Re: -ffile-prefix-map option and reproducibility

[Emilio Pozuelo Monfort <pochu@debian.org>]

On 07/02/2023 20:00, Sven Joachim wrote:
> On 2023-02-07 17:50 +0100, Guillem Jover wrote:
>
> > On Tue, 2023-02-07 at 16:41:47 +0100, Stéphane Glondu wrote:
> > > When building packages, a -ffile-prefix-map option is automatically injected
> > > into CFLAGS. Where does it come from? Since when?
> >
> > This is coming from dpkg-buildflags (in this case probably indirectly
> > via debhelper). AFAICS it was added in dpkg 1.19.1 disabled by default,
> > and then switched to enabled by default in dpkg 1.20.6 (see #974087).
> >
> > > I suspect this was added to improve reproducibility. Ironically, it makes
> > > packages that capture this variable non reproducible, since the build path
> > > seems to be randomized (has it always been the case? since when?). It is the
> > > case of OCaml (see #1030785), and seemingly of R as well (found by grepping
> > > in my /etc). I wouldn't be surprised other packages are affected as well.
> >
> > AFAIR this was considered at the time, yes. If the flag is effectively
> > not fixing anything for the set of packages involved, and is in fact
> > actually making them unreproducible when they would not then, you can
> > disable the fixfilepath feature in the reproducible build flags area,
> > via DEB_BUILD_MAINT_OPTIONS.
>
> This does not help for packages which capture all build flags and store
> them in some file in the package (as is the case here).

What is the purpose of having the build flags in a file in the .deb?

Cheers,
Emilio