = xyes; then
@@ -650,6 +652,13 @@ gld${EMULATION_NAME}_handle_option (int optc)
 	ldelf_emit_note_gnu_build_id = xstrdup (optarg);
       break;
 
+    case OPTION_PACKAGE_METADATA:
+      free ((char *) ldelf_emit_note_fdo_package_metadata);
+      ldelf_emit_note_fdo_package_metadata = NULL;
+      if (optarg != NULL && strlen(optarg) > 0)
+	ldelf_emit_note_fdo_package_metadata = xstrdup (optarg);
+      break;
+
     case OPTION_COMPRESS_DEBUG:
       if (strcasecmp (optarg, "none") == 0)
 	link_info.compress_debug = COMPRESS_DEBUG_NONE;
diff --git a/ld/ld.texi b/ld/ld.texi
index a2b162ce5b5..eabbec8faa9 100644
--- a/ld/ld.texi
+++ b/ld/ld.texi
@@ -2935,6 +2935,18 @@ string identifying the original linked file does not change.
 
 Passing @code{none} for @var{style} disables the setting from any
 @code{--build-id} options earlier on the command line.
+
+@kindex --package-metadata=@var{JSON}
+@item --package-metadata=@var{JSON}
+Request the creation of a @code{.note.package} ELF note section.  The
+contents of the note are in JSON format, as per the package metadata
+specification.  For more information see:
+https://systemd.io/ELF_PACKAGE_METADATA/
+If the JSON argument is missing/empty then this will disable the
+creation of the metadata note, if one had been enabled by an earlier
+occurrence of the --package-metdata option.
+If the linker has been built with libjansson, then the JSON string
+will be validated.
 @end table
 
 @c man end
diff --git a/ld/ldelf.c b/ld/ldelf.c
index 4094640b3f7..e5d1b32db06 100644
--- a/ld/ldelf.c
+++ b/ld/ldelf.c
@@ -39,6 +39,9 @@
 #include <glob.h>
 #endif
 #include "ldelf.h"
+#ifdef HAVE_JANSSON
+#include <jansson.h>
+#endif
 
 struct dt_needed
 {
@@ -49,6 +52,9 @@ struct dt_needed
 /* Style of .note.gnu.build-id section.  */
 const char *ldelf_emit_note_gnu_build_id;
 
+/* Content of .note.package section.  */
+const char *ldelf_emit_note_fdo_package_metadata;
+
 /* These variables are required to pass information back and forth
    between after_open and check_needed and stat_needed and vercheck.  */
 
@@ -1249,7 +1255,8 @@ ldelf_after_open (int use_libpath, int native, int is_linux, int is_freebsd,
 	}
     }
 
-  if (ldelf_emit_note_gnu_build_id != NULL)
+  if (ldelf_emit_note_gnu_build_id != NULL
+      || ldelf_emit_note_fdo_package_metadata != NULL)
     {
       /* Find an ELF input.  */
       for (abfd = link_info.input_bfds;
@@ -1262,11 +1269,18 @@ ldelf_after_open (int use_libpath, int native, int is_linux, int is_freebsd,
       /* PR 10555: If there are no ELF input files do not try to
 	 create a .note.gnu-build-id section.  */
       if (abfd == NULL
-	  || !ldelf_setup_build_id (abfd))
+	  || (ldelf_emit_note_gnu_build_id != NULL && !ldelf_setup_build_id (abfd)))
 	{
 	  free ((char *) ldelf_emit_note_gnu_build_id);
 	  ldelf_emit_note_gnu_build_id = NULL;
 	}
+
+      if (abfd == NULL
+	  || (ldelf_emit_note_fdo_package_metadata != NULL && !ldelf_setup_package_metadata (abfd)))
+	{
+	  free ((char *) ldelf_emit_note_fdo_package_metadata);
+	  ldelf_emit_note_fdo_package_metadata = NULL;
+	}
     }
 
   get_elf_backend_data (link_info.output_bfd)->setup_gnu_properties (&link_info);
@@ -1501,6 +1515,124 @@ ldelf_setup_build_id (bfd *ibfd)
   return false;
 }
 
