Re: [RFC] Documentation: kbuild: Add description of git for reproducible builds

[Nicolas Schier]

On Thu, Oct 20, 2022 at 06:48:20PM -0700, Dan Li wrote:
> 
> 
> On 10/20/22 07:05, Bagas Sanjaya wrote:
> > On Thu, Oct 20, 2022 at 03:38:23AM -0700, Dan Li wrote:
> > > diff --git a/Documentation/kbuild/reproducible-builds.rst b/Documentation/kbuild/reproducible-builds.rst
> > > index 071f0151a7a4..13397f38c358 100644
> > > --- a/Documentation/kbuild/reproducible-builds.rst
> > > +++ b/Documentation/kbuild/reproducible-builds.rst
> > > @@ -119,6 +119,16 @@ To avoid this, you can make the vDSO different for different
> > >   kernel versions by including an arbitrary string of "salt" in it.
> > >   This is specified by the Kconfig symbol ``CONFIG_BUILD_SALT``.
> > > +Git
> > > +-----------------------
> > > +
> > > +Uncommitted changes or different commit ids in git can also lead
> > > +to different compilation results. For example, after executing
> > > +``git reset HEAD^``, even if the code is the same, the
> > > +``include/config/kernel.release`` generated during compilation
> > > +will be different, which will eventually lead to binary differences.
> > > +See ``scripts/setlocalversion`` for details.
> > > +
> > 
> > Briefly read the script, I don't see what the correlation between git
> > reset with LOCALVERSION thing is. Also, does the exact state of git
> > repository required for reproducible builds?
> > 
> 
> Hi Bagas,
> 
> The Makefile has the following code:
> filechk_kernel.release = \
>         echo "$(KERNELVERSION)$$($(CONFIG_SHELL) $(srctree)/scripts/setlocalversion $(srctree))"
> 
> The output of setlocalversion affects kernel.release, while the output
> of setlocalversion is related to the state of git when the git repository
> exists (see function scm_version).
> 
> So changes in git state will result in changes to kernel.release, and
> this information will be included in the final output vmlinux/modules
> and in turn affect reproducible builds.
> 
> For example:
> $ git log
> commit 4cd155a93eec......
> $ make ...
> $ cat include/config/kernel.release
> 6.0.0-rc4-00025-g4cd155a93eec
> 
> $ git reset HEAD^
> $ git log
> commit 7b4d266b0c41......
> $ make ...
> $ cat include/config/kernel.release
> 6.0.0-rc4-00024-g7b4d266b0c41-dirty
> 
> 
> AFAICT, in the presence of a git repository, we can compile a reproducible
> build kernel in any git state, but we need to ensure that the git state is
> always the same between compilations (or the same from the perspective of
> the scm_version function).

yes, that definitely true.  Absence or presence of git tags can change
the output of setlocalversion even more drastically.

I think it is sensible to add a stanza about git in
Documentation/kbuild/reproducible-builds.rst.


Kind regards,
Nicolas