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Date: Thu, 11 Dec 2014 14:54:48 -0500
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Subject: Post hooking opcode execution and accessing userland names of variables
From: argyros.george@gmail.com (George Argyros)

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Hi all,
I am developing a program analysis tool as a PHP extension. The extension
makes use of the zend_set_user_opcode_handler API in order to propagate
information about the variables along the execution.

I have two questions:

1. From my undestanding we can only pre-hook the execution of an opcode. In
other words, we define a user_opcode_handler and after the execution of our
opcode handler finish, then we can dispatch the execution to the original
handler, but there is no way of taking control of the execution after the
original handler is executed. Is that correct? If so, would the following
way of implementing a post-hook work?
- Save (by copying to another memory area) the opcode arguments.
- On the next instruction executed utilize a user opcode handler for the
next instruction to do the post-hook analysis by using the saved arguments.
One reason I would like to use this is to propagate information after, for
example, assignment operations where the result variable is not yet
defined, and therefore no respective zval structure exists. Would it be
possible in this case to access the newly created zval structure with the
post-hook method I described above?


2. Again by what I understand from the code, userland variable names are
stored in the compiled variable structure. Is there a way to access this
structure given the zval pointer of the variable efficiently? Also, in a
similar question, is there a way to access the userland name of a variable
which is passed as an argument to a function of the extension? All I have
found is how to grub the value of the variable or the respective zval
structure using the argument parsing functions.

Thanks,
George

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