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Bug #18780


Incorrect binding receiver for C API rb_eval_string()

Added by daveola (David Stellar) about 1 hour ago.

Target version:
ruby -v:
ruby 2.7.0p0 (2019-12-25 revision 647ee6f091) [x86_64-linux]


% ruby -v
ruby 2.7.0p0 (2019-12-25 revision 647ee6f091) [x86_64-linux]

(Though looking at the source code this problem still exists in ruby 3.0)

The rb_eval_string() is seemingly capable of everything that eval is capable of, with one slight deviation. The binding is oddly setup to be correct except for the receiver/self.

This means that while these both act the same:

ruby: eval("puts someLocalVariable")
C API: rb_eval_string("puts someLocalVariable")

These do not:

ruby: eval("puts @someInstanceVar")
C API: rb_eval_string("puts @someInstanceVar") # nil

And this is because these do not work:

ruby: eval("puts self") # self of calling context
ruby: eval("puts binding().receiver") # self of calling context
C API: rb_eval_string("puts self") # main
C API: rb_eval_string("puts binding().receiver") # main

We can see the problem in the MRI source in ruby_eval_string_from_file() which has:

return eval_string_with_cref(rb_vm_top_self(), rb_str_new2(str), NULL, file, 1);

We've passed in rb_vm_top_self instead of the correct 'self'

Although possibly the issue is in the API itself with the fact that there's no way to plumb through the receiver that you are given in the C extension method function call, i.e.:

    // My C extension that calls eval and knows what it's 'self' is.
    VALUE method_myCMethod(int argc, VALUE *argv, VALUE self) {
            rb_eval_string("...");  // <- no way to be given self?

Having said that, rb_eval_string is able to determine the vast majority of the binding context, since it correctly builds the binding() object except that the receiver is set to main, so perhaps this is something that can be determined. It is something that the builtin eval is able to do, after all. So possibly this is just a failure with MRI. I don't have other rubies to test. (I'm on ruby 2.7.0 but the source relevant to this hasn't changed.)

I would argue this is a bug, because we are essentially given a corrupted result from binding() - one where we may have access to the local variables of an instance method, but one where the self itself is not properly set. That's not an actual legit binding state in the ruby code.

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