[PATCH 23/30] elf: Add endianness markup to ld.so.cache

[Florian Weimer]

Use a reserved byte in the new format cache header to indicate whether
the file is in little endian or big endian format.  Eventually, this
information could be used to provide a unified cache for qemu-user
and similiar scenarios.
---
 elf/cache.c                | 11 ++++++++++
 elf/dl-cache.c             | 20 +++++++++++++++++-
 sysdeps/generic/dl-cache.h | 43 +++++++++++++++++++++++++++++++++++++-
 3 files changed, 72 insertions(+), 2 deletions(-)

diff --git a/elf/cache.c b/elf/cache.c
index d92b4e59c1..c10981641f 100644
--- a/elf/cache.c
+++ b/elf/cache.c
@@ -152,6 +152,14 @@ print_entry (const char *lib, int flag, unsigned int osversion,
   printf (") => %s\n", key);
 }
 
+/* Print an error and exit if the new-file cache is internally
+   inconsistent.  */
+static void
+check_new_cache (struct cache_file_new *cache)
+{
+  if (! cache_file_new_matches_endian (cache))
+    error (EXIT_FAILURE, 0, _("Cache file has wrong endianness.\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.  */
@@ -193,6 +201,7 @@ print_cache (const char *cache_name)
 	  || memcmp (cache_new->version, CACHE_VERSION,
 		      sizeof CACHE_VERSION - 1))
 	error (EXIT_FAILURE, 0, _("File is not a cache file.\n"));
+      check_new_cache (cache_new);
       format = 1;
       /* This is where the strings start.  */
       cache_data = (const char *) cache_new;
@@ -222,6 +231,7 @@ print_cache (const char *cache_name)
 	      && memcmp (cache_new->version, CACHE_VERSION,
 			 sizeof CACHE_VERSION - 1) == 0)
 	    {
+	      check_new_cache (cache_new);
 	      cache_data = (const char *) cache_new;
 	      format = 1;
 	    }
@@ -361,6 +371,7 @@ save_cache (const char *cache_name)
 
       file_entries_new->nlibs = cache_entry_count;
       file_entries_new->len_strings = total_strlen;
+      file_entries_new->flags = cache_file_new_flags_endian;
     }
 
   /* Pad for alignment of cache_file_new.  */
diff --git a/elf/dl-cache.c b/elf/dl-cache.c
index 93d185e788..3aa8ed6c13 100644
--- a/elf/dl-cache.c
+++ b/elf/dl-cache.c
@@ -210,6 +210,11 @@ _dl_load_cache_lookup (const char *name)
 	  && ((cachesize - sizeof *cache_new) / sizeof (struct file_entry_new)
 	      >= ((struct cache_file_new *) file)->nlibs))
 	{
+	  if (! cache_file_new_matches_endian (file))
+	    {
+	      __munmap (file, cachesize);
+	      file = (void *) -1;
+	    }
 	  cache_new = file;
 	  cache = file;
 	}
@@ -231,7 +236,20 @@ _dl_load_cache_lookup (const char *name)
 	  if (cachesize < (offset + sizeof (struct cache_file_new))
 	      || memcmp (cache_new->magic, CACHEMAGIC_VERSION_NEW,
 			 sizeof CACHEMAGIC_VERSION_NEW - 1) != 0)
-	    cache_new = (void *) -1;
+	      cache_new = (void *) -1;
+	  else
+	    {
+	      if (! cache_file_new_matches_endian (cache_new))
+		{
+		  /* The old-format part of the cache is bogus as well
+		     if the endianness does not match.  (But it is
+		     unclear how the new header can be located if the
+		     endianess does not match.)  */
+		  cache = (void *) -1;
+		  cache_new = (void *) -1;
+		  __munmap (file, cachesize);
+		}
+	    }
 	}
       else
 	{
diff --git a/sysdeps/generic/dl-cache.h b/sysdeps/generic/dl-cache.h
index 6b310e9e15..1b04211f6b 100644
--- a/sysdeps/generic/dl-cache.h
+++ b/sysdeps/generic/dl-cache.h
@@ -16,6 +16,11 @@
    License along with the GNU C Library; if not, see
    <https://www.gnu.org/licenses/>.  */
 
+#ifndef _DL_CACHE_H
+#define _DL_CACHE_H
+
+#include <endian.h>
+#include <stdbool.h>
 #include <stdint.h>
 
 #ifndef _DL_CACHE_DEFAULT_ID
@@ -83,21 +88,57 @@ struct file_entry_new
   uint64_t hwcap;		/* Hwcap entry.	 */
 };
 
+/* See flags member of struct cache_file_new below.  */
+enum
+  {
+   cache_file_new_flags_endian = (__BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
+				  ? 2 : 3)
+  };
+
 struct cache_file_new
 {
   char magic[sizeof CACHEMAGIC_NEW - 1];
   char version[sizeof CACHE_VERSION - 1];
   uint32_t nlibs;		/* Number of entries.  */
   uint32_t len_strings;		/* Size of string table. */
-  uint32_t unused[5];		/* Leave space for future extensions
+
+  /* flags & 3 is used to indicate the endianness of the cache.
+     0: no endianness information available
+        (An old ldconfig version without endianness support wrote the file.)
+     1: cache is invalid
+     2: little endian
+     3: big endian
+
+     The remaining bits are unused and should be generated as zero and
+     ignored by readers.  */
+  uint8_t flags;
+
+  uint8_t padding_unsed[3];	/* Not used, for future extensions.  */
+
+  uint32_t unused[4];		/* Leave space for future extensions
 				   and align to 8 byte boundary.  */
   struct file_entry_new libs[0]; /* Entries describing libraries.  */
   /* After this the string table of size len_strings is found.	*/
 };
 
+/* Returns false if *CACHE has the wrong endianness for this
+   architecture, and true if the endianness matches (or is
+   unknown).  */
+static inline bool
+cache_file_new_matches_endian (const struct cache_file_new *cache)
+{
+  /* A zero value for cache->flags means that no endianness
+     information is available.  */
+  return cache->flags == 0
+    || (cache->flags & 3) == cache_file_new_flags_endian;
+}
+
+
 /* Used to align cache_file_new.  */
 #define ALIGN_CACHE(addr)				\
 (((addr) + __alignof__ (struct cache_file_new) -1)	\
  & (~(__alignof__ (struct cache_file_new) - 1)))
 
 extern int _dl_cache_libcmp (const char *p1, const char *p2) attribute_hidden;
+
+#endif /* _DL_CACHE_H */
-- 
2.25.4