Segmentation faults in Apache

If you are running SimpleSAMLphp 1.14.0 or older, and versions of PHP older than 5.6, you might see errors like this in your error_log:

[core:notice] [pid 1234] AH00051: child pid 12345 exit signal Segmentation fault (11), possible coredump in /etc/apache2

If you are really unlucky, you might even get a blank page in your browser or a message from it saying it cannot connect to the server (if you are also using HTTPS).

If you have experienced any of these, you might have hit a bug in PHP as reported in issue #293.

Background

There are a few bugs introduced in PHP when the header_register_callback function was implemented, and they have been fixed over the years and the fixes released through different versions of PHP.

At the same time, there was a race condition in SimpleSAMLphp that could lead to error messages about missing state. In order to fix that issue, we started using the header_register_callback to make sure any change to the session was properly saved before sending any output to the browser, eliminating the race condition.

The problem is that using the header_register_callback makes you vulnerable to those bugs in PHP, so if you are not using a version where all of them have been fixed, you could experience the aforementioned segmentation faults.

Solution

You have two alternatives to fix this issue, each with its pros and cons:

• Update PHP to 5.6 or newer. Pro: you will get rid of both the segmentation faults and the missing state errors. Con: it can be hard in many systems to update to 5.6, as that version might not yet be available in the stable branch of your operating system.

• Disable the header_register_callback function in your php.ini. In order to do that, go to the php.ini file in use by your web server, and locate the disable_functions directive. Append header_register_callback to it (bear in mind that the functions must be comma-separated). Pro: it is way easier than updating PHP, and you can always do this. Con: the missing state errors will eventually be back.