Launchpad has imported 17 comments from the remote bug at https://bz.apache.org/bugzilla/show_bug.cgi?id=60296.
If you reply to an imported comment from within Launchpad, your comment will be sent to the remote bug automatically. Read more about Launchpad's inter-bugtracker facilities at https://help.launchpad.net/InterBugTracking. ------------------------------------------------------------------------ On 2016-10-21T18:32:23+00:00 Max Burke wrote: (I originally filed this with Debian's apache team because when I searched for related information on this issue I turned up something in their bug tracker. Here is the link for more context: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=814980) The anylock structure provided by mod_ldap was set to the type apr_anylock_none, which resulted in multiple threads mutating the shared RMM state at the same time without any concurrency guards. The issue I was seeing was that all the threads on the server were stuck inside the RMM internal function find_block_of_size every 2 or 3 days, requiring the server processes to be killed and restarted. We are using Apache on Windows. I made the following patch which passes in a lock to the RMM pool created in mod_ldap when APR_HAS_THREADS is defined. Since doing so we have not encountered any hangs: diff -Naur httpd-2.4.16\include\util_ldap.h httpd-2.4.16-ea\include\util_ldap.h --- httpd-2.4.16\include\util_ldap.h Mon Jul 14 05:07:55 2014 +++ httpd-2.4.16-ea\include\util_ldap.h Mon Aug 29 10:20:08 2016 @@ -169,6 +169,10 @@ #if APR_HAS_SHARED_MEMORY apr_shm_t *cache_shm; apr_rmm_t *cache_rmm; +#if APR_HAS_THREADS + apr_thread_mutex_t *lock; + apr_anylock_t cache_rmm_anylock; +#endif #endif /* cache ald */ diff -Naur httpd-2.4.16\modules\ldap\util_ldap_cache.c httpd-2.4.16-ea\modules\ldap\util_ldap_cache.c --- httpd-2.4.16\modules\ldap\util_ldap_cache.c Mon Aug 19 04:45:19 2013 +++ httpd-2.4.16-ea\modules\ldap\util_ldap_cache.c Mon Aug 29 10:23:04 2016 @@ -410,6 +410,14 @@ st->cache_shm = NULL; return result; } + +#if APR_HAS_THREADS + apr_thread_mutex_destroy(st->lock); + st->lock = NULL; + st->cache_rmm_anylock.type = apr_anylock_none; + st->cache_rmm_anylock.lock.pm = NULL; +#endif + #endif return APR_SUCCESS; } @@ -436,8 +444,18 @@ /* Determine the usable size of the shm segment. */ size = apr_shm_size_get(st->cache_shm); +#if APR_HAS_THREADS + apr_thread_mutex_create(&st->lock, APR_THREAD_MUTEX_DEFAULT, st->pool); + st->cache_rmm_anylock.type = apr_anylock_threadmutex; + st->cache_rmm_anylock.lock.tm = st->lock; +#else + st->lock = NULL; + st->cache_rmm_anylock.type = apr_anylock_none; + st->cache_rmm_anylock.lock.pm = NULL; +#endif + /* This will create a rmm "handler" to get into the shared memory area */ - result = apr_rmm_init(&st->cache_rmm, NULL, + result = apr_rmm_init(&st->cache_rmm, &st->cache_rmm_anylock, apr_shm_baseaddr_get(st->cache_shm), size, st->pool); if (result != APR_SUCCESS) { Thanks! Reply at: https://bugs.launchpad.net/ubuntu/+source/apache2/+bug/1767105/comments/0 ------------------------------------------------------------------------ On 2016-10-21T19:29:23+00:00 Covener-0 wrote: Hi Max, it appears to me that mod_ldap is trying to manage a global (cross process + cross thread) lock outside of rmm, which is why it tells RMM to not use a lock. Windows is a special case where there is only 1 process and N threads. But passing in a thread mutex only is probably wrong outside of windows. Not having looked too closely, it seems likely that an LDAP_CACHE_LOCK() is missing somewhere where we call into rmm. Have you been able to verify on a later release? Reply at: https://bugs.launchpad.net/ubuntu/+source/apache2/+bug/1767105/comments/1 ------------------------------------------------------------------------ On 2016-10-21T19:47:49+00:00 Max Burke wrote: I have not tried a later release in production, no, only because we have been soaking the change to see