+static bool
+write_package_metadata (bfd *abfd)
+{
+  struct elf_obj_tdata *t = elf_tdata (abfd);
+  const char *json;
+  asection *asec;
+  Elf_Internal_Shdr *i_shdr;
+  unsigned char *contents, *json_bits;
+  bfd_size_type size;
+  file_ptr position;
+  Elf_External_Note *e_note;
+
+  json = t->o->package_metadata.json;
+  asec = t->o->package_metadata.sec;
+  if (bfd_is_abs_section (asec->output_section))
+    {
+      einfo (_("%P: warning: .note.package section discarded,"
+	       " --package-metadata ignored\n"));
+      return true;
+    }
+  i_shdr = &elf_section_data (asec->output_section)->this_hdr;
+
+  if (i_shdr->contents == NULL)
+    {
+      if (asec->contents == NULL)
+	asec->contents = (unsigned char *) xmalloc (asec->size);
+      contents = asec->contents;
+    }
+  else
+    contents = i_shdr->contents + asec->output_offset;
+
+  e_note = (Elf_External_Note *) contents;
+  size = offsetof (Elf_External_Note, name[sizeof "FDO"]);
+  size = (size + 3) & -(bfd_size_type) 4;
+  json_bits = contents + size;
+  size = asec->size - size;
+
+  /* Clear the package metadata field.  */
+  memset (json_bits, 0, size);
+
+  bfd_h_put_32 (abfd, sizeof "FDO", &e_note->namesz);
+  bfd_h_put_32 (abfd, size, &e_note->descsz);
+  bfd_h_put_32 (abfd, FDO_PACKAGING_METADATA, &e_note->type);
+  memcpy (e_note->name, "FDO", sizeof "FDO");
+  memcpy (json_bits, json, strlen(json));
+
+  position = i_shdr->sh_offset + asec->output_offset;
+  size = asec->size;
+  return (bfd_seek (abfd, position, SEEK_SET) == 0
+	  && bfd_bwrite (contents, size, abfd) == size);
+}
+
+/* Make .note.package section.
+   https://systemd.io/ELF_PACKAGE_METADATA/  */
+
+bool
+ldelf_setup_package_metadata (bfd *ibfd)
+{
+  asection *s;
+  bfd_size_type size;
+  size_t json_length;
+  flagword flags;
+
+  /* If the option wasn't specified, silently return. */
+  if (!ldelf_emit_note_fdo_package_metadata)
+    return false;
+
+  /* The option was specified, but it's too short/long, log and return. */
+  json_length = strlen (ldelf_emit_note_fdo_package_metadata);
+  if (json_length == 0)
+    {
+      einfo (_("%P: warning: --package-metadata is empty, ignoring\n"));
+      return false;
+    }
+  if (json_length > SIZE_MAX)
+    {
+      einfo (_("%P: warning: --package-metadata is too long, ignoring\n"));
+      return false;
+    }
+
+#ifdef HAVE_JANSSON
+  json_error_t json_error;
+  json_t *json = json_loads (ldelf_emit_note_fdo_package_metadata, 0, &json_error);
+  if (!json)
+    {
+      einfo (_("%P: warning: --package-metadata=%s does not contain valid "
+	       "JSON, ignoring: %s\n"), ldelf_emit_note_fdo_package_metadata,
+         json_error.text);
+      return false;
+    }
+  else
+    json_decref (json);
+#endif
+
+  size = offsetof (Elf_External_Note, name[sizeof "FDO"]);
+  size += json_length + 1;
+  size = (size + 3) & -(bfd_size_type) 4;
+
+  flags = (SEC_ALLOC | SEC_LOAD | SEC_IN_MEMORY
+	   | SEC_LINKER_CREATED | SEC_READONLY | SEC_DATA);
+  s = bfd_make_section_anyway_with_flags (ibfd, ".note.package",
+					  flags);
+  if (s != NULL && bfd_set_section_alignment (s, 2))
+    {
+      struct elf_obj_tdata *t = elf_tdata (link_info.output_bfd);
+      t->o->package_metadata.after_write_object_contents = &write_package_metadata;
+      t->o->package_metadata.json = ldelf_emit_note_fdo_package_metadata;
+      t->o->package_metadata.sec = s;
+      elf_section_type (s) = SHT_NOTE;
+      s->size = size;
+      return true;
+    }
+
+  einfo (_("%P: warning: cannot create .note.package section,"
+	   " --package-metadata ignored\n"));
+  return false;
+}
+
 /* Look through an expression for an assignment statement.  */
 
