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elf: Always call destructors in reverse constructor order (bug 30785)

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The current implementation of dlclose (and process exit) re-sorts the
link maps before calling ELF destructors.  Destructor order is not the
reverse of the constructor order as a result: The second sort takes
relocation dependencies into account, and other differences can result
from ambiguous inputs, such as cycles.  (The force_first handling in
_dl_sort_maps is not effective for dlclose.)  After the changes in
this commit, there is still a required difference due to
dlopen/dlclose ordering by the application, but the previous
discrepancies went beyond that.

A new global (namespace-spanning) list of link maps,
_dl_init_called_list, is updated right before ELF constructors are
called from _dl_init.

In dl_close_worker, the maps variable, an on-stack variable length
array, is eliminated.  (VLAs are problematic, and dlclose should not
call malloc because it cannot readily deal with malloc failure.)
Marking still-used objects uses the namespace list directly, with
next and next_idx replacing the done_index variable.

After marking, _dl_init_called_list is used to call the destructors
of now-unused maps in reverse destructor order.  These destructors
can call dlopen.  Previously, new objects do not have l_map_used set.
This had to change: There is no copy of the link map list anymore,
so processing would cover newly opened (and unmarked) mappings,
unloading them.  Now, _dl_init (indirectly) sets l_map_used, too.
(dlclose is handled by the existing reentrancy guard.)

After _dl_init_called_list traversal, two more loops follow.  The
processing order changes to the original link map order in the
namespace.  Previously, dependency order was used.  The difference
should not matter because relocation dependencies could already
reorder link maps in the old code.

The changes to _dl_fini remove the sorting step and replace it with
a traversal of _dl_init_called_list.  The l_direct_opencount
decrement outside the loader lock is removed because it appears
incorrect: the counter manipulation could race with other dynamic
loader operations.

tst-audit23 needs adjustments to the changes in LA_ACT_DELETE
notifications.  The new approach for checking la_activity should
make it clearer that la_activty calls come in pairs around namespace
updates.

The dependency sorting test cases need updates because the destructor
order is always the opposite order of constructor order, even with
relocation dependencies or cycles present.

There is a future cleanup opportunity to remove the now-constant
force_first and for_fini arguments from the _dl_sort_maps function.

Fixes commit 1df71d32fe ("elf: Implement
force_first handling in _dl_sort_maps_dfs (bug 28937)").

Reviewed-by: DJ Delorie <dj@redhat.com>
This commit is contained in:
Florian Weimer 2023-09-08 12:32:14 +02:00
parent 434bf72a94
commit 6985865bc3
7 changed files with 177 additions and 171 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