if it improved reliability. I am looking at the mod_ldap changes between our current version (2.4.16) and the latest (2.4.23) and see that the only changes are whitespace, either adding/remove spaces or adding newlines before braces, so I would expect that 2.4.23 is similarly affected. The LDAP_CACHE_LOCK construct is defined only to the util_ldap.c translation unit, it does not seem to be available, at present, to util_ldap_cache_mgr.c where the RMM blocks are allocated and freed. Reply at: https://bugs.launchpad.net/ubuntu/+source/apache2/+bug/1767105/comments/2 ------------------------------------------------------------------------ On 2016-10-22T01:25:19+00:00 Covener-0 wrote: (In reply to Max Burke from comment #2) > I have not tried a later release in production, no, only because we have > been soaking the change to see if it improved reliability. > > I am looking at the mod_ldap changes between our current version (2.4.16) > and the latest (2.4.23) and see that the only changes are whitespace, either > adding/remove spaces or adding newlines before braces, so I would expect > that 2.4.23 is similarly affected. > > The LDAP_CACHE_LOCK construct is defined only to the util_ldap.c translation > unit, it does not seem to be available, at present, to util_ldap_cache_mgr.c > where the RMM blocks are allocated and freed. It's not directly available, but that lock is meant to be held by anyone who would ultimately get into the code that does the allocations. That is usually (AFAICT) via util_ald_cache_insert(). What kind of Require ldap-* do you use? I noticed the debian OP didn't list that bit. Reply at: https://bugs.launchpad.net/ubuntu/+source/apache2/+bug/1767105/comments/3 ------------------------------------------------------------------------ On 2016-10-26T21:52:37+00:00 Max Burke wrote: (In reply to Eric Covener from comment #3) > What kind of Require ldap-* do you use? I noticed the debian OP didn't list > that bit. I'm unfamiliar with what you mean by "Require ldap-*", but here are the snippets from our httpd.conf for the ldap settings: LoadModule authnz_ldap_module modules/mod_authnz_ldap.so LoadModule ldap_module modules/mod_ldap.so LDAPTrustedMode SSL LDAPVerifyServerCert off LDAPSharedCacheSize 20000000 <AuthnProviderAlias ldap eac> AuthLDAPURL "[... snip ... ]" AuthLDAPBindDN "[ ... snip ... ]" # including ldap password from a separate file # AuthLDAPBindPassword secret Include conf/ldap_bind_pwd.conf </AuthnProviderAlias> Reply at: https://bugs.launchpad.net/ubuntu/+source/apache2/+bug/1767105/comments/4 ------------------------------------------------------------------------ On 2017-03-27T16:03:36+00:00 Guido-wischrop wrote: Fixed status - sorry wrong bug. Reply at: https://bugs.launchpad.net/ubuntu/+source/apache2/+bug/1767105/comments/5 ------------------------------------------------------------------------ On 2018-03-01T16:36:14+00:00 Rafael-tinoco wrote: *** Bug 58483 has been marked as a duplicate of this bug. *** Reply at: https://bugs.launchpad.net/ubuntu/+source/apache2/+bug/1767105/comments/6 ------------------------------------------------------------------------ On 2018-03-01T16:38:26+00:00 Rafael-tinoco wrote: Max or Eric, Are there any changes in status of this bug ? I can corroborate to what Max said, and proposed, since I'm analysing a dump that was brought to me, exactly in this situation (and also could find another old bug with the same race condition): https://bz.apache.org/bugzilla/show_bug.cgi?id=58483 These are my notes so far: Problem summary: apr_rmm_init acts as a relocatable memory management initialization it is used in: mod_auth_digest and util_ldap_cache >From the dump was brought to my knowledge, in the following sequence: - util_ldap_compare_node_copy() - util_ald_strdup() - apr_rmm_calloc() - find_block_of_size() Had a "cache->rmm_addr" with no lock at "find_block_of_size()" cache->rmm_addr->lock { type = apr_anylock_none } And an invalid "next" offset (out of rmm->base->firstfree). This rmm_addr was initialized with NULL as a locking mechanism: >From apr-utils: apr_rmm_init() if (!lock) { <-- 2nd argument to apr_rmm_init() nulllock.type = apr_anylock_none; <--- found in the dump nulllock.lock.pm = NULL; lock = &nulllock; } >From apache: # mod_auth_digest sts = apr_rmm_init(&client_rmm, NULL, /* no lock, we'll do the locking ourselves */ apr_shm_baseaddr_get(client_shm), shmem_size, ctx); # util_ldap_cache result = apr_rmm_init(&st->cache_rmm, NULL, apr_shm_baseaddr_get(st->cache_shm), size, st->pool); It appears that the ldap module chose to use "rmm" for memory allocation, using the shared memory approach, but without explicitly definiting a lock to it. Without it, its up to the caller to guarantee that there are locks for rmm synchronization (just like mod_auth_digest does, using global mutexes). Because of that, there was a race condition in "find_block_of_size" and a call touching "rmm->base->firstfree", possibly "move_block()", in a multi-threaded apache environment, since there were no lock guarantees inside rmm logic (lock was "apr_anylock_none" and the locking calls don't do anything). In find_block_of_size: apr_rmm_off_t next = rmm->base->firstfree; We have: rmm->base->firstfree Decimal:356400 Hex:0x57030 But "next" turned into: Name : next Decimal:8320808657351632189 Hex:0x737973636970653d Causing: struct rmm_block_t *blk = (rmm_block_t*)((char*)rmm->base + next); if (blk->size == size) To segfault. Reply at: https://bugs.launchpad.net/ubuntu/+source/apache2/+bug/1767105/comments/7 ------------------------------------------------------------------------ On 2018-03-01T16:41:32+00:00 Covener-0 wrote: I recently re-fixed an issue that causes the lock in mod_ldap to not be used. It will be in 2.4.31. It is a one-line change. http://svn.apache.org/viewvc?view=revision&revision=1824811 Reply at: https://bugs.launchpad.net/ubuntu/+source/apache2/+bug/1767105/comments/8 ------------------------------------------------------------------------ On 2018-03-02T01:40:15+00:00 Rafael-tinoco wrote: Just for a reference on someone that finds this bug. Distributions Fixes (Debian & Ubuntu): https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=814980#15 https://bugs.launchpad.net/ubuntu/+source/apache2/+bug/1752683 With LP: #1752683 being worked on (about to suggest final fix for all Ubuntu versions), it is likely I'll suggest same fix for Bug: #814980, both based in upstream commit: 39ae6cd64268 brought to me by Eric. Reply at: https://bugs.launchpad.net/ubuntu/+source/apache2/+bug/1767105/comments/9 ------------------------------------------------------------------------ On 2018-04-26T12:29:05+00:00 Rafael-tinoco wrote: Eric, Even after your fix... LDAP_CACHE_LOCK() is either missing a barrier or it is not enough for subsequent calls to APR with NULL locking (passed to APR_RMM_INIT). After patch for this bug has been applied, https://bugs.launchpad.net/ubuntu/+source/apache2/+bug/1752683 and the new bug https://bugs.launchpad.net/ubuntu/+source/apache2/+bug/1767105, end user still complains about seg faults and core dumps show same issue: race condition for rmm->base->firstfree in function "find_block_of_size". In the dump, in find_block_of_size(): apr_rmm_off_t next = rmm->base->firstfree; ... while(next) { struct rmm_block_t *blk = (rmm_block_t*)((char*)rmm->base + next); blk gets value 0x5772e56b36226557 because "next" was corrupted (value: 0x57726573553d554f). This happens because the lock APR_ANYLOCK_LOCK(&rmm->lock); in apr_rmm_calloc() is apr_anylock_none, like previously reported by me. For the sake of exercising possibilities, if mod_ldap is calling APR RMM with external locking, it would be using LDAP_CACHE_LOCK. My current stack trace is this: Thread #19 7092 (Suspended : Container) kill() at syscall-template.S:84 0x7ff7e9911767 <signal handler called>() at 