Segmentation Fault When Trying To Fetch Built-In PHP Classes

[Garethp]

Hi

I'm trying to add generalised support for allowing a class to extend a built-in PHP class (Such as \Exception or \DateTime, but when I try to fetch the zend_class_entry with the following

            zend_string *name = zend_string_init(std::string("DateTime").data(), std::string("DateTime").size(), 1);

            zend_class_entry *dateTimeCE = zend_fetch_class(name, ZEND_FETCH_CLASS_DEFAULT TSRMLS_CC);

I get a segmentation fault, with gdb giving the following backtrace

(gdb) bt
#0  0x00005555557c7b47 in zend_hash_find ()
#1  0x00005555557a5bb0 in zend_lookup_class_ex ()
#2  0x00005555557a6555 in zend_fetch_class ()
#3  0x00007fffe824df84 in get_module () from /usr/lib/php/20151012/miro-container.so
#4  0x00005555556f927a in php_load_extension ()
#5  0x00005555557a7afe in zend_llist_apply ()
#6  0x000055555575b4ea in php_ini_register_extensions ()
#7  0x00005555557535d6 in php_module_startup ()
#8  0x0000555555848d7d in ?? ()
#9  0x0000555555637e71 in main ()

Using the zend_fetch_class method does work for classes that I define myself, as I'm later doing the following with no issue

            zend_class_entry *notFoundEntry = zend_fetch_class(zend_string_init(std::string("NotFoundException").data(), std::string("NotFoundException").size(), 1), ZEND_FETCH_CLASS_SILENT);

            zend_throw_exception(notFoundEntry, "Custom Exception Class", 1000 TSRMLS_CC);

I can also get the default exception class with zend_exception_get_default(TSRMLS_C), and register a class as extending from that, so I know that at the very least that's already registered. So my question is why does it segfault when I'm trying to fetch a built-in class, and how can I fetch it without Segfaults?

[sjinks]

It looks like the initialization of the extension takes place right after it is loaded. However, creation of all classes by the PHP Core happens during Module Init phase.

http://lxr.php.net/xref/PHP-7.0/main/main.c#2216 - this is where PHP loads all extensions
http://lxr.php.net/xref/PHP-7.0/main/main.c#2217 - this is where PHP extensions (aka modules) are started.

But the crash itself happens for a different reason: zend_fetch_class() looks up the class name in EG(class_table) which does not yet exist — when PHP modules are started up, only CG(class_table) exists (CG = compiler globals, EG = executor globals).

EG(class_table) gets initialized in init_executor, init_executor() is called from zend_activate, zend_activate is called from php_request_startup which happens during the Request Initialization phase.

Thus the proper way to fetch an internal class would be like this:

#include <iostream>
#include <phpcpp.h>
#include <Zend/zend.h>
#include <Zend/zend_execute.h>

extern "C"
{
        PHPCPP_EXPORT void *get_module()
        {
                // create extension
                static Php::Extension extension("simple","1.0");

                extension.onStartup([]() {
                        zend_string *name = zend_string_init(ZEND_STRL("DateTime"), 1);
                        zend_string *lc_name = zend_string_tolower(name);
                        zend_class_entry *dateTimeCE = static_cast<zend_class_entry*>(zend_hash_find_ptr(CG(class_table), lc_name));
                        zend_string_release(lc_name);

// if dateTimeCE is nullptr here, the class does not exist
                        std::cerr << dateTimeCE << std::endl;
                });

                return extension.module();
        }
}

However, as this is the very first time I am looking at the PHP-CPP code, I cannot suggest you yet how to register a class from onStartup() :-)

[sjinks]

Well, good news is that I have found the way to inherit from an internal class. Bad news is that you still cannot inherit from DateTime.

So, this is how you do that:

#include <cassert>
#include <phpcpp.h>
#include <Zend/zend.h>
#include <Zend/zend_execute.h>
#include <Zend/zend_API.h>
#include <Zend/zend_inheritance.h>

class MyClass : public Php::Base {
public:
        virtual Php::Value method()
        {
                return 42;
        }
};

extern "C"
{
        PHPCPP_EXPORT void *get_module()
        {
                static Php::Extension extension("simple","1.0");

                Php::Class<MyClass> myClass("MyClass");
                myClass.method<&MyClass::method>("method");
                extension.add(myClass);

                extension.onStartup([]() {
                        zend_string *name = zend_string_init(ZEND_STRL("LibXMLError"), 1);
                        zend_string *lc_name = zend_string_tolower(name);
                        zend_class_entry *base_ce = static_cast<zend_class_entry*>(zend_hash_find_ptr(CG(class_table), lc_name));
                        zend_string_release(lc_name);

                        assert(base_ce != nullptr);

                        name = zend_string_init(ZEND_STRL("MyClass"), 1);
                        lc_name = zend_string_tolower(name);
                        zend_class_entry* myclass_ce = static_cast<zend_class_entry*>(zend_hash_find_ptr(CG(class_table), lc_name));
                        zend_string_release(lc_name);

                        assert(myclass_ce != nullptr);
                        zend_do_inheritance(myclass_ce, base_ce);
                });

                return extension.module();
        }
}

Now why you cannot inherit from DateTime: zend_do_inheritance calls do_inherit_parent_constructor:

static void do_inherit_parent_constructor(zend_class_entry *ce) /* {{{ */
{
	ZEND_ASSERT(ce->parent != NULL);

	/* You cannot change create_object */
	ce->create_object = ce->parent->create_object;

that ce->create_object = ce->parent->create_object; spoils everything.

DateTime class is initialized like this:

        INIT_CLASS_ENTRY(ce_date, "DateTime", date_funcs_date);
        ce_date.create_object = date_object_new_date;

And it appears that you can have only one create_object for the entire inheritance chain. Because PHP-CPP needs to use its own create_object function, you cannot inherit from DateTime.

So,

zend_class_entry* myclass_ce = static_cast<zend_class_entry*>(zend_hash_find_ptr(CG(class_table), lc_name));
zend_string_release(lc_name);

assert(myclass_ce != nullptr);

if (!base_ce->create_object || base_ce->create_object == myclass_ce->create_object) {
    zend_do_inheritance(myclass_ce, base_ce);
}
else {
    // FAIL, you cannot inherit from the base class
}