elf: Add glibc-hwcaps subdirectory support to ld.so cache processing

This recognizes the DL_CACHE_HWCAP_EXTENSION flag in cache entries,
and picks the supported cache entry with the highest priority.

The elf/tst-glibc-hwcaps-prepend-cache test documents a non-desired
aspect of the current cache implementation: If the cache selects a DSO
that does not exist on disk, _dl_map_object falls back to open_path,
which may or may not find an alternative implementation.  This is an
existing limitation that also applies to the legacy hwcaps processing
for ld.so.cache.

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

1 files changed, 78 insertions, 0 deletions

diff --git a/elf/dl-hwcaps.c b/elf/dl-hwcaps.c
index f611f3a1a6..5a71f80154 100644
--- a/elf/dl-hwcaps.c
+++ b/elf/dl-hwcaps.c
@@ -89,6 +89,81 @@ copy_hwcaps (struct copy_hwcaps *target, const char *hwcaps,
     }
 }
 
+struct dl_hwcaps_priority *_dl_hwcaps_priorities;
+uint32_t _dl_hwcaps_priorities_length;
+
+/* Allocate _dl_hwcaps_priorities and fill it with data.  */
+static void
+compute_priorities (size_t total_count, const char *prepend,
+		    uint32_t bitmask, const char *mask)
+{
+  _dl_hwcaps_priorities = malloc (total_count
+				  * sizeof (*_dl_hwcaps_priorities));
+  if (_dl_hwcaps_priorities == NULL)
+    _dl_signal_error (ENOMEM, NULL, NULL,
+		      N_("cannot create HWCAP priorities"));
+  _dl_hwcaps_priorities_length = total_count;
+
+  /* First the prepended subdirectories.  */
+  size_t i = 0;
+  {
+    struct dl_hwcaps_split sp;
+    _dl_hwcaps_split_init (&sp, prepend);
+    while (_dl_hwcaps_split (&sp))
+      {
+	_dl_hwcaps_priorities[i].name = sp.segment;
+	_dl_hwcaps_priorities[i].name_length = sp.length;
+	_dl_hwcaps_priorities[i].priority = i + 1;
+	++i;
+      }
+  }
+
+  /* Then the built-in subdirectories that are actually active.  */
+  {
+    struct dl_hwcaps_split_masked sp;
+    _dl_hwcaps_split_masked_init (&sp, _dl_hwcaps_subdirs, bitmask, mask);
+    while (_dl_hwcaps_split_masked (&sp))
+      {
+	_dl_hwcaps_priorities[i].name = sp.split.segment;
+	_dl_hwcaps_priorities[i].name_length = sp.split.length;
+	_dl_hwcaps_priorities[i].priority = i + 1;
+	++i;
+      }
+  }
+  assert (i == total_count);
+}
+
+/* Sort the _dl_hwcaps_priorities array by name.  */
+static void
+sort_priorities_by_name (void)
+{
+  /* Insertion sort.  There is no need to link qsort into the dynamic
+     loader for such a short array.  */
+  for (size_t i = 1; i < _dl_hwcaps_priorities_length; ++i)
+    for (size_t j = i; j > 0; --j)
+      {
+	struct dl_hwcaps_priority *previous = _dl_hwcaps_priorities + j - 1;
+	struct dl_hwcaps_priority *current = _dl_hwcaps_priorities + j;
+
+	/* Bail out if current is greater or equal to the previous
+	   value.  */
+	uint32_t to_compare;
+	if (current->name_length < previous->name_length)
+	  to_compare = current->name_length;
+	else
+	  to_compare = previous->name_length;
+	int cmp = memcmp (current->name, previous->name, to_compare);
+	if (cmp >= 0
+	    || (cmp == 0 && current->name_length >= previous->name_length))
+	  break;
+
+	/* Swap *previous and *current.  */
+	struct dl_hwcaps_priority tmp = *previous;
+	*previous = *current;
+	*current = tmp;
+      }
+}
+
 /* Return an array of useful/necessary hardware capability names.  */
 const struct r_strlenpair *
 _dl_important_hwcaps (const char *glibc_hwcaps_prepend,
@@ -111,6 +186,9 @@ _dl_important_hwcaps (const char *glibc_hwcaps_prepend,
   update_hwcaps_counts (&hwcaps_counts, glibc_hwcaps_prepend, -1, NULL);
   update_hwcaps_counts (&hwcaps_counts, _dl_hwcaps_subdirs,
 			hwcaps_subdirs_active, glibc_hwcaps_mask);
+  compute_priorities (hwcaps_counts.count, glibc_hwcaps_prepend,
+		      hwcaps_subdirs_active, glibc_hwcaps_mask);
+  sort_priorities_by_name ();
 
   /* Each hwcaps subdirectory has a GLIBC_HWCAPS_PREFIX string prefix
      and a "/" suffix once stored in the result.  */