commit 6985865bc3ad5b23147ee73466583dd7fdf65892 Author: Florian Weimer Date: Fri Sep 8 12:32:14 2023 +0200 elf: Always call destructors in reverse constructor order (bug 30785) 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 1df71d32fe5f5905ffd5d100e5e9ca8ad62 ("elf: Implement force_first handling in _dl_sort_maps_dfs (bug 28937)"). Reviewed-by: DJ Delorie diff --git a/elf/dl-close.c b/elf/dl-close.c index 64d82eee05432717..79094f9a48a871e8 100644 --- a/elf/dl-close.c +++ b/elf/dl-close.c @@ -137,30 +137,31 @@ _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. */ - continue; + { + /* Already handled. */ + l = next; + continue; + } /* Check whether this object is still used. */ if (l->l_type == lt_loaded @@ -170,7 +171,10 @@ _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) - continue; + { + l = next; + continue; + } /* We need this object and we handle it now. */ l->l_map_used = 1; @@ -197,8 +201,11 @@ _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; + } } } @@ -218,44 +225,65 @@ _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; + if (jmap->l_idx < next_idx) + { + next = jmap; + next_idx = next->l_idx; + } } } } - } - /* 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); + l = next; + } - /* 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) + /* 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 = maps[i]; + struct link_map *imap = *init_called_head; - /* All elements must be in the same namespace. */ - assert (imap->l_ns == nsid); - - if (!imap->l_map_used) + /* _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; @@ -266,8 +294,8 @@ _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) @@ -403,8 +431,8 @@ _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; } } @@ -475,10 +503,11 @@ _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); @@ -686,7 +715,9 @@ _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; } } diff --git a/elf/dl-fini.c b/elf/dl-fini.c index 9d2c39640275ac35..13308b36099487fb 100644 --- a/elf/dl-fini.c +++ b/elf/dl-fini.c @@ -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. - - 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 - { + /* 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. */ + + /* Protect against concurrent loads and unloads. */ + __rtld_lock_lock_recursive (GL(dl_load_lock)); + + /* 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; + + /* 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)); + + /* 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 - _dl_audit_activity_nsid (ns, LA_ACT_DELETE); + int last_pass = GLRO(dl_naudit) > 0; + Lmid_t last_ns = -1; + for (int do_audit = 0; do_audit <= last_pass; ++do_audit) #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; - - /* 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. */ - __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) - { - struct link_map *l = maps[i]; - - if (l->l_init_called) - { - _dl_call_fini (l); + for (struct link_map *l = local_init_called_list; l != NULL; + l = l->l_init_called_next) + { #ifdef SHARED - /* Auditing checkpoint: another object closed. */ - _dl_audit_objclose (l); + if (GL(dl_ns)[l->l_ns]._ns_loaded->l_auditing != do_audit) + continue; + + /* 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 - } - /* Correct the previous increment. */ - --l->l_direct_opencount; - } + /* 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 - _dl_audit_activity_nsid (ns, LA_ACT_CONSISTENT); + /* Auditing checkpoint: another object closed. */ + _dl_audit_objclose (l); #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" diff --git a/elf/dl-init.c b/elf/dl-init.c index 1d962f4df813e728..e080619a3d1ebe9d 100644 --- a/elf/dl-init.c +++ b/elf/dl-init.c @@ -21,6 +21,7 @@ #include #include +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) diff --git a/elf/dso-sort-tests-1.def b/elf/dso-sort-tests-1.def index 4bf9052db16fb352..61dc54f8ae06d465 100644 --- a/elf/dso-sort-tests-1.def +++ b/elf/dso-sort-tests-1.def @@ -53,21 +53,14 @@ tst-dso-ordering10: {}->a->b->c;soname({})=c output: b>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[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[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[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[];%c(a1());}a1>a>];+b[b1>b>];-b[];%c(a1());}a1>a>];+b[b1>b>];-b[];%c(a1());}