0x7ff7e9cb7390 find_block_of_size() at apr_rmm.c:106 0x7ff7ea10e25a apr_rmm_calloc() at apr_rmm.c:342 0x7ff7ea10ea68 util_ald_alloc() at util_ldap_cache_mgr.c:105 0x7ff7e3369b3d util_ldap_compare_node_copy() at util_ldap_cache.c:257 0x7ff7e3369784 util_ald_cache_insert() at util_ldap_cache_mgr.c:501 0x7ff7e336a310 uldap_cache_compare() at util_ldap.c:1,183 0x7ff7e33662d3 ldapgroup_check_authorization() at mod_authnz_ldap.c:925 0x7ff7e8459937 apply_authz_sections() at mod_authz_core.c:737 0x7ff7e4bb99fa apply_authz_sections() at mod_authz_core.c:751 0x7ff7e4bb9c01 authorize_user_core() at mod_authz_core.c:840 0x7ff7e4bb9dca ap_run_auth_checker() at request.c:91 0x56127e692f00 ap_process_request_internal() at request.c:335 0x56127e695d57 ap_process_async_request() at http_request.c:408 0x56127e6b4690 ap_process_request() at http_request.c:445 0x56127e6b4850 ap_process_http_sync_connection() at http_core.c:210 0x56127e6b091e ap_process_http_connection() at http_core.c:251 0x56127e6b091e ap_run_process_connection() at connection.c:41 0x56127e6a6bf0 ap_process_connection() at connection.c:213 0x56127e6a7000 process_socket() at worker.c:631 0x7ff7e2f51f8b worker_thread() at worker.c:990 0x7ff7e2f51f8b start_thread() at pthread_create.c:333 0x7ff7e9cad6ba clone() at clone.S:109 0x7ff7e99e341d Which means uldap_cache_compare() would have synchronized access to APR RMM calls through LDAP_CACHE_LOCK() macro. This doesn't seem to be the case as the lock doesn't seem to be acquired. LDAP_CACHE_LOCK() translates into: do { if (st->util_ldap_cache_lock) apr_global_mutex_lock(st->util_ldap_cache_lock); } while (0); After the change proposed for this bug (where "util_ldap_cache_lock" would come from the ldap_merge_config), it seems that st has util_ldap_cache_lock and util_ldap_cache all set: Name : util_ldap_cache_lock Hex:0x7ff7ea75aee0 Name : util_ldap_cache Hex:0x7ff7e0e51038 Meaning that it got the ldap_cache and ldap_cache_lock from the merge config function. >From the mutex acquire logic, for the apr_global_mutex_lock() -> apr_proc_mutex_lock(): apr_status_t apr_proc_mutex_lock(apr_proc_mutex_t *mutex) { return mutex->meth->acquire(mutex); } And, from my dump, it would translate into: st->util_ldap_cache_lock->proc_mutex->meth->acquire == proc_mutex_fcntl_acquire() And from that logic: static apr_status_t proc_mutex_fcntl_acquire(apr_proc_mutex_t *mutex) { int rc; do { rc = fcntl(mutex->interproc->filedes, F_SETLKW, &proc_mutex_lock_it); } while (rc < 0 && errno == EINTR); if (rc < 0) { return errno; } mutex->curr_locked=1; return APR_SUCCESS; } Again, from the dump, We would guarantee mutex lock through a file descriptor to the file: "/var/lock/apache2/ldap-cache.1368" (filedes == 15) And the "mutex->curr_locked" would be 1. Unfortunately, considering my stack trace, during the cache insertion: find_block_of_size() at apr_rmm.c:106 0x7ff7ea10e25a apr_rmm_calloc() at apr_rmm.c:342 0x7ff7ea10ea68 util_ald_alloc() at util_ldap_cache_mgr.c:105 0x7ff7e3369b3d util_ldap_compare_node_copy() at util_ldap_cache.c:257 0x7ff7e3369784 util_ald_cache_insert() at util_ldap_cache_mgr.c:501 0x7ff7e336a310 uldap_cache_compare() at util_ldap.c:1,183 0x7ff7e33662d3 Name : st->util_ldap_cache_lock Details:0x7ff7ea75aee0 Default:0x7ff7ea75aee0 Decimal:140702767230688 Hex:0x7ff7ea75aee0 Binary:11111111111011111101010011101011010111011100000 Octal:03777375235327340 Name : proc_mutex Details:0x7ff7ea75aef8 Default:0x7ff7ea75aef8 Decimal:140702767230712 Hex:0x7ff7ea75aef8 Binary:11111111111011111101010011101011010111011111000 Octal:03777375235327370 Name : curr_locked Details:0 Default:0 Decimal:0 Hex:0x0 Binary:0 Octal:0 I have curr_locked = 0 So far.. I can see: The functions (from fcntl type of locking) touching curr_locked are: proc_mutex_fcntl_acquire() -> fcntl(proc_mutex_lock_it), curr_locked = 1 proc_mutex_fcntl_tryacquire() -> unblocking fcntl(proc_mutex_lock_it), curr_locked = 1 