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

[Don Zickus]

On Sat, Dec 10, 2016 at 01:41:03PM +0100, Greg Kroah-Hartman wrote:
> On Fri, Dec 09, 2016 at 11:46:54PM +0100, Dodji Seketeli wrote:
> > Hello,
> > 
> > Nicholas Piggin <npiggin@xxxxxxxxx> a éit:
> > 
> > [...]
> > 
> > > That said, a dwarf based checker tool should be able to do as good a job
> > > (maybe a bit better because report is very informative and it may pick up
> > > compiler alignments or padding options).
> > 
> > So, Nicholas was kind enough to send me the two Linux Kernel binaries
> > that he built with the tiny little interface change that we were
> > discussing earlier.  Here is what the abidiff[1] tools says about that
> > interface change:
> > 
> >     $ time ~/git/libabigail/kabidiff/build/tools/abidiff vmlinux.abi1.abi vmlinux.abi2.abi
> >     Functions changes summary: 0 Removed, 1 Changed, 0 Added function
> >     Variables changes summary: 0 Removed, 0 Changed, 0 Added variable
> > 
> >     1 function with some indirect sub-type change:
> > 
> >       [C]'function int foo(blah*)' at memory.c:82:1 has some indirect sub-type changes:
> >         parameter 1 of type 'blah*' has sub-type changes:
> >           in pointed to type 'struct blah' at memory.c:78:1:
> >             type size changed from 32 to 64 bits
> >             1 data member insertion:
> >               'int blah::y', at offset 0 (in bits) at memory.c:79:1
> >             1 data member change:
> >              'int blah::x' offset changed from 0 to 32 (in bits) (by +32 bits)
> > 
> > 
> > 
> >     real	0m2.595s
> >     user	0m2.489s
> >     sys	0m0.108s
> >     $ 
> > 
> > I kept the timing information to give you an idea of the time it takes
> > on a non-optimized build of abidiff.
> > 
> > One could for instance want that types that are not defined in header
> > files be kept out of the change report.  In that case it's possible to
> > write a little suppression specification file like this one:
> > 
> >     $ cat vmlinux.abignore 
> >     [suppress_type]
> >       source_location_not_regexp = .*\\.h
> >     $
> > 
> > You can then pass that suppression file to the tool:
> > 
> >     $ ~/git/libabigail/kabidiff/build/tools/abidiff --suppr vmlinux.abignore vmlinux.abi1.abi vmlinux.abi2.abi
> >     Functions changes summary: 0 Removed, 0 Changed (1 filtered out), 0 Added function
> >     Variables changes summary: 0 Removed, 0 Changed, 0 Added variable
> > 
> > 
> >     real	0m2.574s
> >     user	0m2.473s
> >     sys	0m0.102s
> >     $
> > 
> > So this is the kind of interface change analysis tool we are working on
> > at the moment.
> > 
> > One could also imagine a tool that would compute a CRC that takes the
> > very same suppression specification files into account, letting people
> > to decide that some interface changes are OK.  That CRC would thus be
> > added to the special ELF sections we already have today.  We could keep
> > the modversion machinery, but with a greater dose of flexibility.
> > Whenever modversion detects a change, abidiff would tell people what the
> > change is exactly.
> > 
> > What do you guys think?
> 
> YES YES YES!!!
> 
> Now I don't work on a distro anymore, but I would think that something
> like this would be really useful, pointing out exactly what changed is
> very important for distro maintainers to determine what they want to do
> (either fix up the abi change with strange hacks, or ignore it due to
> the change being in an area they don't care at all about, i.e. a random
> driver subsystem.)

Well, genksyms does provide this today with the .symref files.  It may not
be as thorough and flexible as libabigail, but RH has been using it for
years to quickly determine what patches broke the abi and more importantly
where (which can be challenging).  I just didn't want to downplay what is
available today.

On the flip side, I do like what libabigail has to offer.  There seems to be
some interesting new ways of handling our abi and I look forward to our kabi
team putting it to use. :-)

> 
> So yes, I think this is really good stuff.  But if the distro
> maintainers correct me and think it's useless, then I need to revisit my
> view of exactly what they do for their customers :)

I also don't want folks to forget that are two parts to this equation.  The
checking above is the first part.  But the second part is what to do about
the stuff you ignored, which leads to the run time checks.

If you don't maintain 100% abi (which RH doesn't), then we need a way to
block drivers from loading that use symbols which we do not maintain and
broke.  The crc checks at load time work great for this.  Hopefully we can
continue to support what modversions is providing today (or something
similar).

I do not think vermagic will be usable at all for us.

Thanks!

Cheers,
Don