@@ -51,6 +51,15 @@ Major new features:
* The ISO C2X macro _PRINTF_NAN_LEN_MAX has been added to <stdio.h>.
+* A new DSO sorting algorithm has been added in the dynamic linker that uses
+ topological sorting by depth-first search (DFS), solving performance issues
+ of the existing sorting algorithm when encountering particular circular
+ object dependency cases.
+
+* A new tunable, glibc.rtld.dynamic_sort, can be used to select between the two
+ DSO sorting algorithms. The default setting of '1' uses the current existing
+ algorithm, while a value of '2' selects the new DFS-based algorithm.
+
Deprecated and removed features, and other changes affecting compatibility:
* The r_version update in the debugger interface makes the glibc binary
@@ -167,8 +167,6 @@ _dl_close_worker (struct link_map *map, bool force)
bool any_tls = false;
const unsigned int nloaded = ns->_ns_nloaded;
- char used[nloaded];
- char done[nloaded];
struct link_map *maps[nloaded];
/* Run over the list and assign indexes to the link maps and enter
@@ -176,24 +174,21 @@ _dl_close_worker (struct link_map *map, bool force)
int idx = 0;
for (struct link_map *l = ns->_ns_loaded; l != NULL; l = l->l_next)
{
+ l->l_map_used = 0;
+ l->l_map_done = 0;
l->l_idx = idx;
maps[idx] = l;
++idx;
-
}
assert (idx == nloaded);
- /* Prepare the bitmaps. */
- memset (used, '\0', sizeof (used));
- memset (done, '\0', sizeof (done));
-
/* Keep track of the lowest index link map we have covered already. */
int done_index = -1;
while (++done_index < nloaded)
{
struct link_map *l = maps[done_index];
- if (done[done_index])
+ if (l->l_map_done)
/* Already handled. */
continue;
@@ -204,12 +199,12 @@ _dl_close_worker (struct link_map *map, bool force)
/* See CONCURRENCY NOTES in cxa_thread_atexit_impl.c to know why
acquire is sufficient and correct. */
&& atomic_load_acquire (&l->l_tls_dtor_count) == 0
- && !used[done_index])
+ && !l->l_map_used)
continue;
/* We need this object and we handle it now. */
- done[done_index] = 1;
- used[done_index] = 1;
+ l->l_map_used = 1;
+ l->l_map_done = 1;
/* Signal the object is still needed. */
l->l_idx = IDX_STILL_USED;
@@ -225,9 +220,9 @@ _dl_close_worker (struct link_map *map, bool force)
{
assert ((*lp)->l_idx >= 0 && (*lp)->l_idx < nloaded);
- if (!used[(*lp)->l_idx])
+ if (!(*lp)->l_map_used)
{
- used[(*lp)->l_idx] = 1;
+ (*lp)->l_map_used = 1;
/* If we marked a new object as used, and we've
already processed it, then we need to go back
and process again from that point forward to
@@ -250,9 +245,9 @@ _dl_close_worker (struct link_map *map, bool force)
{
assert (jmap->l_idx >= 0 && jmap->l_idx < nloaded);
- if (!used[jmap->l_idx])
+ if (!jmap->l_map_used)
{
- used[jmap->l_idx] = 1;
+ jmap->l_map_used = 1;
if (jmap->l_idx - 1 < done_index)
done_index = jmap->l_idx - 1;
}
@@ -262,8 +257,7 @@ _dl_close_worker (struct link_map *map, bool force)
/* Sort the entries. We can skip looking for the binary itself which is
at the front of the search list for the main namespace. */
- _dl_sort_maps (maps + (nsid == LM_ID_BASE), nloaded - (nsid == LM_ID_BASE),
- used + (nsid == LM_ID_BASE), true);
+ _dl_sort_maps (maps, nloaded, (nsid == LM_ID_BASE), true);
/* Call all termination functions at once. */
#ifdef SHARED
@@ -280,7 +274,7 @@ _dl_close_worker (struct link_map *map, bool force)
/* All elements must be in the same namespace. */
assert (imap->l_ns == nsid);
- if (!used[i])
+ if (!imap->l_map_used)
{
