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

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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>
This commit is contained in:
Chung-Lin Tang 2021-10-21 21:41:22 +08:00 committed by Adhemerval Zanella
parent e6fd79f379
commit 15a0c5730d
14 changed files with 269 additions and 42 deletions
Show stats Download patch file Download diff file Expand all files Collapse all files

9
NEWS
View file

@ -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

34
elf/dl-close.c
View file

@ -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);

7
elf/dl-deps.c
View file

@ -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;

3
elf/dl-fini.c
View file

@ -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

208
elf/dl-sort-maps.c
View file

@ -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. */

2
elf/dl-support.c
View file

@ -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;

3
elf/dl-sysdep.c
View file

@ -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

9
elf/dl-tunables.list
View file

@ -156,4 +156,13 @@ glibc {
security_level: SXID_IGNORE
}
}
rtld {
dynamic_sort {
type: INT_32
minval: 1
maxval: 2
default: 1
}
}
}

2
elf/dso-sort-tests-1.def
View file

@ -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];}

3
elf/rtld.c
View file

@ -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. */

1
elf/tst-rtld-list-tunables.exp
View file

@ -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]+)

5
include/link.h
View file

@ -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

11
manual/tunables.texi
View file

@ -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

14
sysdeps/generic/ldsodefs.h
View file

@ -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;