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Exception and error handling in SimpleSAMLphp

Table of Contents

This document describes the way errors and exceptions are handled in authentication sources and authentication processing filters. The basic goal is to be able to throw an exception during authentication, and then have that exception transported back to the SP in a way that the SP understands.

This means that internal SimpleSAMLphp exceptions must be mapped to transport specific error codes for the various transports that are supported by SimpleSAMLphp. E.g.: When a SimpleSAML_Error_NoPassive error is thrown by an authentication processing filter in a SAML 2.0 IdP, we want to map that exception to the urn:oasis:names:tc:SAML:2.0:status:NoPassive status code. That status code should then be returned to the SP.

1 Throwing exceptions

How you throw an exception depends on where you want to throw it from. The simplest case is if you want to throw it during the authenticate()-method in an authentication module or during the process()-method in a processing filter. In those methods, you can just throw an exception:

public function process(&$state) {
    if ($state['something'] === FALSE) {
        throw new SimpleSAML_Error_Exception('Something is wrong...');
    }
}

Exceptions thrown at this stage will be caught and delivered to the appropriate error handler.

If you want to throw an exception outside of those methods, i.e. after you have done a redirect, you need to use the SimpleSAML_Auth_State::throwException() function:

<?php
$id = $_REQUEST['StateId'];
$state = SimpleSAML_Auth_State::loadState($id, 'somestage...');
SimpleSAML_Auth_State::throwException($state,
    new SimpleSAML_Error_Exception('Something is wrong...'));
?>

The SimpleSAML_Auth_State::throwException function will then transfer your exception to the appropriate error handler.

1.1 Note

Note that we use the SimpleSAML_Error_Exception class in both cases. This is because the delivery of the exception may require a redirect to a different web page. In those cases, the exception needs to be serialized. The normal Exception class in PHP isn't always serializable.

If you throw an exception that isn't a subclass of the SimpleSAML_Error_Exception class, your exception will be converted to an instance of SimpleSAML_Error_UnserializableException. The SimpleSAML_Auth_State::throwException function does not accept any exceptions that does not subclass the SimpleSAML_Error_Exception class.

2 Returning specific SAML 2 errors

By default, all thrown exceptions will be converted to a generic SAML 2 error. In some cases, you may want to convert the exception to a specific SAML 2 status code. For example, the SimpleSAML_Error_NoPassive exception should be converted to a SAML 2 status code with the following properties:

The sspmod_saml_Error class represents SAML 2 errors. It represents a SAML 2 status code with three elements: the top-level status code, the second-level status code and the status message. The second-level status code and the status message is optional, and can be NULL.

The sspmod_saml_Error class contains a helper function named fromException. The fromException() function is used by www/saml2/idp/SSOService.php to return SAML 2 errors to the SP. The function contains a list which maps various exceptions to specific SAML 2 errors. If it is unable to convert the exception, it will return a generic SAML 2 error describing the original exception in its status message.

To return a specific SAML 2 error, you should:

2.1 Note

While it is possible to throw SAML 2 errors directly from within authentication sources and processing filters, this practice is discouraged. Throwing SAML 2 errors will tie your code directly to the SAML 2 protocol, and it may be more difficult to use with other protocols.

3 Converting SAML 2 errors to normal exceptions

On the SP side, we want to convert SAML 2 errors to SimpleSAMLphp exceptions again. This is handled by the toException() method in sspmod_saml_Error. The assertion consumer script of the SAML 2 authentication source (modules/saml2/sp/acs.php) uses this method. The result is that generic exceptions are thrown from that authentication source.

For example, NoPassive errors will be converted back to instances of SimpleSAML_Error_NoPassive.

4 Other protocols

The error handling code has not yet been added to other protocols, but the framework should be easy to adapt for other protocols. To eventually support other protocols was a goal when designing this framework.

5 Technical details

This section attempts to describe the internals of the error handling framework.

5.1 SimpleSAML_Error_Exception

The SimpleSAML_Error_Exception class extends the normal PHP Exception class. It makes the exceptions serializable by overriding the __sleep() method. The __sleep() method returns all variables in the class which should be serialized when saving the class.

To make sure that the class is serializable, we remove the $trace variable from the serialization. The $trace variable contains the full stack trace to the point where the exception was instantiated. This can be a problem, since the stack trace also contains the parameters to the function calls. If one of the parameters in unserializable, serialization of the exception will fail.