proc_mutex_fcntl_create() -> apr_file_open(), curr_locked = 0 proc_mutex_fcntl_release() -> curr_locked = 0, fcntl(proc_mutex_unlock_it) proc_mutex_fcntl_cleanup() -> if (cur_locked = 1) { mutex_fcntl_release() } -> apr_file_close() Meaning that likely _release() might have raced with _acquire() somehow. One thing that comes to my thoughts, right away, is the fact that function proc_mutex_fcntl_release(), from apr, sets "curr_locked" to 0 before actually the fcntl() F_SETLKW succeeds BUT the locking is guaranteed using the file lock with fcntl() call and not due to the variable value change, so it would be okay-ish. I can't see any other obvious reason why the lock is not acquired in this current execution. Still investigating... any ideas ? Reply at: https://bugs.launchpad.net/ubuntu/+source/apache2/+bug/1767105/comments/11 ------------------------------------------------------------------------ On 2018-04-26T13:26:50+00:00 Covener-0 wrote: Is fcntl being forced here somehow, or is it the default (apachectl -V|grep APR_USE) ? I wonder if an affected user could try a different mechanism, via e.g. https://httpd.apache.org/docs/2.4/mod/core.html#mutex with "ldap-cache" as the parm (or if they already are somehow?) Reply at: https://bugs.launchpad.net/ubuntu/+source/apache2/+bug/1767105/comments/13 ------------------------------------------------------------------------ On 2018-05-07T13:08:27+00:00 Rafael-tinoco wrote: Thread #19 7092 (Suspended : Container) kill() at syscall-template.S:84 0x7ff7e9911767 <signal handler called>() at 0x7ff7e9cb7390 find_block_of_size() at apr_rmm.c:106 0x7ff7ea10e25a apr_rmm_calloc() at apr_rmm.c:342 0x7ff7ea10ea68 util_ald_alloc() at util_ldap_cache_mgr.c:105 0x7ff7e3369b3d util_ldap_compare_node_copy() at util_ldap_cache.c:257 0x7ff7e3369784 util_ald_cache_insert() at util_ldap_cache_mgr.c:501 0x7ff7e336a310 uldap_cache_compare() at util_ldap.c:1,183 0x7ff7e33662d3 ldapgroup_check_authorization() at mod_authnz_ldap.c:925 0x7ff7e8459937 apply_authz_sections() at mod_authz_core.c:737 0x7ff7e4bb99fa apply_authz_sections() at mod_authz_core.c:751 0x7ff7e4bb9c01 authorize_user_core() at mod_authz_core.c:840 0x7ff7e4bb9dca ap_run_auth_checker() at request.c:91 0x56127e692f00 ap_process_request_internal() at request.c:335 0x56127e695d57 ap_process_async_request() at http_request.c:408 0x56127e6b4690 ap_process_request() at http_request.c:445 0x56127e6b4850 ap_process_http_sync_connection() at http_core.c:210 0x56127e6b091e ap_process_http_connection() at http_core.c:251 0x56127e6b091e ap_run_process_connection() at connection.c:41 0x56127e6a6bf0 ap_process_connection() at connection.c:213 0x56127e6a7000 process_socket() at worker.c:631 0x7ff7e2f51f8b worker_thread() at worker.c:990 0x7ff7e2f51f8b start_thread() at pthread_create.c:333 0x7ff7e9cad6ba clone() at clone.S:109 0x7ff7e99e341d -------------------------------------------------------------------------------- static apr_rmm_off_t find_block_of_size(apr_rmm_t *rmm, apr_size_t size) { apr_rmm_off_t next = rmm->base->firstfree; apr_rmm_off_t best = 0; apr_rmm_off_t bestsize = 0; while (next) { struct rmm_block_t *blk = (rmm_block_t*)((char*)rmm->base + next); -------------------------------------------------------------------------------- The following code touches "rmm->base->firstfree" (capable of racing): apr_rmm_destroy() - ... apr_rmm_init() - ... find_block_of_size() - apr_rmm_calloc() - apr_rmm_malloc() move_block() - apr_rmm_calloc() - apr_rmm_free() - apr_rmm_malloc() -------------------------------------------------------------------------------- All APR calls to RMM have to have: APR_ANYLOCK_LOCK(&rmm->lock) and lock can't be apr_anylock_none ORELSE the lock has to be guaranteed by the caller, in our case, uldap_cache_compare() using the LDAP_CACHE_LOCK() directive. LDAP_CACHE_LOCK() directive is: do { if (st->util_ldap_cache_lock) apr_global_mutex_lock(st->util_ldap_cache_lock); } while (0); Where st is a ldap state struct, got by ap_get_module_config() on the ldap mod. "util_ldap_cache_lock" is the global apr mutex for this module and it is set by: st->util_ldap_cache_lock = base->util_ldap_cache_lock; in "util_ldap_merge_config" AND (re)-initialized in util_ldap_post_config if, still after ldap merge, it does NOT contain "cache_shm" and APR_HAS_SHARED_MEMOR was defined (cache settings are inherited in the virtual host, all server use the same shared memory cache). If re-initialized by util_ldap_post_config, util_ldap_cache_lock is a mutex created with ap_global_mutex_create using a "ldap-cache" type. #if APR_HAS_SHARED_MEMORY if (!st->cache_shm) { #endif If APR_HAS_SHARED_MEMORY is not set, it always initializes the lock like this. result = ap_global_mutex_create(&st->util_ldap_cache_lock, NULL, ldap_cache_mutex_type, NULL, s, p, 0); Since the util_ldap_cache_lock->proc_mutex->fname is this: 0x7ff7ea75af38 == "/var/lock/apache2/ldap-cache.1368" We know for sure that the locking being used is "ldap-cache" and was set like above since the name is originated from "lock type" string. -------------------------------------------------------------------------------- We can say for sure that the ldap global lock: st->util_ldap_cache_lock was created by util_ldap_post_config() function, and the locking method was obtained from the "mxcfg_lookup()" logic when doing the ap_global_mutex_create() with the type "ldap_cache_mutex_type" ("ldap-cache" string). ldap_cache_mutex_type variable ("ldap-cache") is a global, configured or used by functions coming from the util_ldap_register_hooks: util_ldap_child_init() - n/a util_ldap_post_config() - creates util_ldap_cache_lock based on it util_ldap_pre_config() - registers the ldap_cache_mutex_type util_ldap_cache_lock is used for all uldap_cache_XXXXX functions calling LDAP_CACHE_LOCK() and by the following functions: util_ldap_child_init() - n/a util_ldap_merge_config() - set to base (server) config util_ldap_post_config() - created based on ldap_cache_mutex_type -------------------------------------------------------------------------------- Inside util_ldap_post_config() -> ap_global_mutex_create() we called the functio get_futex_filename(). It checks wether the mutex mech(anism) needs a backing file or not (only if mech == APR_LOCK_FLOCK or APR_LOCK_FCNTL) My dump had st->util_ldap_cache_lock->proc_mutex->fname = /var/lock/apache2/ldap-cache.1368 Meaning that my lock was, for sure, APR_LOCK_FLOCK or, likely, APR_LOCK_FCNTL. -------------------------------------------------------------------------------- >From our compilation flags: $ apachectl -V Server version: Apache/2.4.18 (Ubuntu) Server built: 2018-03-01T18:29:12 Server's Module Magic Number: 20120211:52 Server loaded: APR 1.5.2, APR-UTIL 1.5.4 Compiled using: APR 1.5.2, APR-UTIL 1.5.4 Architecture: 64-bit Server MPM: event threaded: yes (fixed thread count) forked: yes (variable process count) Server compiled with.... -D APR_HAS_SENDFILE -D APR_HAS_MMAP -D APR_HAVE_IPV6 (IPv4-mapped addresses enabled) -D APR_USE_SYSVSEM_SERIALIZE -D APR_USE_PTHREAD_SERIALIZE -D SINGLE_LISTEN_UNSERIALIZED_ACCEPT -D APR_HAS_OTHER_CHILD -D AP_HAVE_RELIABLE_PIPED_LOGS -D DYNAMIC_MODULE_LIMIT=256 -D HTTPD_ROOT="/etc/apache2" -D SUEXEC_BIN="/usr/lib/apache2/suexec" -D DEFAULT_PIDLOG="/var/run/apache2.pid" -D DEFAULT_SCOREBOARD="logs/apache_runtime_status" -D DEFAULT_ERRORLOG="logs/error_log" -D AP_TYPES_CONFIG_FILE="mime.types" -D SERVER_CONFIG_FILE="apache2.conf" -------------------------------------------------------------------------------- Since we've registered this locking mechanism, ldap_cache_mutex_type, as "APR_LOCK_DEFAULT", we have to check how the APR_LOCK_DEFAULT logic is defined inside APR lib. The full call path is this: util_ldap_post_config() >>> ap_global_mutex_create() >>> apr_global_mutex_create() >>> apr_proc_mutex_create() >>> proc_mutex_create() >>> proc_mutex_choose_method() And