assert (imap->l_type == lt_loaded && !imap->l_nodelete_active);
@@ -333,7 +327,7 @@ _dl_close_worker (struct link_map *map, bool force)
if (i < first_loaded)
first_loaded = i;
}
- /* Else used[i]. */
+ /* Else imap->l_map_used. */
else if (imap->l_type == lt_loaded)
{
struct r_scope_elem *new_list = NULL;
@@ -560,7 +554,7 @@ _dl_close_worker (struct link_map *map, bool force)
for (unsigned int i = first_loaded; i < nloaded; ++i)
{
struct link_map *imap = maps[i];
- if (!used[i])
+ if (!imap->l_map_used)
{
assert (imap->l_type == lt_loaded);
@@ -613,10 +613,9 @@ Filters not supported with LD_TRACE_PRELINKING"));
/* If libc.so.6 is the main map, it participates in the sort, so
that the relocation order is correct regarding libc.so.6. */
- if (l_initfini[0] == GL (dl_ns)[l_initfini[0]->l_ns].libc_map)
- _dl_sort_maps (l_initfini, nlist, NULL, false);
- else
- _dl_sort_maps (&l_initfini[1], nlist - 1, NULL, false);
+ _dl_sort_maps (l_initfini, nlist,
+ (l_initfini[0] != GL (dl_ns)[l_initfini[0]->l_ns].libc_map),
+ false);
/* Terminate the list of dependencies. */
l_initfini[nlist] = NULL;
@@ -92,8 +92,7 @@ _dl_fini (void)
/* Now we have to do the sorting. We can skip looking for the
binary itself which is at the front of the search list for
the main namespace. */
- _dl_sort_maps (maps + (ns == LM_ID_BASE), nmaps - (ns == LM_ID_BASE),
- NULL, true);
+ _dl_sort_maps (maps, nmaps, (ns == LM_ID_BASE), true);
/* We do not rely on the linked list of loaded object anymore
from this point on. We have our own list here (maps). The
@@ -16,16 +16,24 @@
License along with the GNU C Library; if not, see
<https://www.gnu.org/licenses/>. */
+#include <assert.h>
#include <ldsodefs.h>
+#include <elf/dl-tunables.h>
+/* Note: this is the older, "original" sorting algorithm, being used as
+ default up to 2.35.
-/* Sort array MAPS according to dependencies of the contained objects.
- Array USED, if non-NULL, is permutated along MAPS. If FOR_FINI this is
- called for finishing an object. */
-void
-_dl_sort_maps (struct link_map **maps, unsigned int nmaps, char *used,
- bool for_fini)
+ Sort array MAPS according to dependencies of the contained objects.
+ If FOR_FINI is true, this is called for finishing an object. */
+static void
+_dl_sort_maps_original (struct link_map **maps, unsigned int nmaps,
+ unsigned int skip, bool for_fini)
{
+ /* Allows caller to do the common optimization of skipping the first map,
+ usually the main binary. */
+ maps += skip;
+ nmaps -= skip;
+
/* A list of one element need not be sorted. */
if (nmaps <= 1)
return;
@@ -66,14 +74,6 @@ _dl_sort_maps (struct link_map **maps, unsigned int nmaps, char *used,
(k - i) * sizeof (maps[0]));
maps[k] = thisp;
- if (used != NULL)
- {
- char here_used = used[i];
- memmove (&used[i], &used[i + 1],
- (k - i) * sizeof (used[0]));
- used[k] = here_used;
- }
-
if (seen[i + 1] > nmaps - i)
{
++i;
@@ -120,3 +120,183 @@ _dl_sort_maps (struct link_map **maps, unsigned int nmaps, char *used,
next:;
}
}
+
+#if !HAVE_TUNABLES
+/* In this case, just default to the original algorithm. */
+strong_alias (_dl_sort_maps_original, _dl_sort_maps);
+#else
+
+/* We use a recursive function due to its better clarity and ease of
+ implementation, as well as faster execution speed. We already use
+ alloca() for list allocation during the breadth-first search of
+ dependencies in _dl_map_object_deps(), and this should be on the
+ same order of worst-case stack usage.
