linux-kernel - Re: MTD: How to get actual image size from MTD partition

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Message-ID: <CAOuPNLhti3tocN-_D7Q0QaAx5acHpb3AQyWaUKgQPNW3XWu58g@mail.gmail.com>
Date:   Tue, 20 Jul 2021 13:31:55 +0530
From:   Pintu Agarwal <pintu.ping@...il.com>
To:     Richard Weinberger <richard@....at>
Cc:     Greg KH <greg@...ah.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-mtd <linux-mtd@...ts.infradead.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Phillip Lougher <phillip@...ashfs.org.uk>,
        Sean Nyekjaer <sean@...nix.com>,
        Kernelnewbies <kernelnewbies@...nelnewbies.org>
Subject: Re: MTD: How to get actual image size from MTD partition

On Tue, 20 Jul 2021 at 12:10, Richard Weinberger <richard@....at> wrote:

> > Anyways, I will create a separate thread for dm-verity issue and keep
> > this thread still open for UBI image size issue.
> > We may use dm-verify for rootfs during booting, but still we need to
> > perform integrity check for other nand partitions and UBI volumes.
> >
> > So, instead of calculating the checksum for the entire partition, is
> > it possible to perform checksum only based on the image size ?
> > Right now, we are still exploring what are the best possible
> > mechanisms available for this.
>
> I still don't fully understand what you are trying to achieve.
> Is it about cryptographic integrity of your storage or detecting
> errors after the flashing process?
>
Yes, it is about md5 checksum verification for every partition to
check its integrity before updates.


> But let me advertise ubiblock a second time.
Sorry, I could not understand about the ubiblock request. Is it
possible to elaborate little more ?
We are already using squashfs on top of our UBI volumes (including
rootfs mounting).
This is the kernel command line we pass:
rootfstype=squashfs root=/dev/mtdblock44 ubi.mtd=40,0,30
And CONFIG_MTD_UBI_BLOCK=y is already enabled in our kernel.
Do we need to do something different for ubiblock ?

> If you place your squashfs on a UBI static volume, UBI knows the exact length and you can checksum it
> more easily.
Yes, we use squashfs on UBI volumes, but our volume type is still dynamic.
Also, you said, UBI knows the exact length, you mean the whole image length ?
How can we get this length at runtime ?
Also, how can we get the checksum of the entire UBI volume content
(ignoring the erased/empty/bad block content) ?

Or, you mean to say, the whole checksum logic is in-built inside the
UBI layer and users don't need to worry about the integrity at all ?


Thanks,
Pintu