Bug #63380 [Asn]: Allocation via libxml does not use PHP's per-request allocator
Edit report at https://bugs.php.net/bug.php?id=63380&edit=1
ID: 63380
Updated by: tstarling@php.net
Reported by: tstarling@php.net
Summary: Allocation via libxml does not use PHP's per-request
allocator
Status: Assigned
Type: Bug
Package: XML related
Operating System: Linux
PHP Version: master-Git-2012-10-29 (Git)
Assigned To: tstarling
Block user comment: N
Private report: N
New Comment:
Do you know of a specific case where request-local allocations could bleed into mod_perl and cause
memory corruption?
I have reviewed all of libxml's global variables and ensured that they cannot be made to hold
pointers to request-local allocations. The hooks are disabled at post-deactivate via a thread-local
variable, so a perl request will not get request-local pointers from xmlMalloc() whether it runs in
its own thread concurrently, or in the same thread as PHP but at a later time. TSRM_FETCH() should
give default global variables, with local request allocation disabled, even if it is called from a
thread where PHP has never been used.
Either way, I would be happy to make this configurable, off by default, since the robustness of the
solution depends on details of global pointer storage in libxml which may change in the future. So
my patch does introduce a maintenance burden, with a risk of dangling pointers if that maintenance
is not kept up to date. I'm just not keen on having the documentation say that there are known
issues with interaction with other Apache modules unless that is truly the case.
Previous Comments:
------------------------------------------------------------------------
[2012-12-05 11:08:41] rrichards@php.net
There is a major problem with doing this and why I didn't end tying into the PHP
memory allocator. Depending upon setup, it is extremely likely to be able to hit
memory corruption and/or mix memory allocations between modules. i.e. using
mod_perl and mod_php will cause PHP to override the libxml memory handling
functions (which are global) and bleed into mod_perl (or any others that are
using libxml2) causing any number of results (crashes, security issues, etc..).
The only way to be able to do something like this would be to make it compile
time option which is disabled by default allowing those who know their
environment intimately can utilize this at their own risk, Don't know if you
want to write a patch for that or not. Otherwise I don't see any way this could
safely be added,
------------------------------------------------------------------------
[2012-10-29 21:55:03] tstarling@php.net
https://github.com/php/php-src/pull/223
------------------------------------------------------------------------
[2012-10-29 03:25:17] tstarling@php.net
Description:
------------
Allocation via libxml does not use PHP's per-request allocator. So any memory used by libxml
will not be accounted against memory_get_usage() or memory_limit.
At Wikimedia we use libxml DOM trees to store wikitext parse trees, because they are more compact in
memory than the equivalent pure-PHP data structures. When these parse trees are cached, the memory
requirements can become excessive, and the memory is typically not returned to the system after
request termination. Using xmlMemSetup() to set hook functions which use PHP's per-request
allocation functions will allow us to more effectively monitor and limit the use of libxml in
production.
I've developed a patch and will submit it to GitHub as a pull request.
Test script:
---------------
$doc = new DOMDocument;
for ( $i = 0; $i < 1000000 ; $i++ ) {
$doc->appendChild($doc->createElement('foo'));
}
print memory_get_usage()."\n";
Expected result:
----------------
312331440 (with debug and ZTS)
Actual result:
--------------
694256
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