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Re: Updating dpkg-buildflags to enable reproducible=+fixfilepath by default

On 2021-01-09, Lisandro Damián Nicanor Pérez Meyer wrote:
> Oh, I have sadly forgotten to mention another thing.
> On Sat, 9 Jan 2021 at 15:53, Lisandro Damián Nicanor Pérez Meyer
> <perezmeyer@gmail.com> wrote:
>> # __FILE__ is a public, well defined API
> According to:
> Adrian Bunks mentions it in
> https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=876901#10
> Holger Levsen in https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=876901#42
> Mattia Rizzolo on https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=876901#192
> The ability of gcc to change __FILE__ was a patch that, at the time of
> those writings, wasn't yet accepted.

That is no longer the case. The fixfilepath feature enabled in dpkg only
uses features (e.g. -ffile-prefix-map) available in both upstream GCC
(>=9? or 8? ~2018) and Clang (>= 10), possibly other compilers as
well. There are no special patches to toolchains needed to enable this

> Even more, Ximion Luo wrote on
> https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=876901#212
>   The GCC patch (neither the previous nor the planned version) does
>   not change the default behaviour of __FILE__, and was never intended
>   to. Instead, it gives users the ability to rewrite __FILE__, more
>   specifically a prefix of it.
> makes it clear that the default behaviour is not changed. So if the
> patch got accepted (did it get accepted?) it was only to allow
> reproducible people to break API in order to get reproducibility.

Since then an alternate implementation was implemented that the
reproducible=+fixfilepath feature in dpkg takes advantage of, in order
to implement this feature in another distribution, if I recall

It didn't address all the cases of the old GCC patches that Ximin
submitted, but the Reproducible Builds team decided in 2018 to make use
of upstream supported features only and dropped all of our patches to

The notable difference is that it not longer makes use of an environment
variable; it requires passing the -ffile-prefix-map option to the
compiler. The dpkg patch simply adds this to the default relevent *FLAGS

(For historical completeness, though somewhat an aside to the topic at
hand, the -ffile-prefix-map approach does not address all the cases of
the former patches to GCC as the compiler flags sometimes end up in
various shipped artifacts in *some* cases, though certainly not all.)

> This in itself, if something has not changed in the meantime, marks
> another reference that __FILE__ is a public, well defined API.

I think reading #876901 demonstrates that the case can be made either
way; it not as well defined as you make it out to be.

live well,

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