[Crash-utility] Extensions ABI compatibility

[HAGIO KAZUHITO(萩尾 一仁)]

-----Original Message-----
> Hi, Mathias
> 
> Thank you for discussing this topic and describing its backgrounds.
> 
> 在 2020年08月19日 23:45, Mathias Krause 写道:
> > 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.
> >
> Sounds good to me.

hmm, Lianbo, are you Red Hat ready to recompile and release crash extension
packages (almost) every time releasing a crash package?  As far as I know,
at least Red Hat has not done so, so your crash release work will have to
be changed.  And you will have to specify the versions of extension modules
corresponding to a crash package for users somehow or unite those packages.

I think this way would cost distributors some effort to change their release
operations like above.  Users also will lose some convenience, even if we
adopt a warning, usually they will not be able to use an extension emitting
it, though they can ignore it in almost all cases..

I wonder if there are merits enough to warn and cost them for now.

But the implementation is nice, and might be useful as debugging information
in case something happens.  Is it acceptable to move it into CRASHDEBUG(1)
and "help -e"?

Thanks,
Kazu

> 
> > 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,
> 
> Recently, we are considering putting the crash guidelines on the wiki in
> order to help developers avoid making 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?
> >
> 
> Let's wait for more comments or suggestions.
> 
> Thanks.
> Lianbo
> 
> > Regards,
> > Mathias
> >
> > [1]: https://www.redhat.com/archives/crash-utility/2020-August/msg00024.html
> >
> 
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