Re: pahole and libdwarves linking/versioning issues

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Fri, Nov 13, 2020 at 3:45 AM Arnaldo Carvalho de Melo
<arnaldo.melo@gmail.com> wrote:
>
> Em Tue, Nov 10, 2020 at 04:14:16PM -0800, Andrii Nakryiko escreveu:
> > Hey Arnaldo,
>
> Hi Andrii,
>
> > We use pahole pretty extensively here at Facebook, as you might
> > imagine, given it's a required tool to compile Linux kernel with BTF.
>
> > There are a few problems we periodically run into, all of which seems
> > to be related to the fact that pahole is just a thin wrapper binary on
> > top of libdwarves shared library.
>
> > One very common problem is when we upgrade the dwarves package from
> > one minor version to another one. Typically people would do
>
> > yum upgrade dwarves
>
> > And that would succeed to update the dwarves package (say 1.13 to
> > 1.16). But when you run
>
> > pahole --version
>
> > You'll still see 1.13 (and still have 1.13 functionality, of
> > course)... So what happens is that pahole *the binary* was updated,
> > but libdwarves1 package, which is a dependency of dwarves package,
> > wasn't updated. And the actual version string all pretty much all of
> > the functionality is coming from libdwarves1. So users have to
>
> > yum upgrade libdwarves1
>
> > explicitly. Which is, of course, not an obvious and frustrating experience.
>
> > Also, whenever pahole is using some new API from libdwarves and
> > libdwarves is outdated, that results in spectacular failure during
> > dynamic linking time. Which makes sense because libdwarves doesn't
> > maintain API stability per se and is supposed to match 1:1 w/ pahole
> > version. But it all causes real logistics issues.
>
> > So I have a few questions:
> > 1. Is there anything on the RPM spec side that could be done to
> > trigger libdwarves1 update when dwarves is updated and keep them in
> > sync down to the minor version?
>
> So, there is this in the reference .spec file that is in pahole's git
> repo:
>
> [acme@five pahole]$ egrep '^(Requires|Name)' rpm/SPECS/dwarves.spec
> Name: dwarves
> Requires: %{libname}%{libver} = %{version}-%{release}
> Requires: %{libname}%{libver} = %{version}-%{release}
> [acme@five pahole]$ egrep '^(Requires|Name|%package)' rpm/SPECS/dwarves.spec
> Name: dwarves
> Requires: %{libname}%{libver} = %{version}-%{release}
> %package -n %{libname}%{libver}
> %package -n %{libname}%{libver}-devel
> Requires: %{libname}%{libver} = %{version}-%{release}

Ok, so I would have never noticed it, but Matt Mullins has an eagle
eye! This Requirement applies only to libdwarves1-devel package!

%package -n %{libname}%{libver} doesn't have this requirement!

Arnaldo, if you can roll it into 1.19 along the changelog timestamp
fix, that would be awesome!

> [acme@five pahole]$
>
> Someone else also reported this recently... Fedora has:
>
> [acme@five pahole]$ rpm -qR dwarves | grep ^libdwarves1
> libdwarves1 = 1.17-4.fc33
> [acme@five pahole]$
>
> So it includes both ${version} and %{release}, which should get it in
> lockstep.
>
> Having said that I'm not aware of anything using libdwarves and it was
> intended just to share those among the tools that comes in the
> 'dwarves' package, so I should have really made it internal, installed
> in /usr/libexec/dwarves/ :-\
>
> > 2. If not, can we add an option of statically linking libdwarves into
> > pahole and other tools? That would eliminate all the problems
> > altogether at the expense of a pretty negligible slight increase in
> > the binary sizes.
>
> Yeah, this looks like the way to proceed, i.e. we statically link it
> while keeping the .so around in case someone uses it, while adding some
> warning that that mode is deprecated and is going away.
>
> - Arnaldo