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SubjectRe: [PATCH 2/3] dm: add support for passing through inline crypto support
On Mon, Sep 21 2020 at  8:32pm -0400,
Eric Biggers <ebiggers@kernel.org> wrote:

> On Wed, Sep 09, 2020 at 11:44:21PM +0000, Satya Tangirala wrote:
> > From: Eric Biggers <ebiggers@google.com>
> >
> > Update the device-mapper core to support exposing the inline crypto
> > support of the underlying device(s) through the device-mapper device.
> >
> > This works by creating a "passthrough keyslot manager" for the dm
> > device, which declares support for encryption settings which all
> > underlying devices support. When a supported setting is used, the bio
> > cloning code handles cloning the crypto context to the bios for all the
> > underlying devices. When an unsupported setting is used, the blk-crypto
> > fallback is used as usual.
> >
> > Crypto support on each underlying device is ignored unless the
> > corresponding dm target opts into exposing it. This is needed because
> > for inline crypto to semantically operate on the original bio, the data
> > must not be transformed by the dm target. Thus, targets like dm-linear
> > can expose crypto support of the underlying device, but targets like
> > dm-crypt can't. (dm-crypt could use inline crypto itself, though.)
> >
> > When a key is evicted from the dm device, it is evicted from all
> > underlying devices.
> >
> > Signed-off-by: Eric Biggers <ebiggers@google.com>
> > Co-developed-by: Satya Tangirala <satyat@google.com>
> > Signed-off-by: Satya Tangirala <satyat@google.com>
>
> Looks good as far as Satya's changes from my original patch are concerned.
>
> Can the device-mapper maintainers take a look at this?

In general it looks like these changes were implemented very carefully
and are reasonable if we _really_ want to enable passing through inline
crypto.

I do have concerns about the inability to handle changes at runtime (due
to a table reload that introduces new devices without the encryption
settings the existing devices in the table are using). But the fallback
mechanism saves it from being a complete non-starter.

Can you help me better understand the expected consumer of this code?
If you have something _real_ please be explicit. It makes justifying
supporting niche code like this more tolerable.

Thanks,
Mike

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