[systemd-devel] Storing package metadata in ELF objects

Fri May 14 13:41:32 UTC 2021

On Fri, 2021-05-14 at 12:41 +0200, Guillem Jover wrote:
> On Sat, 2021-04-10 at 13:38:31 +0100, Luca Boccassi wrote:
> > On Sat, 2021-04-10 at 13:29 +0100, Luca Boccassi wrote:
> > > After an initial discussion [0], recently we have been working on a new
> > > specification [0] to encode rich package-level metadata inside ELF
> > > objects, so that it can be included automatically in generated coredump
> > > files. The prototype to parse this in systemd-coredump and store the
> > > information in systemd-journal is ready for testing and merged
> > > upstream. We are now seeking further comments/opinions/suggestions, as
> > > we have a few months before the next release and thus there's plenty of
> > > time to make incompatible changes to the format and implementation, if
> > > required.
> 
> I've skimmed over the discussion at [0], and while having this data
> seems like it might be "nice", I've to agree with the comments there
> voicing that there does not really seem to be an actual need and the
> overhead and additional work do not seem worth it, TBH, at least
> in the Debian context.

Hi Guillem, thanks for having a look, much appreciated!

Just to clarify, the need is there - this is not an experimental
exercise, but it is borne out of an actual need&requirement, and it is
undergoing testing right now before deployment in a large scale
production infrastructure.
Not _everybody_ will need it, and not everywhere - that's absolutely
fair, and discussions on whether the ovearhead is worth it for
something that is not universally needed, but only in certain use
cases, is perfectly reasonable and welcome. I know Zbigniew is going to
try and get some raw numbers on the kind of overhead we are talking
about, that will hopefully help frame the discussion with more
precision.

> > > The Fedora Wiki and the systemd.io document have more details, but to
> > > make a long story short, a new .notes.package section with a JSON
> > > payload will be included in ELF objects, encoding various package-
> > > build-time information like distro name&version, package name&version,
> > > etc.
> > > 
> > > To summarize from the discussion, the main reasons why we believe this
> > > is useful are as following:
> > > 
> > > 1) minimal containers: the rpm database is not installed in the
> > > containers. The information about build-ids needs to be stored
> > > externally, so package name information is not available immediately,
> > > but only after offline processing. The new note doesn't depend on the
> > > rpm db in any way.
> 
> In the Debian context, the build-ids data is going to be available
> in the affected executables, and in debug symbols packages and the
> Packages metaindices listing them, so there's no need for access to
> any local dpkg database. Given that someone needing to install debug
> packages will need access to those indices (either with outgoing network
> access or with a repository mirror), these can be queried at that time.
> Not to mention that AFAIR the Debian debug symbol servers make it
> possible to query for specific build-ids.

This is not strictly related to debug packages, though? In fact, on
systems where this could be of most use you explicitly do _not_ install
debug packages (or anything at all). Or even if you wanted to, you
could not - corefiles are not handled inside the container, but
outside. Even if you wanted to and were allowed to (which for many
environments it's not the case), you can't install a Debian debug
package on a CoreOS host or Mariner host or a Flatcar host.

> > > 2) handling of a core from a container, where the container and host
> > > have different distros
> 
> How each distribution handles debug packages and debug symbols is
> going to be different, so it seems there will be a need for explicit
> handling of these, at which point the above mentioned querying can be
> implemented as well, w/o the need to encode the packaging data inside
> the executable.

Again, matching to debug symbols is not the main goal here, build-id
works for that. The main goal is to have useful metadata immediately
available in all occasions, regardless of where the core was generated
on the host, without reaching out to external services, so that it is
directly included and collated in the system journal when the core file
is handled.

With a common metadata definition, there's no need to query or
explicitly handle anything - this already works if you use systemd-
coredump built from the main branch, and handle a core file from
different containers running different distros with binaries having
this metadata in the ELF file, and it just works. This is tested, not
theoretical.

> > > 3) self-built and external packages: unless a lot of care is taken to
> > > keep access to the debuginfo packages, this information may be lost.
> > > The new note is available even if the repository metadata gets lost.
> > > Users can easily provide equivalent information in a format that makes
> > > sense in their own environment. It should work even when rpms and debs
> > > and other formats are mixed, e.g. during container image creation.
> 
> I'm not sure I see the problem here. Either these self-built 3rd-party
> packages are kept in repos that also provide the debug symbols
> somewhere for all historically released versions or these will not be
> accessible anyway. If they are, they can as well be located as per
> above from the Packages metaindices, and even if the repository
> metadata gets lost, as long as the debug symbol packages are present
> (if they are not what's the point anyway) the build-ids can always be
> re-scanned from them as they are part of the Build-Ids field in the
> .deb control file.

I think you are thinking about being confined to single distro here - I
am talking about mixed environments. Maybe the build-id can be used to
trace it back - maybe it is available on some federated server. Most
likely it is not, in these cases - it would require customers to go and
upload their sources and symbols and information to random third party
services, a bit unlikely. You have no reference that indicates where to
go look for. Just a filename, which can be anything, and a random id
which might never be published anywhere. What do you do with it?

> > > Other than in Fedora, we are already making the required code changes
> > > at Microsoft to use the same format&specification for internally-built
> > > binaries, and for tools that parse core files and logs.
> > > 
> > > Tools for RPM and DEB (debhelper) integration are also available [3].
> 
> So, to conclude, I don't really see the point of this in the Debian
> context. (Not to mention the problems with encoding binary versions
> that might be wrong, and the busy work involved.)

Sorry, what do you mean here by encoding binary versions?

> > > [0] https://github.com/systemd/systemd/issues/18433
> > > [1] https://systemd.io/COREDUMP_PACKAGE_METADATA/
> > > [2] https://fedoraproject.org/wiki/Changes/Package_information_on_ELF_objects
> > > [3] https://github.com/systemd/package-notes
> 
> Thanks,
> Guillem

-- 
Kind regards,
Luca Boccassi
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