115
elf/dl-close.c
View File

@ -138,30 +138,31 @@ _dl_close_worker (struct link_map *map, bool force)
bool any_tls = false;
const unsigned int nloaded = ns->_ns_nloaded;
struct link_map *maps[nloaded];
/* Run over the list and assign indexes to the link maps and enter
them into the MAPS array. */
/* Run over the list and assign indexes to the link maps. */
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);
/* Keep track of the lowest index link map we have covered already. */
int done_index = -1;
while (++done_index < nloaded)
/* Keep marking link maps until no new link maps are found. */
for (struct link_map *l = ns->_ns_loaded; l != NULL; )
{
struct link_map *l = maps[done_index];
/* next is reset to earlier link maps for remarking. */
struct link_map *next = l->l_next;
int next_idx = l->l_idx + 1; /* next->l_idx, but covers next == NULL. */
if (l->l_map_done)
{
/* Already handled. */
l = next;
continue;
}
/* Check whether this object is still used. */
if (l->l_type == lt_loaded
@ -171,7 +172,10 @@ _dl_close_worker (struct link_map *map, bool force)
acquire is sufficient and correct. */
&& atomic_load_acquire (&l->l_tls_dtor_count) == 0
&& !l->l_map_used)
{
l = next;
continue;
}
/* We need this object and we handle it now. */
l->l_map_used = 1;
@ -198,8 +202,11 @@ _dl_close_worker (struct link_map *map, bool force)
already processed it, then we need to go back
and process again from that point forward to
ensure we keep all of its dependencies also. */
if ((*lp)->l_idx - 1 < done_index)
done_index = (*lp)->l_idx - 1;
if ((*lp)->l_idx < next_idx)
{
next = *lp;
next_idx = next->l_idx;
}
}
}
@ -219,44 +226,65 @@ _dl_close_worker (struct link_map *map, bool force)
if (!jmap->l_map_used)
{
jmap->l_map_used = 1;
if (jmap->l_idx - 1 < done_index)
done_index = jmap->l_idx - 1;
}
}
}
}
/* 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, nloaded, (nsid == LM_ID_BASE), true);
/* Call all termination functions at once. */
bool unload_any = false;
bool scope_mem_left = false;
unsigned int unload_global = 0;
unsigned int first_loaded = ~0;
for (unsigned int i = 0; i < nloaded; ++i)
if (jmap->l_idx < next_idx)
{
struct link_map *imap = maps[i];
next = jmap;
next_idx = next->l_idx;
}
}
}
}
/* All elements must be in the same namespace. */
assert (imap->l_ns == nsid);
l = next;
}
if (!imap->l_map_used)
/* Call the destructors in reverse constructor order, and remove the
closed link maps from the list. */
for (struct link_map **init_called_head = &_dl_init_called_list;
*init_called_head != NULL; )
{
struct link_map *imap = *init_called_head;
/* _dl_init_called_list is global, to produce a global odering.
Ignore the other namespaces (and link maps that are still used). */
if (imap->l_ns != nsid || imap->l_map_used)
init_called_head = &imap->l_init_called_next;
else
{
assert (imap->l_type == lt_loaded && !imap->l_nodelete_active);
/* Call its termination function. Do not do it for
half-cooked objects. Temporarily disable exception
handling, so that errors are fatal. */
if (imap->l_init_called)
/* _dl_init_called_list is updated at the same time as
l_init_called. */
assert (imap->l_init_called);
if (imap->l_info[DT_FINI_ARRAY] != NULL
|| imap->l_info[DT_FINI] != NULL)
_dl_catch_exception (NULL, _dl_call_fini, imap);
#ifdef SHARED
/* Auditing checkpoint: we remove an object. */
_dl_audit_objclose (imap);
#endif
/* Unlink this link map. */
*init_called_head = imap->l_init_called_next;
}
}
bool unload_any = false;
bool scope_mem_left = false;
unsigned int unload_global = 0;
/* For skipping un-unloadable link maps in the second loop. */
struct link_map *first_loaded = ns->_ns_loaded;
/* Iterate over the namespace to find objects to unload. Some
unloadable objects may not be on _dl_init_called_list due to
dlopen failure. */
for (struct link_map *imap = first_loaded; imap != NULL; imap = imap->l_next)
{
if (!imap->l_map_used)
{
/* This object must not be used anymore. */
imap->l_removed = 1;
@ -267,8 +295,8 @@ _dl_close_worker (struct link_map *map, bool force)
++unload_global;
/* Remember where the first dynamically loaded object is. */
if (i < first_loaded)
first_loaded = i;
if (first_loaded == NULL)
first_loaded = imap;
}
/* Else imap->l_map_used. */
else if (imap->l_type == lt_loaded)
@ -404,8 +432,8 @@ _dl_close_worker (struct link_map *map, bool force)
imap->l_loader = NULL;
/* Remember where the first dynamically loaded object is. */
if (i < first_loaded)
first_loaded = i;
if (first_loaded == NULL)
first_loaded = imap;
}
}
@ -476,10 +504,11 @@ _dl_close_worker (struct link_map *map, bool force)
/* Check each element of the search list to see if all references to
it are gone. */
for (unsigned int i = first_loaded; i < nloaded; ++i)
for (struct link_map *imap = first_loaded; imap != NULL; )
{
struct link_map *imap = maps[i];
if (!imap->l_map_used)
if (imap->l_map_used)
imap = imap->l_next;
else
{
assert (imap->l_type == lt_loaded);
@ -690,7 +719,9 @@ _dl_close_worker (struct link_map *map, bool force)
if (imap == GL(dl_initfirst))
GL(dl_initfirst) = NULL;
struct link_map *next = imap->l_next;
free (imap);
imap = next;
}
}