 static void
diff --git a/ld/ldelf.h b/ld/ldelf.h
index efa8b45851c..a3ded3dd04c 100644
--- a/ld/ldelf.h
+++ b/ld/ldelf.h
@@ -19,6 +19,7 @@
    MA 02110-1301, USA.  */
 
 extern const char *ldelf_emit_note_gnu_build_id;
+extern const char *ldelf_emit_note_fdo_package_metadata;
 
 extern void ldelf_after_parse (void);
 extern bool ldelf_load_symbols (lang_input_statement_type *);
@@ -26,6 +27,7 @@ extern void ldelf_before_plugin_all_symbols_read (int, int, int, int,
 						  int, const char *);
 extern void ldelf_after_open (int, int, int, int, int, const char *);
 extern bool ldelf_setup_build_id (bfd *);
+extern bool ldelf_setup_package_metadata (bfd *);
 extern void ldelf_append_to_separated_string (char **, char *);
 extern void ldelf_before_allocation (char *, char *, const char *);
 extern bool ldelf_open_dynamic_archive
diff --git a/ld/lexsup.c b/ld/lexsup.c
index 82c459adb51..9225f71b3ce 100644
--- a/ld/lexsup.c
+++ b/ld/lexsup.c
@@ -2144,6 +2144,8 @@ elf_static_list_options (FILE *file)
   fprintf (file, _("\
   --build-id[=STYLE]          Generate build ID note\n"));
   fprintf (file, _("\
+  --package-metadata[=JSON]   Generate package metadata note\n"));
+  fprintf (file, _("\
   --compress-debug-sections=[none|zlib|zlib-gnu|zlib-gabi]\n\
                               Compress DWARF debug sections using zlib\n"));
 #ifdef DEFAULT_FLAG_COMPRESS_DEBUG
diff --git a/ld/testsuite/ld-elf/package-note.exp b/ld/testsuite/ld-elf/package-note.exp
new file mode 100644
index 00000000000..c4239098691
--- /dev/null
+++ b/ld/testsuite/ld-elf/package-note.exp
@@ -0,0 +1,45 @@
+# Expect script for --package-note tests.
+#   Copyright (C) 2022 Free Software Foundation, Inc.
+#
+# This file is part of the GNU Binutils.
+#
+# 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, write to the Free Software
+# Foundation, Inc., 51 Franklin Street - Fifth Floor, Boston,
+# MA 02110-1301, USA.
+#
+
+# Exclude non-ELF targets.
+
+if ![is_elf_format] {
+    return
+}
+
+if { !([istarget *-*-linux*]
+       || [istarget arm*-*-uclinuxfdpiceabi]
+       || [istarget *-*-nacl*]
+       || [istarget *-*-gnu*]) } then {
+    return
+}
+
+run_ld_link_tests [list \
+    [list \
+	"package-note.o" \
+	"--package-metadata='{\"foo\":\"bar\"}'" \
+	"" \
+	"" \
+	{start.s} \
+	{{readelf {--notes} package-note.rd}} \
+	"package-note.o" \
+    ] \
+]
diff --git a/ld/testsuite/ld-elf/package-note.rd b/ld/testsuite/ld-elf/package-note.rd
new file mode 100644
index 00000000000..c65dd21cdac
--- /dev/null
+++ b/ld/testsuite/ld-elf/package-note.rd
@@ -0,0 +1,6 @@
+#...
+Displaying notes found in: \.note\.package
+  Owner                Data size 	Description
+  FDO                  0x00000010	FDO_PACKAGING_METADATA
+    Packaging Metadata: {"foo":"bar"}
+#pass
-- 
2.35.1