the "mech" used in util_ldap_post_config(), to decide type of mutex, comes from the ap_global_mutex_create() function, doing the mxcfg_lookup() with the hash type "ldap-cache" (as I showed before, APR_LOCK_DEFAULT). Then the proc_mutex_choose_method() function takes its decision on what locking type to use: switch (mech) { ... case APR_LOCK_DEFAULT: #if APR_USE_FLOCK_SERIALIZE new_mutex->inter_meth = &mutex_flock_methods; #elif APR_USE_SYSVSEM_SERIALIZE new_mutex->inter_meth = &mutex_sysv_methods; #elif APR_USE_FCNTL_SERIALIZE new_mutex->inter_meth = &mutex_fcntl_methods; #elif APR_USE_PROC_PTHREAD_SERIALIZE new_mutex->inter_meth = &mutex_proc_pthread_methods; #elif APR_USE_POSIXSEM_SERIALIZE new_mutex->inter_meth = &mutex_posixsem_methods; #else return APR_ENOTIMPL; #endif Meaning that default is set compile time and, in our case, it was FCNTL based, based on the logic above AND the dump file containing the: st->util_ldap_cache_lock->proc_mutex->inter_meth->*_mutex_fcntl_* pointers This decision path would have to have APR_USE_FCNTL_SERIALIZE set and this does not happen for me, since the decision comes from the autoconf script: # See which lock mechanism we'll select by default on this system. # The last APR_DECIDE to execute sets the default. # At this stage, we match the ordering in Apache 1.3 # which is (highest to lowest): sysvsem -> fcntl -> flock. # POSIX semaphores and cross-process pthread mutexes are not # used by default since they have less desirable behaviour when # e.g. a process holding the mutex segfaults. # The BEOSSEM decision doesn't require any substitutions but is # included here to prevent the fcntl() branch being selected # from the decision making. >From configure.in: case $ac_decision in USE_FLOCK_SERIALIZE ) flockser="1" ;; USE_FCNTL_SERIALIZE ) fcntlser="1" ;; USE_SYSVSEM_SERIALIZE ) sysvser="1" ;; USE_POSIXSEM_SERIALIZE ) posixser="1" ;; USE_PROC_PTHREAD_SERIALIZE ) procpthreadser="1" ;; USE_BEOSSEM ) beossem="1" ;; esac And those variables are the ones defining the headers: $ grep -ri APR_USE_FCNTL_SERIALIZE * ... debian/build/include/apr.h:#define APR_USE_FCNTL_SERIALIZE 0 include/apr.h.in:#define APR_USE_FCNTL_SERIALIZE @fcntlser@ ... This is what would make proc_mutex_choose_method() to decide to use fcntl locking mechanism and you can see that, in my case, fakeroot debian/rules was build-arch did NOT set that variable. In the configure output, you can see: decision on apr_lock implementation method... SysV IPC semget() Which means that the APR_DECIDE was sysvser, like described, as we can see: $ grep -ri APR_USE_SYSVSEM_SERIALIZE * | grep -v html debian/build/include/apr.h:#define APR_USE_SYSVSEM_SERIALIZE 1 include/apr.h.in:#define APR_USE_SYSVSEM_SERIALIZE @sysvser@ in the build dir. -------------------------------------------------------------------------------- So, how was fcntl choosen as locking mech(anism) if APR_USE_FCNTL_SERIALIZE is not set ? The only possibility I see is that in the beginning of proc_mutex_choose_method, mech == APR_LOCK_FCNTL was passed as an argument, and NOT APR_LOCK_DEFAULT, like it should have been in the "ldap-cache" type of mutex, when being created. OR for the APR_LOCK_DEFAULT to be something else. With that, I went to check what defined it and found apache's function: ap_set_mutex(), called by apache "init core commands" logic. This function will set the mechanism for APR_LOCK_DEFAULT depending on the configuration file stanza (derd!) and with that I went for it and found: Mutex file:${APACHE_LOCK_DIR} default -------------------------------------------------------------------------------- SUMMARY: Now it makes more sense. So the "bug" here now is to find out why fcntl locking did not guarantee atomicity for the LDAP operations, or even, if it should have done (based on its premises). Another question that raises is, why to use fcntl as a