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

From: Michal Marek
Date: Tue Nov 29 2016 - 04:14:48 EST

Dne 29.11.2016 v 03:31 Nicholas Piggin napsal(a):
> On Tue, 29 Nov 2016 01:15:48 +0000
> Ben Hutchings <ben@xxxxxxxxxxxxxxx> wrote:
>> [I've had to guess at the cc list for this, because we no longer have
>> mail archives that preserve them.]
> You got it about right.
>> On Fri, 2016-11-25 at 10:01 -0800, Linus Torvalds wrote:
>>> On Thu, Nov 24, 2016 at 4:40 PM, Nicholas Piggin <npiggin@xxxxxxxxx> wrote:
>>>>> Yes, manual "marking" is never going to be a viable solution.
>>>> I guess it really depends on how exactly you want to use it. For distros
>>>> that do stable ABI but rarely may have to break something for security
>>>> reasons, it should work and give exact control.
>> This is roughly how Debian handles the kernel module ABI during a
>> stable release.
>>> No. Because nobody else will care, so unless it's like a single symbol
>>> or something, it will just be a maintenance nightmare.
>> I agree with this. We can explicitly "version" individual symbols
>> anyway by doing something like:
>> -int foo(void);
>> +#define foo foo_2
>> +int foo_2(int);
> Yeah... Benefit being it's very simple and everybody can see exactly
> what it does and knows how it will work.
>>>> What else do people *actually* use it for? Preventing mismatched modules
>>>> when .git version is not attached and release version of the kernel has
>>>> not been bumped. Is that it?
>>> It used to be very useful for avoiding loading stale modules and then
>>> wasting days on debugging something that wasn't the case when you had
>>> forgotten to do "make modules_install". Change some subtle internal
>>> ABI issue (add/remove a parameter, whatever) and it would really help.
>>> These days, for me, LOCALVERSION_AUTO and module signing are what I
>>> personally tend to use.
>>> The modversions stuff may just be too painful to bother with. Very few
>>> people probably use it, and the ones that do likely don't have any
>>> overriding reason why.
>> [...]
>> Debian has some strong reasons:

I guess many distros have similar reasons.

>> 1. Changing the release string requires any out-of-tree modules to be
>> upgraded (at least rebuilt) on end-user systems. So we try to avoid
>> doing that during the lifetime of a stable release, i.e. we don't let
>> the release string change. Also, the release string is reflected in
>> package names (e.g. linux-image-4.8.0-1-amd64), and introducing new
>> package names requires manual approval by the Debian archive team.
> This is something I've noticed. Would it be better if the module loader
> ignores the kernel version and instead used some internal ABI version
> string to check against? Otherwise (AFAICT) you always have 4.8.0 versions
> despite being 4.8.7 kernel, and you can't upgrade a point release without
> overwriting your old kernel and modules.

The thing is - to maintain an ABI version string, you need some level of
certainty that two given ABIs are really interchangeable. Which means
you need to check whether the symbols _and_ types exposed are unchanged.
Which is a thing that genksyms, the tool behind CONFIG_MODVERSIONS, does
quite well. So yes, you could do a testbuild with CONFIG_MODVERSIONS=y
and a production build with some global ABI string, but what's the point

> It would be nice to get upstream to the point where 4.9 modversions
> works if you just patch out depends BROKEN. That would require reverting
> a few more of Al's arch patches.
> Then in 4.10 we can re-add all those arch patches (which are less
> controversial without the asm-prototypes.h workaround), and implement a
> simple stable ABI version string check, and then in 4.11 we can remove
> modversions.

I'd rather change the kconfig to

depends on BROKEN || <archs that have asm/asm-prototypes.h>

and eventuallly remove the dependency again. PPC has the header already,
so it can be added right away. I do not know why the x86 patch has not
been merged yet.