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

[Nicholas Piggin <npiggin@gmail.com>]

On Thu, 24 Nov 2016 10:56:22 +0100
Greg Kroah-Hartman <gregkh@linuxfoundation.org> wrote:

> On Thu, Nov 24, 2016 at 06:53:22PM +1100, Nicholas Piggin wrote:
> > On Thu, 24 Nov 2016 08:36:39 +0100
> > Greg Kroah-Hartman <gregkh@linuxfoundation.org> wrote:
> >   
> > > On Thu, Nov 24, 2016 at 06:20:26PM +1100, Nicholas Piggin wrote:  
> > > > But still, modversions is pretty complicated for what it gives us. It sends
> > > > preprocessed C into a C parser that makes CRCs using type definitions of
> > > > exported symbols, then turns those CRCs into a linker script which which is
> > > > used to link the .o file with. What we get in return is a quite limited and
> > > > symbol "versioning" system.
> > > > 
> > > > What if we ripped all that out and just attached an explicit version to
> > > > each export, and incompatible changes require an increment?    
> > > 
> > > How would that work for structures?  Would that be required for every
> > > EXPORT_SYMBOL* somehow?  
> > 
> > Yeah just have EXPORT_SYMBOL take another parameter which attaches a version
> > number and use that as the value for the __crc_ symbol versions rather than
> > a calculated CRC.
> > 
> > Yes it would require some level of care from developers and may be a small
> > annoyance when changing exports. But making people think a tiny bit more
> > before chnaging exported ABI shouldn't be the end of the world.  
> 
> That wouldn't work at all for structures that change, as we never
> explicitly "mark" them for export anywhere.

Well, the module arrives at the objects one way or another via an exported
symbol. Although it can be by following a lot of pointers so yes it's
probably near impossible to do well.

>  You need a tool that looks
> at either the source code (what we have today), or looks at the

What we have today only looks at the type of the exported function or
variable I think (or does it? I didn't look that far into the parser).
Does not follow down all possible derivable pointer types.

> object/debugging code (like the link I pointed at.)
> 
> > > > Google tells me
> > > > Linus is not a neutral bystander on the topic of symbol versioning, so I'm
> > > > bracing for a robust response :) (actually I don't much care either way, I'm
> > > > happy to put a couple of bandaids on it and keep it going)    
> > > 
> > > There are tools that people are working on to make it more obvious where
> > > API breaks happen by looking at the .o debug data instead of our crazy
> > > current system (which is really better than nothing), perhaps we should
> > > start using them instead?
> > > 
> > > See here for more details about this:
> > > 	https://kernel-recipes.org/en/2016/talks/would-an-abi-changes-visualization-tool-be-useful-to-linux-kernel-maintenance/  
> > 
> > Hmm. I guess it's basically similar to modversions, so has downsides of not
> > detecting a semantic change unless it changes the type. But still, if we could
> > replace our custom code with a tool like this for modversions functionality,
> > that alone would be a massive improvement. But requiring debug info might be
> > a bit of a show stopper. I also don't know if that would handle asm functions.  
> 
> I think we can live without asm functions changing their arguments as
> that is usually very rare.  And maybe debugging info being a requirement
> for those that want modversions (i.e. the distros), is ok as they
> already generate that as part of their build.

Maybe. I'd like to know how people really care about it. Linus post from

http://yarchive.net/comp/linux/modversions.html

Seem to be that he just likes it to prevent module loading if the git version
is not available. Fair usage, but could we do better with less effort? Maybe
ship with a source version that can do the same job. If you take care of that
case, then what is left?

> 
> But more importantly, that's a much longer-term solution, fixing what we
> have today to at least start working again is much more important before
> we start bikeshedding the whole mess :)

Oh yeah, we're fixing it. I just thought I'd bring it up since I have a few
important ears :)

Thanks,
Nick