backing mechanism for the LDAP locking ? If the lock was supposed to be guaranteed among different nodes, then backing the lock in a file, IF the name was based also on the instance id, which in this case is not, it could make sense. But in a shared-threaded only environment, why ? TODOs: (1) I would like end user to test the LDAP caching operations/locking without fcntl based locking. According to: https://httpd.apache.org/docs/2.4/mod/core.html#mutex It is possible to set a different locking mechanism to each of the subsystems described in table "Mutex name, Module(s), Protected Resource". This, per se, will likely guarantee the atomicity of the LDAP cache operations. (2) To investigate if there is some catch in regarding to using fcntl() in an intensive I/O environment for backing up mutexes implementation. Is there any fix related to fcntl() that should be picked OR any type of I/O configuration (including page cache, filesystem mounting options, need for direct-io) in order for it to be used ? Eric, A colleague of mine, or I, will get back to you with some answers on all this investigation. Tks a lot! Reply at: https://bugs.launchpad.net/ubuntu/+source/apache2/+bug/1767105/comments/14 ------------------------------------------------------------------------ On 2018-05-07T13:16:31+00:00 Covener-0 wrote: > Another question that raises is, why to use fcntl as a backing mechanism for > the > LDAP locking ? If the lock was supposed to be guaranteed among different > nodes, > then backing the lock in a file, IF the name was based also on the instance > id, > which in this case is not, it could make sense. But in a shared-threaded only > environment, why ? the locking is only meant to be among the N children of a single instance / parent process. You should find the PID here is the parent pid. So I think this aspect is OK. I have seen fcntl related errors on other OS'es, where APR might use it by default, related to EDEADLOCK being returned when two unrelated threads use two unrelated fcntl mutexes. Reply at: https://bugs.launchpad.net/ubuntu/+source/apache2/+bug/1767105/comments/17 ------------------------------------------------------------------------ On 2018-05-08T12:12:49+00:00 Covener-0 wrote: In a local test I set: `Mutex fcntl:/tmp/run default` globally and added an assert to the two lock/unlock macros and got several aborts. The asserts with sysvsem never fired. I will be removing the macros and adding inline funcitons that log and abort on failure. Reply at: https://bugs.launchpad.net/ubuntu/+source/apache2/+bug/1767105/comments/18 ------------------------------------------------------------------------ On 2018-05-08T12:27:52+00:00 Covener-0 wrote: [Tue May 08 08:26:42.418299 2018] [ldap:crit] [pid 68897:tid 700008b10000] (11)Resource deadlock avoided: [client 127.0.0.1:60404] LDAP cache lock failed Reply at: https://bugs.launchpad.net/ubuntu/+source/apache2/+bug/1767105/comments/19 ------------------------------------------------------------------------ On 2018-06-12T13:00:31+00:00 Covener-0 wrote: Hi Rafael -- out of curiosity did you ever find how fcntl() came to be used here? Reply at: https://bugs.launchpad.net/ubuntu/+source/apache2/+bug/1767105/comments/20 ** Changed in: apache2 Status: Unknown => Incomplete ** Changed in: apache2 Importance: Unknown => Medium ** Bug watch added: Debian Bug tracker #814980 https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=814980 ** Bug watch added: bz.apache.org/bugzilla/ #58483 https://bz.apache.org/bugzilla/show_bug.cgi?id=58483 -- You received this bug notification because you are a member of Ubuntu Bugs, which is subscribed to Ubuntu. https://bugs.launchpad.net/bugs/1767105 Title: race condition on rmm for module ldap (ldap cache) - part II To manage notifications about this bug go to: https://bugs.launchpad.net/apache2/+bug/1767105/+subscriptions -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs
