Bug #65822 [Com]: crash on shutdown because of zend extension cleanup order
Edit report at https://bugs.php.net/bug.php?id=65822&edit=1
ID: 65822
Comment by: askalski at gmail dot com
Reported by: askalski at gmail dot com
Summary: crash on shutdown because of zend extension cleanup
order
Status: Open
Type: Bug
Package: Reproducible crash
Operating System: Linux
PHP Version: 5.4.20
Block user comment: N
Private report: N
New Comment:
Turns out it's not so simple. It never is...
Modules need to be shutdown before the global function/class tables are destroyed. Extensions need
to be shutdown after. Consequently there is no valid way to interleave extension and module
shutdown.
Because this issue likely stems from APC meddling with PHP internals in a manner in which modules
were never intended to meddle, I see no real reason to act upon this ticket... with one exception:
One thing that still ought to be fixed is zend_shutdown_extensions() should apply
zend_extension_shutdown over the zend_extensions list in *reverse* order, not forward order. (This
would involve adding a zend_llist_reverse_apply_* family of functions to zend_llist.c)
Previous Comments:
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[2013-10-03 16:00:22] askalski at gmail dot com
Spent some time digging into this. It's not as simple as calling zend_shutdown_extensions
before zend_destroy_modules; there are a few issues at play which complicate matters.
The two Zend Extensions I investigated (OpCache and XDebug) implement both the zend_extension and
zend_module API. Both of these extensions load the zend_module portion by calling
zend_startup_module in the zend_extension startup function. Also, it is possible to load modules at
runtime using the dl() userland function.
Consequently, the shutdown order is not a simple matter of "extensions first, then
modules". Because startup order of extensions and modules can be interleaved, this precise
order must be recorded during initialization. The implementation might be something as simple as a
stack of enumerated values: { MODULE, MODULE, MODULE, MODULE, EXTENSION, MODULE, EXTENSION, MODULE }
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[2013-10-02 22:09:12] askalski at gmail dot com
Description:
------------
php_module_startup() initializes modules first, extensions second.
However, zend_shutdown() destroys them in the same order (modules first, extensions second), rather
than in stack order as one would expect.
Furthermore, it seems (based on reading the zend_startup_extensions() zend_shutdown_extensions()
functions) that if multiple zend extensions are loaded, they are destroyed in the wrong order as
well.
Multiple modules work fine; they are destroyed in stack order.
To reproduce the issue, load an extension and module which both override the same Zend structure.
For example, loading both OpCache 7.0.2 and APC 3.1.13 will cause a segfault on shutdown because of
improper cleanup order of orig_interned_strings_start, old_interned_strings_start, and
compiler_globals.interned_strings_start.
I'm aware that the example sounds like a bizarre combination of modules and extensions here;
I'm reporting the bug because it points at an issue in PHP itself. The specific use case for
loading both APC and OpCache is to use OpCache for opcodes and APC with apc.cache_by_default=0 for
the apc_store/apc_fetch userland functions.
Test script:
---------------
zend_extension=/usr/lib64/php/modules/opcache.so
extension=apc.so
Load both OpCache and APC in mod_php in Apache "prefork" mode. Send SIGTERM to one of the
workers (or simply send enough requests to make Apache reap the worker), and watch for the
Segmentation fault in Apache's error_log.
Expected result:
----------------
No crash.
Actual result:
--------------
Segmentation fault.
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