elf: Add extension mechanism to ld.so.cache

A previously unused new-format header field is used to record
the address of an extension directory.

This change adds a demo extension which records the version of
ldconfig which builds a file.

Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>

1 files changed, 89 insertions, 0 deletions

diff --git a/elf/cache.c b/elf/cache.c
index ffecbe6..549e04c 100644
--- a/elf/cache.c
+++ b/elf/cache.c
@@ -15,6 +15,7 @@
    You should have received a copy of the GNU General Public License
    along with this program; if not, see <https://www.gnu.org/licenses/>.  */
 
+#include <assert.h>
 #include <errno.h>
 #include <error.h>
 #include <dirent.h>
@@ -33,6 +34,7 @@
 
 #include <ldconfig.h>
 #include <dl-cache.h>
+#include <version.h>
 
 struct cache_entry
 {
@@ -161,6 +163,21 @@ check_new_cache (struct cache_file_new *cache)
     error (EXIT_FAILURE, 0, _("Cache file has wrong endianness.\n"));
 }
 
+/* Print the extension information at the cache at start address
+   FILE_BASE, of length FILE_SIZE bytes.  The new-format cache header
+   is at CACHE, and the file name for diagnostics is CACHE_NAME.  */
+static void
+print_extensions (struct cache_extension_all_loaded *ext)
+{
+  if (ext->sections[cache_extension_tag_generator].base != NULL)
+    {
+      fputs (_("Cache generated by: "), stdout);
+      fwrite (ext->sections[cache_extension_tag_generator].base, 1,
+	      ext->sections[cache_extension_tag_generator].size, stdout);
+      putchar ('\n');
+    }
+}
+
 /* Print the whole cache file, if a file contains the new cache format
    hidden in the old one, print the contents of the new format.  */
 void
@@ -250,6 +267,11 @@ print_cache (const char *cache_name)
     }
   else if (format == 1)
     {
+      struct cache_extension_all_loaded ext;
+      if (!cache_extension_load (cache_new, cache, cache_size, &ext))
+	error (EXIT_FAILURE, 0,
+	       _("Malformed extension data in cache file %s\n"), cache_name);
+
       printf (_("%d libs found in cache `%s'\n"),
 	      cache_new->nlibs, cache_name);
 
@@ -260,6 +282,7 @@ print_cache (const char *cache_name)
 		     cache_new->libs[i].osversion,
 		     cache_new->libs[i].hwcap,
 		     cache_data + cache_new->libs[i].value);
+      print_extensions (&ext);
     }
   /* Cleanup.  */
   munmap (cache, cache_size);
@@ -301,6 +324,45 @@ compare (const struct cache_entry *e1, const struct cache_entry *e2)
   return res;
 }
 
+/* Size of the cache extension directory.  All tags are assumed to be
+   present.  */
+enum
+  {
+   cache_extension_size = (offsetof (struct cache_extension, sections)
+			   + (cache_extension_count
+			      * sizeof (struct cache_extension_section)))
+  };
+
+/* Write the cache extensions to FD.  The extension directory is
+   assumed to be located at CACHE_EXTENSION_OFFSET.  */
+static void
+write_extensions (int fd, uint32_t cache_extension_offset)
+{
+  assert ((cache_extension_offset % 4) == 0);
+
+  struct cache_extension *ext = xmalloc (cache_extension_size);
+  ext->magic = cache_extension_magic;
+  ext->count = cache_extension_count;
+
+  for (int i = 0; i < cache_extension_count; ++i)
+    {
+      ext->sections[i].tag = i;
+      ext->sections[i].flags = 0;
+    }
+
+  const char *generator
+    = "ldconfig " PKGVERSION RELEASE " release version " VERSION;
+  ext->sections[cache_extension_tag_generator].offset
+    = cache_extension_offset + cache_extension_size;
+  ext->sections[cache_extension_tag_generator].size = strlen (generator);
+
+  if (write (fd, ext, cache_extension_size) != cache_extension_size
+      || write (fd, generator, strlen (generator)) != strlen (generator))
+    error (EXIT_FAILURE, errno, _("Writing of cache extension data failed"));
+
+  free (ext);
+}
+
 /* Save the contents of the cache.  */
 void
 save_cache (const char *cache_name)
@@ -435,6 +497,25 @@ save_cache (const char *cache_name)
       && idx_old < cache_entry_old_count)
     file_entries->libs[idx_old] = file_entries->libs[idx_old - 1];
 
+  /* Compute the location of the extension directory.  This
+     implementation puts the directory after the string table.  The
+     size computation matches the write calls below.  The extension
+     directory does not exist with format 0, so the value does not
+     matter.  */
+  uint32_t extension_offset = 0;
+  if (opt_format != opt_format_new)
+    extension_offset += file_entries_size;
+  if (opt_format != opt_format_old)
+    {
+      if (opt_format != opt_format_new)
+	extension_offset += pad;
+      extension_offset += file_entries_new_size;
+    }
+  extension_offset += total_strlen;
+  extension_offset = roundup (extension_offset, 4); /* Provide alignment.  */
+  if (opt_format != opt_format_old)
+    file_entries_new->extension_offset = extension_offset;
+
   /* Write out the cache.  */
 
   /* Write cache first to a temporary file and rename it later.  */
@@ -473,6 +554,14 @@ save_cache (const char *cache_name)
   if (write (fd, strings, total_strlen) != (ssize_t) total_strlen)
     error (EXIT_FAILURE, errno, _("Writing of cache data failed"));
 
+  if (opt_format != opt_format_old)
+    {
+      /* Align file position to 4.  */
+      off64_t old_offset = lseek64 (fd, extension_offset, SEEK_SET);
+      assert ((unsigned long long int) (extension_offset - old_offset) < 4);
+      write_extensions (fd, extension_offset);
+    }
+
   /* Make sure user can always read cache file */
   if (chmod (temp_name, S_IROTH|S_IRGRP|S_IRUSR|S_IWUSR))
     error (EXIT_FAILURE, errno,