Re: [PATCH v1 1/2] No insert of the build timestamp into the x86 xen efi binary

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 30/10/2020 12:48, Jan Beulich wrote:
> On 30.10.2020 13:23, Marek Marczykowski-Górecki wrote:
>> On Fri, Oct 30, 2020 at 01:08:44PM +0100, Jan Beulich wrote:
>>> On 30.10.2020 13:03, Frédéric Pierret (fepitre) wrote:
>>>
>>>> --- a/xen/arch/x86/Makefile
>>>> +++ b/xen/arch/x86/Makefile
>>>> @@ -170,6 +170,7 @@ EFI_LDFLAGS += --major-image-version=$(XEN_VERSION)
>>>>  EFI_LDFLAGS += --minor-image-version=$(XEN_SUBVERSION)
>>>>  EFI_LDFLAGS += --major-os-version=2 --minor-os-version=0
>>>>  EFI_LDFLAGS += --major-subsystem-version=2 --minor-subsystem-version=0
>>>> +EFI_LDFLAGS += --no-insert-timestamp
>>> Generally I prefer binaries to carry timestamps, when they are
>>> intended to do so (i.e. when they have a respective field). So
>>> I think if no timestamp is wanted, it should be as an option
>>> (not sure about the default).
>> What about setting it to the SOURCE_DATE_EPOCH[1] variable value, if
>> present? Of course if there is an option to set explicit timestamp
>> value.
>>
>> [1] https://reproducible-builds.org/docs/source-date-epoch/
> Why not.

SOURCE_DATE_EPOCH is the right way to fix this.

It probably wants to default to something sane in the root Makefile, so
it covers tools as well.

>>> This said, I didn't think time stamps got meaningfully in the
>>> way of reproducible builds - ignoring the minor differences
>>> cause by them, especially when they sit at well known offsets
>>> in the binaries, shouldn't be a big deal.
>> It is a big deal. There is a huge difference between running sha256sum
>> (or your other favorite hash) on two build artifacts, and using a
>> specialized tool/script to compare each file separately. Note the
>> xen.efi may be buried very deep in the thing you compare, for example
>> inside deb/rpm and then ISO image (installation image), at which point
>> it's far from "they sit at well known offsets".
> If you care about checking images / blobs where binaries with time
> stamps are merely constituent parts, why not strip the time stamps
> at the time of creation of those images / blobs (or as a minor
> intermediate step, in case you want to e.g. record the hashes for
> later reference)?

Because that is a disaster to maintain.  A critical part of reproducible
builds is not needing custom comparison logic for every binary artefact.

~Andrew