+
+ Note: the '*rpo' parameter is supposed to point to one past the
+ last element of the array where we save the sort results, and is
+ decremented before storing the current map at each level. */
+
+static void
+dfs_traversal (struct link_map ***rpo, struct link_map *map,
+ bool *do_reldeps)
+{
+ if (map->l_visited)
+ return;
+
+ map->l_visited = 1;
+
+ if (map->l_initfini)
+ {
+ for (int i = 0; map->l_initfini[i] != NULL; i++)
+ {
+ struct link_map *dep = map->l_initfini[i];
+ if (dep->l_visited == 0
+ && dep->l_main_map == 0)
+ dfs_traversal (rpo, dep, do_reldeps);
+ }
+ }
+
+ if (__glibc_unlikely (do_reldeps != NULL && map->l_reldeps != NULL))
+ {
+ /* Indicate that we encountered relocation dependencies during
+ traversal. */
+ *do_reldeps = true;
+
+ for (int m = map->l_reldeps->act - 1; m >= 0; m--)
+ {
+ struct link_map *dep = map->l_reldeps->list[m];
+ if (dep->l_visited == 0
+ && dep->l_main_map == 0)
+ dfs_traversal (rpo, dep, do_reldeps);
+ }
+ }
+
+ *rpo -= 1;
+ **rpo = map;
+}
+
+/* Topologically sort array MAPS according to dependencies of the contained
+ objects. */
+
+static void
+_dl_sort_maps_dfs (struct link_map **maps, unsigned int nmaps,
+ unsigned int skip __attribute__ ((unused)), bool for_fini)
+{
+ for (int i = nmaps - 1; i >= 0; i--)
+ maps[i]->l_visited = 0;
+
+ /* We apply DFS traversal for each of maps[i] until the whole total order
+ is found and we're at the start of the Reverse-Postorder (RPO) sequence,
+ which is a topological sort.
+
+ We go from maps[nmaps - 1] backwards towards maps[0] at this level.
+ Due to the breadth-first search (BFS) ordering we receive, going
+ backwards usually gives a more shallow depth-first recursion depth,
+ adding more stack usage safety. Also, combined with the natural
+ processing order of l_initfini[] at each node during DFS, this maintains
+ an ordering closer to the original link ordering in the sorting results
+ under most simpler cases.
+
+ Another reason we order the top level backwards, it that maps[0] is
+ usually exactly the main object of which we're in the midst of
+ _dl_map_object_deps() processing, and maps[0]->l_initfini[] is still
+ blank. If we start the traversal from maps[0], since having no
+ dependencies yet filled in, maps[0] will always be immediately
+ incorrectly placed at the last place in the order (first in reverse).
+ Adjusting the order so that maps[0] is last traversed naturally avoids
+ this problem.
+
+ Further, the old "optimization" of skipping the main object at maps[0]
+ from the call-site (i.e. _dl_sort_maps(maps+1,nmaps-1)) is in general
+ no longer valid, since traversing along object dependency-links
+ may "find" the main object even when it is not included in the initial
+ order (e.g. a dlopen()'ed shared object can have circular dependencies
+ linked back to itself). In such a case, traversing N-1 objects will
+ create a N-object result, and raise problems.
+
+ To summarize, just passing in the full list, and iterating from back
+ to front makes things much more straightforward. */
+
+ /* Array to hold RPO sorting results, before we copy back to maps[]. */
+ struct link_map *rpo[nmaps];
+
+ /* The 'head' position during each DFS iteration. Note that we start at
+ one past the last element due to first-decrement-then-store (see the
+ bottom of above dfs_traversal() routine). */
+ struct link_map **rpo_head = &rpo[nmaps];
+
+ bool do_reldeps = false;
+ bool *do_reldeps_ref = (for_fini ? &do_reldeps : NULL);
+
+ for (int i = nmaps - 1; i >= 0; i--)
+ {
+ dfs_traversal (&rpo_head, maps[i], do_reldeps_ref);
+
+ /* We can break early if all objects are already placed. */
+ if (rpo_head == rpo)
+ goto end;
+ }
+ assert (rpo_head == rpo);
+
+ end:
+ /* Here we may do a second pass of sorting, using only l_initfini[]
+ static dependency links. This is avoided if !FOR_FINI or if we didn't
+ find any reldeps in the first DFS traversal.
+
+ The reason we do this is: while it is unspecified how circular
+ dependencies should be handled, the presumed reasonable behavior is to
+ have destructors to respect static dependency links as much as possible,
+ overriding reldeps if needed. And the first sorting pass, which takes
+ l_initfini/l_reldeps links equally, may not preserve this priority.
+
+ Hence we do a 2nd sorting pass, taking only DT_NEEDED links into account
+ (see how the do_reldeps argument to dfs_traversal() is NULL below). */
+ if (do_reldeps)
+ {
+ for (int i = nmaps - 1; i >= 0; i--)
+ rpo[i]->l_visited = 0;
+
+ struct link_map **maps_head = &maps[nmaps];
+ for (int i = nmaps - 1; i >= 0; i--)
+ {
+ dfs_traversal (&maps_head, rpo[i], NULL);
+
+ /* We can break early if all objects are already placed.
+ The below memcpy is not needed in the do_reldeps case here,
+ since we wrote back to maps[] during DFS traversal. */
+ if (maps_head == maps)
+ return;
+ }
+ assert (maps_head == maps);
+ return;
+ }
+
+ memcpy (maps, rpo, sizeof (struct link_map *) * nmaps);
+}
+
+void
+_dl_sort_maps_init (void)
+{
+ int32_t algorithm = TUNABLE_GET (glibc, rtld, dynamic_sort, int32_t, NULL);
+ GLRO(dl_dso_sort_algo) = algorithm == 1 ? dso_sort_algorithm_original
+ : dso_sort_algorithm_dfs;
+}
+
+void
+_dl_sort_maps (struct link_map **maps, unsigned int nmaps,
+ unsigned int skip, bool for_fini)
+{
+ /* It can be tempting to use a static function pointer to store and call
+ the current selected sorting algorithm routine, but experimentation
+ shows that current processors still do not handle indirect branches
+ that efficiently, plus a static function pointer will involve
+ PTR_MANGLE/DEMANGLE, further impairing performance of small, common
+ input cases. A simple if-case with direct function calls appears to
+ be the fastest. */
+ if (__glibc_likely (GLRO(dl_dso_sort_algo) == dso_sort_algorithm_original))
+ _dl_sort_maps_original (maps, nmaps, skip, for_fini);
+ else
+ _dl_sort_maps_dfs (maps, nmaps, skip, for_fini);
+}
+
+#endif /* HAVE_TUNABLES. */
@@ -166,6 +166,8 @@ size_t _dl_phnum;
uint64_t _dl_hwcap;
uint64_t _dl_hwcap2;
+enum dso_sort_algorithm _dl_dso_sort_algo;
+
/* The value of the FPU control word the kernel will preset in hardware. */
fpu_control_t _dl_fpu_control = _FPU_DEFAULT;
@@ -231,6 +231,9 @@ _dl_sysdep_start (void **start_argptr,
__tunables_init (_environ);
+ /* Initialize DSO sorting algorithm after tunables. */
+ _dl_sort_maps_init ();
+
#ifdef DL_SYSDEP_INIT
DL_SYSDEP_INIT;
