linux-kernel - Re: [PATCH v2 2/4] firmware: qcom: uefisecapp: add support for R/O UEFI vars

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Message-ID: <aFlps9iUcD42vN4w@hovoldconsulting.com>
Date: Mon, 23 Jun 2025 16:50:27 +0200
From: Johan Hovold <johan@...nel.org>
To: Dmitry Baryshkov <dmitry.baryshkov@....qualcomm.com>
Cc: Bjorn Andersson <andersson@...nel.org>,
	Maximilian Luz <luzmaximilian@...il.com>,
	Konrad Dybcio <konradybcio@...nel.org>,
	Rob Herring <robh@...nel.org>,
	Krzysztof Kozlowski <krzk+dt@...nel.org>,
	Conor Dooley <conor+dt@...nel.org>, linux-arm-msm@...r.kernel.org,
	linux-kernel@...r.kernel.org, devicetree@...r.kernel.org
Subject: Re: [PATCH v2 2/4] firmware: qcom: uefisecapp: add support for R/O
 UEFI vars

On Mon, Jun 23, 2025 at 04:45:30PM +0200, Johan Hovold wrote:
> On Sat, Jun 21, 2025 at 10:56:11PM +0300, Dmitry Baryshkov wrote:
> > For some platforms (e.g. Lenovo Yoga C630) we don't yet know a way to
> > update variables in the permanent storage. However being able to read
> > the vars is still useful as it allows us to get e.g. RTC offset.
> > 
> > Add a quirk for QSEECOM specifying that UEFI variables for this platform
> > should be registered in read-only mode.
> > 
> > Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@....qualcomm.com>
> > ---
> >  drivers/firmware/qcom/qcom_qseecom_uefisecapp.c | 18 +++++++++++++++++-
> >  include/linux/firmware/qcom/qcom_qseecom.h      |  2 ++
> >  2 files changed, 19 insertions(+), 1 deletion(-)
> > 
> > diff --git a/drivers/firmware/qcom/qcom_qseecom_uefisecapp.c b/drivers/firmware/qcom/qcom_qseecom_uefisecapp.c
> > index 98a463e9774bf04f2deb0f7fa1318bd0d2edfa49..05f700dcb8cf3189f640237ff0e045564abb8264 100644
> > --- a/drivers/firmware/qcom/qcom_qseecom_uefisecapp.c
> > +++ b/drivers/firmware/qcom/qcom_qseecom_uefisecapp.c
> > @@ -792,6 +792,12 @@ static efi_status_t qcuefi_query_variable_info(u32 attr, u64 *storage_space, u64
> >  	return status;
> >  }
> >  
> > +static const struct efivar_operations qcom_efivars_ro_ops = {
> > +	.get_variable = qcuefi_get_variable,
> > +	.get_next_variable = qcuefi_get_next_variable,
> > +	.query_variable_info = qcuefi_query_variable_info,
> > +};
> 
> It looks like the efivars implementation does not support read-only
> efivars and this will lead to NULL pointer dereferences whenever you try
> to write a variable.

Ok, efivarfs seems to support it, but you'd crash when setting a
variable from the kernel (e.g. from the RTC driver).

> Also not sure how useful it is to only be able to read variables,
> including for the RTC where you'll end up with an RTC that's always
> slightly off due to drift (even if you can set it when booting into
> Windows or possibly from the UEFI setup).
> 
> Don't you have any SDAM blocks in the PMICs that you can use instead for
> a proper functioning RTC on these machines?

Johan