128
elf/dl-fini.c
View File

@ -24,116 +24,68 @@
void
_dl_fini (void)
{
/* Lots of fun ahead. We have to call the destructors for all still
loaded objects, in all namespaces. The problem is that the ELF
specification now demands that dependencies between the modules
are taken into account. I.e., the destructor for a module is
called before the ones for any of its dependencies.
/* Call destructors strictly in the reverse order of constructors.
This causes fewer surprises than some arbitrary reordering based
on new (relocation) dependencies. None of the objects are
unmapped, so applications can deal with this if their DSOs remain
in a consistent state after destructors have run. */
To make things more complicated, we cannot simply use the reverse
order of the constructors. Since the user might have loaded objects
using `dlopen' there are possibly several other modules with its
dependencies to be taken into account. Therefore we have to start
determining the order of the modules once again from the beginning. */
/* We run the destructors of the main namespaces last. As for the
other namespaces, we pick run the destructors in them in reverse
order of the namespace ID. */
#ifdef SHARED
int do_audit = 0;
again:
#endif
for (Lmid_t ns = GL(dl_nns) - 1; ns >= 0; --ns)
{
/* Protect against concurrent loads and unloads. */
__rtld_lock_lock_recursive (GL(dl_load_lock));
unsigned int nloaded = GL(dl_ns)[ns]._ns_nloaded;
/* No need to do anything for empty namespaces or those used for
auditing DSOs. */
if (nloaded == 0
#ifdef SHARED
|| GL(dl_ns)[ns]._ns_loaded->l_auditing != do_audit
#endif
)
__rtld_lock_unlock_recursive (GL(dl_load_lock));
else
{
#ifdef SHARED
_dl_audit_activity_nsid (ns, LA_ACT_DELETE);
#endif
/* Now we can allocate an array to hold all the pointers and
copy the pointers in. */
struct link_map *maps[nloaded];
unsigned int i;
struct link_map *l;
assert (nloaded != 0 || GL(dl_ns)[ns]._ns_loaded == NULL);
for (l = GL(dl_ns)[ns]._ns_loaded, i = 0; l != NULL; l = l->l_next)
/* Do not handle ld.so in secondary namespaces. */
if (l == l->l_real)
{
assert (i < nloaded);
maps[i] = l;
l->l_idx = i;
++i;
/* Ignore objects which are opened during shutdown. */
struct link_map *local_init_called_list = _dl_init_called_list;
for (struct link_map *l = local_init_called_list; l != NULL;
l = l->l_init_called_next)
/* Bump l_direct_opencount of all objects so that they
are not dlclose()ed from underneath us. */
++l->l_direct_opencount;
}
assert (ns != LM_ID_BASE || i == nloaded);
assert (ns == LM_ID_BASE || i == nloaded || i == nloaded - 1);
unsigned int nmaps = i;
/* 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, 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
various members of this list cannot vanish since the open
count is too high and will be decremented in this loop. So
we release the lock so that some code which might be called
from a destructor can directly or indirectly access the
lock. */
/* After this point, everything linked from local_init_called_list
cannot be unloaded because of the reference counter update. */
__rtld_lock_unlock_recursive (GL(dl_load_lock));
/* 'maps' now contains the objects in the right order. Now
call the destructors. We have to process this array from
the front. */
for (i = 0; i < nmaps; ++i)
/* Perform two passes: One for non-audit modules, one for audit
modules. This way, audit modules receive unload notifications
for non-audit objects, and the destructors for audit modules
still run. */
#ifdef SHARED
int last_pass = GLRO(dl_naudit) > 0;
Lmid_t last_ns = -1;
for (int do_audit = 0; do_audit <= last_pass; ++do_audit)
#endif
for (struct link_map *l = local_init_called_list; l != NULL;
l = l->l_init_called_next)
{
struct link_map *l = maps[i];
#ifdef SHARED
if (GL(dl_ns)[l->l_ns]._ns_loaded->l_auditing != do_audit)
continue;
if (l->l_init_called)
/* Avoid back-to-back calls of _dl_audit_activity_nsid for the
same namespace. */
if (last_ns != l->l_ns)
{
if (last_ns >= 0)
_dl_audit_activity_nsid (last_ns, LA_ACT_CONSISTENT);
_dl_audit_activity_nsid (l->l_ns, LA_ACT_DELETE);
last_ns = l->l_ns;
}
#endif
/* There is no need to re-enable exceptions because _dl_fini
is not called from a context where exceptions are caught. */
_dl_call_fini (l);
#ifdef SHARED
/* Auditing checkpoint: another object closed. */
_dl_audit_objclose (l);
#endif
}
/* Correct the previous increment. */
--l->l_direct_opencount;
}
#ifdef SHARED
_dl_audit_activity_nsid (ns, LA_ACT_CONSISTENT);
#endif
}
}
#ifdef SHARED
if (! do_audit && GLRO(dl_naudit) > 0)
{
do_audit = 1;
goto again;
}
if (last_ns >= 0)
_dl_audit_activity_nsid (last_ns, LA_ACT_CONSISTENT);
if (__glibc_unlikely (GLRO(dl_debug_mask) & DL_DEBUG_STATISTICS))
_dl_debug_printf ("\nruntime linker statistics:\n"

