Adverse ABIs
============

.. articleMetaData::
   :Where: London, UK
   :Date: 2019-02-19 09:15 Europe/London
   :Tags: blog, php, xdebug
   :Short: abi-woes

After last week's `release candidate`_ I started receiving some `bug reports`_
that Xdebug nearly instantaneously crashed PHP. Upon further investigation, it
turned out to only be a problem on macOS, which is good as that's not
affecting everybody, but bad as I don't have a macOS machine to try to
reproduce things on.

.. _`release candidate`: https://xdebug.org/updates.php#x_2_7_0rc1
.. _`bug reports`: https://bugs.xdebug.org/view.php?id=1625

I did some screen sharing with `Frédéric Marand`_ in order to dive down what the
problem was, but we couldn't really get to the problem. It looked like there
was nothing really wrong, until Xdebug called the ``php_addslashes`` C
function (which is the internal implementation of addslashes_). This matched
the exact same location as `David Vuorio`_ showed in his `stack trace`_ in the bug
tracker.

.. _`Frédéric Marand`: https://twitter.com/OSInet
.. _addslashes: https://php.net/addslashes
.. _`David Vuorio`: https://github.com/dv-ds
.. _`stack trace`: https://bugs.xdebug.org/view.php?id=1625#c4888

Still tearing my hairs out, I contacted `Nikita Popov`_ to see if he knew
whether there was anything special about ``php_addslashes``. His reply:

.. image:: images/nikita-suspicion.png
   :alt: I have some suspicions...

And his suspicion turned out to be right.

.. _`Nikita Popov`: https://twitter.com/nikita_ppv/

PHP 7.3 has received many optimisations. One of them is the use of specific
CPU instructions to speed up certain tasks. In this case, the PCMPESTRM_
instruction is used to find whether a string has characters that need to be
escaped_.

.. _PCMPESTRM: https://en.wikipedia.org/wiki/SSE4#SSE4.2
.. _escaped: https://github.com/nikic/php-src/blob/5634df7f8dbf104d4e828ead31c78c5ab819eecc/ext/standard/string.c#L3954

Upon build time, PHP's ``./configure`` script checks whether the CPU it is
built on supports these newer CPU instructions. If so, it then uses some
`header magic`_ to use a specialised version of ``php_addslashes``. This works
fine if both PHP **and** the extension that uses ``php_addslashes`` are
compiled on the same machine. However, if PHP is compiled on a machine without
support, and the extension (Xdebug) is compiled on a machine **with** support,
then the ABI_ (calling convention) of the ``php_addslashes`` C function is
different. A crash will then occur when the extension tries to call the PHP
``php_addslashes`` function "in the wrong way". And that is exactly what
happened here: the PHP binary that came from Homebrew uses a different ABI
then the Xdebug binary that users compiled themselves. Modifying_ the installed
headers made the problem go away.

.. _`header magic`: https://github.com/nikic/php-src/blob/5634df7f8dbf104d4e828ead31c78c5ab819eecc/ext/standard/string.c#L3885
.. _ABI: https://en.wikipedia.org/wiki/Application_binary_interface
.. _Modifying: https://bugs.xdebug.org/view.php?id=1625#c4909

Nikita soon fixed this in the PHP 7.3 branch with a hack_ to make sure that
the ABI among PHP 7.3 versions stays the same, and a proper fix_ for PHP 7.4
and later to prevent this situation from occurring in the feature. Xdebug
works around it for PHP 7.3.0 and PHP 7.3.1. Instead of relying on PHP's
implementation of ``php_addslashes`` (`xdebug_add_slashes`_), it uses its own.
It also uses its own implementations_ of ``php_base64_encode`` and
``php_base64_decode`` which exhibit a similar issue. As Xdebug's
implementations are slightly modified to work better for Xdebug, there is a
minimal performance improvement too.

.. _hack: https://github.com/php/php-src/pull/3826/commits/6093364d1979a776a0e76b3226246dcfd1191fae
.. _fix: https://github.com/php/php-src/pull/3828/commits/5634df7f8dbf104d4e828ead31c78c5ab819eecc
.. _xdebug_add_slashes: https://github.com/xdebug/xdebug/blob/master/xdebug_compat.c#L231-L294
.. _implementations: https://github.com/xdebug/xdebug/blob/master/xdebug_compat.c#L77-L229

Time for Release Candidate 2!