Re: -ffile-prefix-map option and reproducibility
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). With
DEB_BUILD_MAINT_OPTIONS=reproducible=-fixfilepath, dpkg-buildflags falls
back to "-fdebug-prefix-map=<BUILDDIR>=.", and you have the same
problem. If you disable that as well via
DEB_BUILD_MAINT_OPTIONS=reproducible=-fixfilepath,-fixdebugpath, the
-dbgsym packages will most likely end up unreproducible.
Cheers,
Sven
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