[Crash-utility] Extensions ABI compatibility

Hi list, during development of the PaX module patch[1], Lianbo pointed out an issue about adding new members to struct offset_table. They should be added to the end to ensure binary compatibility with already compiled extensions, as these won't be aware of the new members and, even worse, might be using wrong offsets within the offset_table if we didn't. As this seems to be rather fragile, here's a proposal to make it less so. To not blindly make already compiled extensions misbehave by using the wrong offsets when new members get added in the middle of shared types, we need to have some validation step, preferably during extension registration to catch these. If we want to keep binary compatibility, we need to add members to the end. That's a rule maintainers can enforce but as we all make mistakes, code can help us here. Changing a shared data structure has, at least, implications on its size. So we can use this as a base to build checks on. If the size of a data structure used during compilation of an extension differs from the size of that same data structure as seen within 'crash', we have an ABI issue. The response should be, at least, a warning to inform the user to recompile its extension to ensure ABI compatibility. Another option would be to decline loading the extension but that might be inconvenient for users, if we indeed made sure to add members only at the end of the involved data structures. These checks can be implemented "transparently" as sketched in the attached patch. It changes register_extension() to be a macro that stores the sizes of critical types in a data structure that gets passed to _register_extension() which is implemented in 'crash' and will do the size checks against its version of the involved types. The patch also takes care about backward compatibility by providing a stub register_extension() function that passes a NULL structure, as all current extension modules refer to register_extension() instead of the newly introduced _register_extension(). Open TODOs: 1/ Compile a list of all types we want to protect and add checks for these. 2/ Put these types into a 'shared.h' header file and use that exclusively within extensions -- maybe even enforce it. Do we want to follow that approach or look for alternatives? Regards, Mathias [1]: https://www.redhat.com/archives/crash-utility/2020-August/msg00024.html