[PATCH v2 3/3] tpm: allocate tpm event log if missing

[Ilias Apalodimas]

On Mon, 15 Jun 2026 at 16:09, Ilias Apalodimas
<ilias.apalodimas at linaro.org> wrote:
>
> Hi Ludwig,
>
> [...]
>
> > >>>
> > >>> I'd like the eventlog to be initialized before calling this (look below).
> > >>
> > >> What if I removed the log parameter from those tcg2_* functions instead?
> > >> The event log belongs to the tpm device IMO, so should be initialized
> > >> with the device.
> > >
> > > Measured boot EventLogs etc are primarily described by the TCG specs &
> > > EFI and we only have a specification for it there. The DT describes on
> > > how to pass it over to linux with sml-base etc.
> > > However, in the EFI case the Eventlog is allocated as ACPI memory. So
> > > you cannot allocate the EventLog a priori, until you figure out what
> > > method you are using to boot. We also have use cases people using it
> > > only for RNG, due to limited memory, so we can't just allocate it on
> > > the device bringup. And lastly... Some first stage bootloaders do not
> > > initialize the TPM & drivers. What they do is prepare an EventLog and
> > > expect U-Boot to replay it once the TPM is up and running. In that
> > > case we obviously need to preserve what we have instead of starting
> > > from scratch.
> >
> > Ack, we are on the same page here.
> >
> > > My main concern is that I *don't* want the EFI and non-EFI code to
> > > deviate. I have some patches that I am playing around for quite some
> > > time and rebased them last week for this reason [1]. If you think
> > > those help your use case or make your life easier feel free to include
> > > them.
> >
> > Good idea and orthogonal to what I am doing I think. This duplication in
> > EFI stressed my brain too but I can't care about EFI right now :-)
>
> It is, but the deviation in EFI is pretty minimal atm.
> - EFI calls tcg2_log_prepare_buffer instead of tcg2_measurement_init()
> -- That's what my patches fix
> - EFI has it's own version of tcg2_measure_event() -- That's easy to
> fix as well,.
>
> >
> > IIUC you are centering the change around calling
> > tcg2_log_prepare_buffer() in efi_init_event_log(). What do you think
> > about aiming for tcg2_measurement_init() as central starting point
> > instead?
> > That would allow to remove even more duplicated code.
> > It should be sufficient to have tcg2_measurement_init() accept an
> > optional pointer to pre-allocated log buffer memory. Attached patch
> > maybe explains the idea better (just as idea, won't compile).
>
> I agree. In fact I mistyped the functions names on my previous
> email.... I really meant tcg2_measurement_init(). The patchset I
> pointed you to already converts tcg2_log_prepare_buffer() to static.

Replying to myself here, but.. In order to do that you do need the
unification patches. tcg2_measurement_init() will need to allocate the
appropriate buffer depending on EFI or not.

Cheers
/Ilias
>
> >
> > >> I am not sure what the intention of the current api is tbh. The event
> > >> log basically documents when and how PCR registers were extended. We
> > >> have to build that log ourselves as the TPM doesn't do it for us. The
> > >> api weirdly allows to have partial local logs even though there's only
> > >> one set of PCR registers.
> > >
> > > Where?
> >
> > The elog parameters of tcg2_log_prepare_buffer() and tcg2_measure_data()
> > are not related to the dev parameter. So it looks like the intention is
> > to be able to call it on the same tpm but with different event logs.
>
> Ah no. The reason was that the non EFI code was retrofitted way later,
> and it was eiaster to just allocate the buffer on the spot. I always
> had the idea of abstracting this even more in the future
>
> Cheers
> /Ilias
> >
> > cu
> > Ludwig
> >
> > --
> > Ludwig Nussel
> > Siemens AG
> > www.siemens.com