16
elf/dl-init.c
View File

@ -21,6 +21,7 @@
#include <ldsodefs.h>
#include <elf-initfini.h>
struct link_map *_dl_init_called_list;
static void
call_init (struct link_map *l, int argc, char **argv, char **env)
@ -42,6 +43,21 @@ call_init (struct link_map *l, int argc, char **argv, char **env)
dependency. */
l->l_init_called = 1;
/* Help an already-running dlclose: The just-loaded object must not
be removed during the current pass. (No effect if no dlclose in
progress.) */
l->l_map_used = 1;
/* Record execution before starting any initializers. This way, if
the initializers themselves call dlopen, their ELF destructors
will eventually be run before this object is destructed, matching
that their ELF constructors have run before this object was
constructed. _dl_fini uses this list for audit callbacks, so
register objects on the list even if they do not have a
constructor. */
l->l_init_called_next = _dl_init_called_list;
_dl_init_called_list = l;
/* Check for object which constructors we do not run here. */
if (__builtin_expect (l->l_name[0], 'a') == '\0'
&& l->l_type == lt_executable)

19
elf/dso-sort-tests-1.def
View File

@ -53,21 +53,14 @@ tst-dso-ordering10: {}->a->b->c;soname({})=c
output: b>a>{}<a<b
# Complex example from Bugzilla #15311, under-linked and with circular
# relocation(dynamic) dependencies. While this is technically unspecified, the
# presumed reasonable practical behavior is for the destructor order to respect
# the static DT_NEEDED links (here this means the a->b->c->d order).
# The older dynamic_sort=1 algorithm does not achieve this, while the DFS-based
# dynamic_sort=2 algorithm does, although it is still arguable whether going
# beyond spec to do this is the right thing to do.
# The below expected outputs are what the two algorithms currently produce
# respectively, for regression testing purposes.
# relocation(dynamic) dependencies. For both sorting algorithms, the
# destruction order is the reverse of the construction order, and
# relocation dependencies are not taken into account.
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
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];}
output: {+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<e<a<b<c<d];}
# Test that even in the presence of dependency loops involving dlopen'ed
# object, that object is initialized last (and not unloaded prematurely).
# Final destructor order is indeterminate due to the cycle.
# Final destructor order is the opposite of constructor order.
tst-bz28937: {+a;+b;-b;+c;%c};a->a1;a->a2;a2->a;b->b1;c->a1;c=>a1
output(glibc.rtld.dynamic_sort=1): {+a[a2>a1>a>];+b[b1>b>];-b[<b<b1];+c[c>];%c(a1());}<a<a2<c<a1
output(glibc.rtld.dynamic_sort=2): {+a[a2>a1>a>];+b[b1>b>];-b[<b<b1];+c[c>];%c(a1());}<a2<a<c<a1
output: {+a[a2>a1>a>];+b[b1>b>];-b[<b<b1];+c[c>];%c(a1());}<c<a<a1<a2

