RE: [PATCH] sbuild: Fix low hit ratio for ccache on linux kernel rebuild

["Moessbauer, Felix" <felix.moessbauer@siemens.com>]

> -----Original Message-----
> From: isar-users@googlegroups.com <isar-users@googlegroups.com> On
> Behalf Of Baurzhan Ismagulov
> Sent: Friday, July 15, 2022 3:50 PM
> To: isar-users@googlegroups.com
> Subject: Re: [PATCH] sbuild: Fix low hit ratio for ccache on linux kernel rebuild
> 
> On Fri, Jul 15, 2022 at 04:44:29PM +0300, Uladzimir Bely wrote:
> > It can be moved to the kernel recipe, but you left me a bit confused.
> > Why changing of build directory from some random (from build to build)
> > path to some fixed one (the same hash on every rebuild) makes the
> > build less reproducible?
> 
> Debian wants to have identical binary package contents from different builds,
> see
> https://eur01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fwiki.debi
> an.org%2FReproducibleBuilds&amp;data=05%7C01%7Cfelix.moessbauer%40sie
> mens.com%7C21efa7957915433beaaf08da6668f548%7C38ae3bcd95794fd4add
> ab42e1495d55a%7C1%7C0%7C637934898219710974%7CUnknown%7CTWFpb
> GZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6M
> n0%3D%7C3000%7C%7C%7C&amp;sdata=1AgsTG9AeVSOlofWSjrURw3%2BsNv
> zI1NcGcJcVZLClwE%3D&amp;reserved=0. I don't know by heart whether / how
> that works for debug info, though. Seems that at least absolute paths should be
> a no-go.

Exactly. We do not want to have any absolute paths in the binary.
By that, the build-dir shall not be relevant (neither for caching, nor for reproducibility).

Regarding relocatable debug info: the debhelpers normally handle that for you.
You can play around by running dpkg-buildflags with various settings (via DEB_BUILD_MAINT_OPTIONS, e.g. DEB_BUILD_MAINT_OPTIONS= reproducible=-timeless).

On bullseye, the -ffile-prefix-map=/tmp/foo=. -fdebug-prefix-map=/tmp/foo=. (building in /tmp/foo) flags are passed to CFLAGS, which strip the specified prefix from the data in the debug info.
However, this does not work for the kernel build, as the CFLAGS are ignored but only KCFLAGS are considered.
By that, we have to manually pass some flags to KCFLAGS.
The official Debian kernel build recipe also does that, but for our custom kernels this might be "too much".

Maybe we could use the dpkg-buildflags to generate some reasonable flags and also enable the reproducibility ones:
export DEB_BUILD_MAINT_OPTIONS="hardening=-all reproducible"
Then, we can pass the generated flags to KCFLAGS.

Felix

> 
> With kind regards,
> Baurzhan
> 
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