linux-kernel - Re: [PATCH v3 1/3] rpmb: add Replay Protected Memory Block (RPMB) subsystem

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Date: Tue, 5 Mar 2024 13:29:38 +0100
From: Linus Walleij <linus.walleij@...aro.org>
To: Jens Wiklander <jens.wiklander@...aro.org>
Cc: linux-kernel@...r.kernel.org, linux-mmc@...r.kernel.org, 
	op-tee@...ts.trustedfirmware.org, 
	Shyam Saini <shyamsaini@...ux.microsoft.com>, Ulf Hansson <ulf.hansson@...aro.org>, 
	Jerome Forissier <jerome.forissier@...aro.org>, Sumit Garg <sumit.garg@...aro.org>, 
	Ilias Apalodimas <ilias.apalodimas@...aro.org>, Bart Van Assche <bvanassche@....org>, 
	Randy Dunlap <rdunlap@...radead.org>, Ard Biesheuvel <ardb@...nel.org>, Arnd Bergmann <arnd@...db.de>, 
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>, Tomas Winkler <tomas.winkler@...el.com>, 
	Alex Bennée <alex.bennee@...aro.org>
Subject: Re: [PATCH v3 1/3] rpmb: add Replay Protected Memory Block (RPMB) subsystem

Hi Jens,

I realized there is one thing I wonder about:

On Tue, Feb 27, 2024 at 4:31 PM Jens Wiklander
<jens.wiklander@...aro.org> wrote:

> +struct rpmb_frame {
> +       u8     stuff[196];
> +       u8     key_mac[32];
> +       u8     data[256];
> +       u8     nonce[16];
> +       __be32 write_counter;
> +       __be16 addr;
> +       __be16 block_count;
> +       __be16 result;
> +       __be16 req_resp;
> +} __packed;

I didn't quite get why these things are encoded big-endian?

As on the producer side (the eMMC backend) it seems we are anyway
calling cpu_to_be* to convert them into this format.

If this is a requirement on the consumer side (such as TEE) I think
the consumer should swap the bytes rather than the producer,
but I guess that kind of assumes that we foresee there will be other
consumers in the first place.

Yours,
Linus Walleij