Since preserving the stack trace can be useful for debugging, we save a variant of the stack trace in the $backtrace variable. This variable can be accessed through the getBacktrace() method. It returns an array with one line of text for each function call in the stack, ending on the point where the exception was created.

5.1.1 Note

Since we lose the original $trace variable during serialization, PHP will fill it with a new stack trace when the exception is unserialized. This may be confusing since the new stack trace leads into the unserialize() function. It is therefore recommended to use the getBacktrace() method.

5.2 SimpleSAML_Auth_State

There are two methods in this class that deals with exceptions:

5.2.1 throwException

This method delivers the exception to the code that initialized the exception handling in the authentication state. That would be SimpleSAML_Auth_Default for authtentication sources, and www/saml2/idp/SSOService.php for processing filters. To configure how and where the exception should be delivered, there are two fields in the state-array which can be set:

If the exception is delivered by a function call, the function will be called with two parameters: The exception and the state array.

If the exception is delivered by a redirect, SimpleSAML_Auth_State will save the exception in a field in the state array, pass a parameter with the id of the state array to the URL. The SimpleSAML_Auth_State::EXCEPTION_PARAM constant contains the name of that parameter, while the SimpleSAML_Auth_State::EXCEPTION_DATA constant holds the name of the field where the exception is saved.

5.2.2 loadException

To retrieve the exception, the application should check for the state parameter in the request, and then retrieve the state array by calling SimpleSAML_Auth_State::loadExceptionState(). The exception can be located in a field named SimpleSAML_Auth_State::EXCEPTION_DATA. The following code illustrates this behaviour:

if (array_key_exists(SimpleSAML_Auth_State::EXCEPTION_PARAM, $_REQUEST)) {
    $state = SimpleSAML_Auth_State::loadExceptionState();
    $exception = $state[SimpleSAML_Auth_State::EXCEPTION_DATA];

    /* Process exception. */
}

5.3 SimpleSAML_Auth_Default

This class accepts an $errorURL parameter to the initLogin() function. This parameter is stored in the SimpleSAML_Auth_State::EXCEPTION_HANDLER_URL of the state array. Exceptions thrown by the authentication source will be delivered to that URL.

It also wraps the call to the authenticate() function inside a try-catch block. Any exceptions thrown during that function call will be delivered to the URL specified in the $errorURL parameter. This is done for consistency, since SimpleSAML_Auth_Default never transfers control back to the caller by returning.

5.4 SimpleSAML_Auth_ProcessingChain

This class requires the caller to add the error handler to the state array before calling the processState() function. Exceptions thrown by the processing filters will be delivered directly to the caller of processState() if possible. However, if one of the filters in the processing chain redirected the user away from the caller, exceptions will be delivered through the error handler saved in the state array.

This is the same behaviour as normal processing filters. The result will be delivered directly if it is possible, but if not, it will be delivered through a redirect.

The code for handling this becomes something like:

if (array_key_exists(SimpleSAML_Auth_State::EXCEPTION_PARAM, $_REQUEST)) {
    $state = SimpleSAML_Auth_State::loadExceptionState();
    $exception = $state[SimpleSAML_Auth_State::EXCEPTION_DATA];

    /* Handle exception... */
    [...]
}

$procChain = [...];

$state = array(
    'ReturnURL' => SimpleSAML_Utilities::selfURLNoQuery(),
    SimpleSAML_Auth_State::EXCEPTION_HANDLER_URL => SimpleSAML_Utilities::selfURLNoQuery(),
    [...],
)

try {
    $procChain->processState($state);
} catch (SimpleSAML_Error_Exception $e) {
    /* Handle exception. */
    [...];
}

5.4.1 Note

An exception which isn't a subclass of SimpleSAML_Error_Exception will be converted to the SimpleSAML_Error_UnserializedException class. This happens regardless of whether the exception is delivered directly or through the error handler. This is done to be consistent in what the application receives - now it will always receive the same exception, regardless of whether it is delivered directly or through a redirect.

5.5 Custom error show function

Optional custom error show function, called from SimpleSAML_Error_Error::show, is defined with 'errors.show_function' in config.php.

Example code for this function, which implements the same functionality as SimpleSAML_Error_Error::show, looks something like:

public static function show(SimpleSAML_Configuration $config, array $data) {
    $t = new SimpleSAML_XHTML_Template($config, 'error.php', 'errors');
    $t->data = array_merge($t->data, $data);
    $t->show();
    exit;
}