11–20 of 39 posts
It's hardly news but I guess it makes this common cracking technique more accessible.
> The C++ standard does not specify how virtual functions should be implemented. In practice, however, compilers generate a virtual function table and place a pointer to it as the first member of a class. wishful thinking: https://gcc.godbolt.org/z/qWEe9r
Not sure what you are trying to show, the object still has a vtable and is placed as the first member (and in your example, only) of the class, so that quote is correct. Obviously if you enable optimizations and one of those optimizations is avoiding the virtual call when the compiler thinks it isn't necessary, then sure you wont get a virtual call everywhere. But if your code is relying on implementation assumptions…
I was wondering whether such a thing is possible for JVM based languages and it turns out it is: https://stackoverflow.com/questions/8273685/is-it-possible-t...
I believe classes, once loaded by the JVM, cannot be changed. https://stackoverflow.com/a/43653466/
I was wondering whether such a thing is possible for JVM based languages and it turns out it is: https://stackoverflow.com/questions/8273685/is-it-possible-t...
So, is it possible? I get the impression you can create a new class using a bytecode-manipulation library, but that's not the same as modifying an existing class at runtime. I believe classes, once loaded by the JVM, cannot be changed. https://stackoverflow.com/a/43653466/
Earlier quoted context omitted.
So, is it possible? I get the impression you can create a new class using a bytecode-manipulation library, but that's not the same as modifying an existing class at runtime. I believe classes, once loaded by the JVM, cannot be changed. https://stackoverflow.com/a/43653466/
It's generally possible to rewrite loaded classes. Some changes are possible (e.g. replacing a method's body), others are not (adding new methods). The state of the art library for doing this at the application layer is ByteBuddy https://github.com/raphw/byte-buddy#changing-existing-classe... but this functionality is used by plenty of tooling - profilers such as YourKit rewrite methods to add telemetry - I've seen s…
AutoCad does this in their ObjectARX technology, with a fixed compiler version, to support user or vendor provided plugins to extend classes. At runtime. For decades.
Had to go a step further in a project and patch static functions in a codebase with no source. It’s certainly enlightening how much you can do with just a symbol map and type info. I don’t think the articles vtable layout is entirely accurate for gcc though - usually you’ll get 2 destructors at the start of the vtable (assuming the first virtual func declared is the destructor).
Earlier quoted context omitted.
Not sure what you are trying to show, the object still has a vtable and is placed as the first member (and in your example, only) of the class, so that quote is correct. Obviously if you enable optimizations and one of those optimizations is avoiding the virtual call when the compiler thinks it isn't necessary, then sure you wont get a virtual call everywhere. But if your code is relying on implementation assumptions…
In other words if your program relies on undefined behavior then make sure you look at your compiler's documentation and then you might be able to make it implementation defined.
The fact that inside a single CU an optimizing compiler can determine the targets of polymorphic dispatch statically is unrelated, because most usage scenarios (more than one CU, external modules) preclude such optimizations anyway.
vtables aren't undefined, they are implementation defined and part of the C++ ABI. As soon as you expose something through the C++ ABI the compiler has to use the ABI's definitions.
Had to go a step further in a project and patch static functions in a codebase with no source. It’s certainly enlightening how much you can do with just a symbol map and type info. I don’t think the articles vtable layout is entirely accurate for gcc though - usually you’ll get 2 destructors at the start of the vtable (assuming the first virtual func declared is the destructor).
Are you willing to share any info on how you got stuck with this project, and what this "codebase" was? Can you call it a codebase is it doesn't have source code?
> The C++ standard does not specify how virtual functions should be implemented. In practice, however, compilers generate a virtual function table and place a pointer to it as the first member of a class. wishful thinking: https://gcc.godbolt.org/z/qWEe9r
Cool. Is this due to "final" and the fact that the compiler can figure out the target at compile time?