elf: Fix slow DSO sorting behavior in dynamic loader (BZ #17645)

This second patch contains the actual implementation of a new sorting algorithm
for shared objects in the dynamic loader, which solves the slow behavior that
the current "old" algorithm falls into when the DSO set contains circular
dependencies.

The new algorithm implemented here is simply depth-first search (DFS) to obtain
the Reverse-Post Order (RPO) sequence, a topological sort. A new l_visited:1
bitfield is added to struct link_map to more elegantly facilitate such a search.

The DFS algorithm is applied to the input maps[nmap-1] backwards towards
maps[0]. This has the effect of a more "shallow" recursion depth in general
since the input is in BFS. Also, when combined with the natural order of
processing l_initfini[] at each node, this creates a resulting output sorting
closer to the intuitive "left-to-right" order in most cases.

Another notable implementation adjustment related to this _dl_sort_maps change
is the removing of two char arrays 'used' and 'done' in _dl_close_worker to
represent two per-map attributes. This has been changed to simply use two new
bit-fields l_map_used:1, l_map_done:1 added to struct link_map. This also allows
discarding the clunky 'used' array sorting that _dl_sort_maps had to sometimes
do along the way.

Tunable support for switching between different sorting algorithms at runtime is
also added. A new tunable 'glibc.rtld.dynamic_sort' with current valid values 1
(old algorithm) and 2 (new DFS algorithm) has been added. At time of commit
of this patch, the default setting is 1 (old algorithm).

Signed-off-by: Chung-Lin Tang  <cltang@codesourcery.com>
Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>

1 files changed, 5 insertions, 0 deletions

diff --git a/include/link.h b/include/link.h
index 484ee6cb1b..c1c382ccfa 100644
--- a/include/link.h
+++ b/include/link.h
@@ -181,6 +181,11 @@ struct link_map
     unsigned int l_init_called:1; /* Nonzero if DT_INIT function called.  */
     unsigned int l_global:1;	/* Nonzero if object in _dl_global_scope.  */
     unsigned int l_reserved:2;	/* Reserved for internal use.  */
+    unsigned int l_main_map:1;  /* Nonzero for the map of the main program.  */
+    unsigned int l_visited:1;   /* Used internally for map dependency
+				   graph traversal.  */
+    unsigned int l_map_used:1;  /* These two bits are used during traversal */
+    unsigned int l_map_done:1;  /* of maps in _dl_close_worker. */
     unsigned int l_phdr_allocated:1; /* Nonzero if the data structure pointed
 					to by `l_phdr' is allocated.  */
     unsigned int l_soname_added:1; /* Nonzero if the SONAME is for sure in