Re: [PATCH] x86/kbuild: enable modversions for symbols exported from asm

From: Stanislav Kozina
Date: Mon Dec 12 2016 - 04:48:59 EST

A runtime check is still done, with per-module vermagic which distros
can change when they bump the ABI version. Is it really necessary to
have more than that (i.e., per-symbol versioning)?
From my point of view, it is. We need to allow changing ABI for some
modules while maintaining it for others.
In fact I think that there should be version not only for every exported
symbol (in the EXPORT_SYMBOL() sense), but also for every public type
(in the sense of eg. structure defined in the public header file).
Well the distro can just append _v2, _v3 to the name of the function
or type if it has to break compat for some reason. Would that be enough?
There are other ways that distros can work around when upstream "breaks"
the ABI, sometimes they can rename functions, and others they can
"preload" structures with padding in anticipation for when/if fields get
added to them. But that's all up to the distros, no need for us to
worry about that at all :)

Currently, the ABI version (checksum) is stored outside of the actual code in the __ksymtab section. That means that the distributions can still apply upstream patches cleanly and only update the version checksum if these break ABI.

With the _v2, _v3 suffixes (or similar solutions) we'd be effectively storing the ABI versions directly in the code and that would cause conflicts when pulling further patches from upstream.

My view is that it would be than easier to maintain out-of-tree modversions (or similar tool) rather than to solve all these conflicts.

Warm Regards,