#endif
@@ -156,4 +156,13 @@ glibc {
security_level: SXID_IGNORE
}
}
+
+ rtld {
+ dynamic_sort {
+ type: INT_32
+ minval: 1
+ maxval: 2
+ default: 1
+ }
+ }
}
@@ -62,5 +62,5 @@ output: b>a>{}<a<b
# The below expected outputs are what the two algorithms currently produce
# respectively, for regression testing purposes.
tst-bz15311: {+a;+e;+f;+g;+d;%d;-d;-g;-f;-e;-a};a->b->c->d;d=>[ba];c=>a;b=>e=>a;c=>f=>b;d=>g=>c
-xfail_output(glibc.rtld.dynamic_sort=1): {+a[d>c>b>a>];+e[e>];+f[f>];+g[g>];+d[];%d(b(e(a()))a()g(c(a()f(b(e(a()))))));-d[];-g[];-f[];-e[];-a[<a<c<d<g<f<b<e];}
+output(glibc.rtld.dynamic_sort=1): {+a[d>c>b>a>];+e[e>];+f[f>];+g[g>];+d[];%d(b(e(a()))a()g(c(a()f(b(e(a()))))));-d[];-g[];-f[];-e[];-a[<a<c<d<g<f<b<e];}
output(glibc.rtld.dynamic_sort=2): {+a[d>c>b>a>];+e[e>];+f[f>];+g[g>];+d[];%d(b(e(a()))a()g(c(a()f(b(e(a()))))));-d[];-g[];-f[];-e[];-a[<g<f<a<b<c<d<e];}
@@ -1391,6 +1391,9 @@ dl_main (const ElfW(Phdr) *phdr,
main_map->l_name = (char *) "";
*user_entry = main_map->l_entry;
+ /* Set bit indicating this is the main program map. */
+ main_map->l_main_map = 1;
+
#ifdef HAVE_AUX_VECTOR
/* Adjust the on-stack auxiliary vector so that it looks like the
binary was executed directly. */
@@ -10,5 +10,6 @@ glibc.malloc.tcache_max: 0x0 (min: 0x0, max: 0x[f]+)
glibc.malloc.tcache_unsorted_limit: 0x0 (min: 0x0, max: 0x[f]+)
glibc.malloc.top_pad: 0x0 (min: 0x0, max: 0x[f]+)
glibc.malloc.trim_threshold: 0x0 (min: 0x0, max: 0x[f]+)
+glibc.rtld.dynamic_sort: 1 (min: 1, max: 2)
glibc.rtld.nns: 0x4 (min: 0x1, max: 0x10)
glibc.rtld.optional_static_tls: 0x200 (min: 0x0, max: 0x[f]+)
@@ -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
@@ -309,6 +309,17 @@ changed once allocated at process startup. The default allocation of
optional static TLS is 512 bytes and is allocated in every thread.
@end deftp
+@deftp Tunable glibc.rtld.dynamic_sort
+Sets the algorithm to use for DSO sorting, valid values are @samp{1} and
+@samp{2}. For value of @samp{1}, an older O(n^3) algorithm is used, which is
+long time tested, but may have performance issues when dependencies between
+shared objects contain cycles due to circular dependencies. When set to the
+value of @samp{2}, a different algorithm is used, which implements a
+topological sort through depth-first search, and does not exhibit the
+performance issues of @samp{1}.
+
+The default value of this tunable is @samp{1}.
+@end deftp
@node Elision Tunables
@section Elision Tunables
@@ -245,6 +245,13 @@ enum allowmask
};
+/* DSO sort algorithm to use (check dl-sort-maps.c). */
+enum dso_sort_algorithm
+ {
+ dso_sort_algorithm_original,
+ dso_sort_algorithm_dfs
+ };
+
struct audit_ifaces
{
void (*activity) (uintptr_t *, unsigned int);
@@ -678,6 +685,8 @@ struct rtld_global_ro
platforms. */
EXTERN uint64_t _dl_hwcap2;
+ EXTERN enum dso_sort_algorithm _dl_dso_sort_algo;
+
#ifdef SHARED
/* We add a function table to _rtld_global which is then used to
call the function instead of going through the PLT. The result
@@ -1104,7 +1113,7 @@ extern void _dl_fini (void) attribute_hidden;
/* Sort array MAPS according to dependencies of the contained objects. */
extern void _dl_sort_maps (struct link_map **maps, unsigned int nmaps,
- char *used, bool for_fini) attribute_hidden;
+ unsigned int skip, bool for_fini) attribute_hidden;
/* The dynamic linker calls this function before and having changing
any shared object mappings. The `r_state' member of `struct r_debug'
@@ -1235,6 +1244,9 @@ extern struct link_map * _dl_get_dl_main_map (void)
# endif
#endif
+/* Initialize the DSO sort algorithm to use. */
+extern void _dl_sort_maps_init (void) attribute_hidden;
+
/* Initialization of libpthread for statically linked applications.
If libpthread is not linked in, this is an empty function. */
void __pthread_initialize_minimal (void) weak_function;