40
elf/tst-audit23.c
View File

@ -98,6 +98,8 @@ do_test (int argc, char *argv[])
char *lname;
uintptr_t laddr;
Lmid_t lmid;
uintptr_t cookie;
uintptr_t namespace;
bool closed;
} objs[max_objs] = { [0 ... max_objs-1] = { .closed = false } };
size_t nobjs = 0;
@ -117,6 +119,9 @@ do_test (int argc, char *argv[])
size_t buffer_length = 0;
while (xgetline (&buffer, &buffer_length, out))
{
*strchrnul (buffer, '\n') = '\0';
printf ("info: subprocess output: %s\n", buffer);
if (startswith (buffer, "la_activity: "))
{
uintptr_t cookie;
@ -125,29 +130,26 @@ do_test (int argc, char *argv[])
&cookie);
TEST_COMPARE (r, 2);
/* The cookie identifies the object at the head of the link map,
so we only add a new namespace if it changes from the previous
one. This works since dlmopen is the last in the test body. */
if (this_act == LA_ACT_ADD && acts[nacts] != cookie)
{
/* The cookie identifies the object at the head of the
link map, so we only add a new namespace if it
changes from the previous one. This works since
dlmopen is the last in the test body. */
if (cookie != last_act_cookie && last_act_cookie != -1)
TEST_COMPARE (last_act, LA_ACT_CONSISTENT);
if (this_act == LA_ACT_ADD && acts[nacts] != cookie)
{
acts[nacts++] = cookie;
last_act_cookie = cookie;
}
/* The LA_ACT_DELETE is called in the reverse order of LA_ACT_ADD
at program termination (if the tests adds a dlclose or a library
with extra dependencies this will need to be adapted). */
/* LA_ACT_DELETE is called multiple times for each
namespace, depending on destruction order. */
else if (this_act == LA_ACT_DELETE)
{
last_act_cookie = acts[--nacts];
TEST_COMPARE (acts[nacts], cookie);
acts[nacts] = 0;
}
last_act_cookie = cookie;
else if (this_act == LA_ACT_CONSISTENT)
{
TEST_COMPARE (cookie, last_act_cookie);
last_act_cookie = -1;
/* LA_ACT_DELETE must always be followed by an la_objclose. */
if (last_act == LA_ACT_DELETE)
@ -179,6 +181,8 @@ do_test (int argc, char *argv[])
objs[nobjs].lname = lname;
objs[nobjs].laddr = laddr;
objs[nobjs].lmid = lmid;
objs[nobjs].cookie = cookie;
objs[nobjs].namespace = last_act_cookie;
objs[nobjs].closed = false;
nobjs++;
@ -201,6 +205,12 @@ do_test (int argc, char *argv[])
if (strcmp (lname, objs[i].lname) == 0 && lmid == objs[i].lmid)
{
TEST_COMPARE (objs[i].closed, false);
TEST_COMPARE (objs[i].cookie, cookie);
if (objs[i].namespace == -1)
/* No LA_ACT_ADD before the first la_objopen call. */
TEST_COMPARE (acts[0], last_act_cookie);
else
TEST_COMPARE (objs[i].namespace, last_act_cookie);
objs[i].closed = true;
break;
}
@ -209,13 +219,9 @@ do_test (int argc, char *argv[])
/* la_objclose should be called after la_activity(LA_ACT_DELETE) for
the closed object's namespace. */
TEST_COMPARE (last_act, LA_ACT_DELETE);
if (!seen_first_objclose)
{
TEST_COMPARE (last_act_cookie, cookie);
seen_first_objclose = true;
}
}
}
for (size_t i = 0; i < nobjs; i++)
{

4
include/link.h
View File

@ -278,6 +278,10 @@ struct link_map
/* List of object in order of the init and fini calls. */
struct link_map **l_initfini;
/* Linked list of objects in reverse ELF constructor execution
order. Head of list is stored in _dl_init_called_list. */
struct link_map *l_init_called_next;
/* List of the dependencies introduced through symbol binding. */
struct link_map_reldeps
{

4
sysdeps/generic/ldsodefs.h
View File

@ -1037,6 +1037,10 @@ extern int _dl_check_map_versions (struct link_map *map, int verbose,
extern void _dl_init (struct link_map *main_map, int argc, char **argv,
char **env) attribute_hidden;
/* List of ELF objects in reverse order of their constructor
invocation. */
extern struct link_map *_dl_init_called_list attribute_hidden;
/* Call the finalizer functions of all shared objects whose
initializer functions have completed. */
extern void _dl_fini (void) attribute_hidden;