ld: add --package-metadata

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 it is explicitly
enabled), link ld to it, and use it to validate that the input is
correct JSON, to avoid writing garbage to the file. The
configure option --enable-jansson has to be used to explicitly enable
it (error out when not found). This allows bootstrappers (or others who
are not interested) to seamlessly skip it without issues.

1 files changed, 133 insertions, 2 deletions

diff --git a/ld/ldelf.c b/ld/ldelf.c
index 4094640..bfa0d54 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 +1269,20 @@ 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 +1517,121 @@ 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 empty, 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;
+    }
+
+#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