Re: [PATCH] qapi: Craft the BlockdevCreateOptionsLUKS comment

[Daniel P . Berrangé]

On Wed, Feb 28, 2024 at 11:17:37AM +0100, Kevin Wolf wrote:
> Am 28.02.2024 um 07:43 hat Markus Armbruster geschrieben:
> > Yong Huang <yong.huang@smartx.com> writes:
> > 
> > > On Wed, Feb 21, 2024 at 4:26 PM Markus Armbruster <armbru@redhat.com> 
> > > wrote:
> > >
> > >> Yong Huang <yong.huang@smartx.com> writes:
> > >>
> > >> > On Wed, Feb 21, 2024 at 2:43 PM Markus Armbruster <armbru@redhat.com>
> > >> wrote:
> > >> >
> > >> >> Hyman Huang <yong.huang@smartx.com> writes:
> > >> >>
> > >> >> > Add comment in detail for commit 433957bb7f (qapi:
> > >> >> > Make parameter 'file' optional for
> > >> >> > BlockdevCreateOptionsLUKS).
> > >> >> >
> > >> >> > Signed-off-by: Hyman Huang <yong.huang@smartx.com>
> > >> >> > ---
> > >> >> >  qapi/block-core.json | 20 +++++++++++++++++++-
> > >> >> >  1 file changed, 19 insertions(+), 1 deletion(-)
> > >> >> >
> > >> >> > diff --git a/qapi/block-core.json b/qapi/block-core.json
> > >> >> > index ab5a93a966..42b0840d43 100644
> > >> >> > --- a/qapi/block-core.json
> > >> >> > +++ b/qapi/block-core.json
> > >> >> > @@ -4973,7 +4973,25 @@
> > >> >> >  ##
> > >> >> >  # @BlockdevCreateOptionsLUKS:
> > >> >> >  #
> > >> >> > -# Driver specific image creation options for LUKS.
> > >> >> > +# Driver specific image creation options for LUKS. Note that
> > >> >> > +# @file is required if @preallocation is specified and equals
> > >> >> > +# PREALLOC_MODE_ON. The following three scenarios determine how
> > >> >> > +# creation logic behaves when @preallocation is either equal to
> > >> >> > +# PREALLOC_MODE_OFF or is not given:
> > >> >> > +#
> > >> >> > +#  1) When @file is given only, format the block device referenced
> > >> >> > +#     by @file as the LUKS specification and trunk it to the @size.
> > >> >>
> > >> >> Do you mean "truncate it to @size"?
> > >> >>
> > >> > Yes, :( sorry for the spelling mistake.
> > >>
> > >> Writing good documentation in a second language is *hard*.  All we can
> > >> reasonably expect from contributors to try their best.  And then we
> > >> improve the text together in review.  Just like we do for code :)
> > >>
> > >> >> > +#     In this case, the @size should reflect amount of space made
> > >> >> > +#     available to the guest, so the trunk size must take account
> > >> >> > +#     of that which will be used by the crypto header.
> > >> >> > +#
> > >> >> > +#  2) When @header is given only, just format the block device
> > >> >> > +#     referenced by @header as the LUKS specification.
> > >> >> > +#
> > >> >> > +#  3) When both @file and @header are given, block device
> > >> >> > +#     referenced by @file should be trunked to @size, and block
> > >> >> > +#     device referenced by @header should be formatted as the LUKS
> > >> >> > +#     specification.
> > >> >> >  #
> > >> >> >  # @file: Node to create the image format on, mandatory except when
> > >> >> >  #        'preallocation' is not requested
> > >> >>
> > >> >> Let's see whether I understand.
> > >> >>
> > >> >> blockdev-create with "driver": "luks" can work in three different 
> > >> >> ways:
> > >> >>
> > >> >> 1. Create an image with a LUKS header
> > >> >>
> > >> >> 2. Create just a detached LUKS header
> > >> >>
> > >> >> 3. Create an image and a detached LUKS header
> > >> >>
> > >> >> Correct?
> > >> >>
> > >> >
> > >> > Yes
> > >> >
> > >> >
> > >> >> @file and @header are BlockdevRef, which means they refer to existing
> > >> >> images with arbitrary driver.  Could be "file", "qcow2", or anything.
> > >> >>
> > >> >> Correct?
> > >> >>
> > >> > Yes
> > >> >
> > >> >
> > >> >>
> > >> >> To get 1., specify @file, but not @header.
> > >> >>
> > >> >> To get 2., specify @header, but not @file.
> > >> >>
> > >> >> To get 3., specify both.
> > >> >>
> > >> >> Specifying neither is an error.
> > >> >>
> > >> >> Correct?
> > >> >>
> > >> >
> > >> > Yes
> > >> >
> > >> >
> > >> >> In any case, @size is the logical size of the image (how much data it
> > >> >> can hold).
> > >> >>
> > >> >
> > >> > Yes
> > >> >
> > >> >
> > >> >>
> > >> >> With 1., the actual image size is a bit larger due to the LUKS header.
> > >> >> The @file image is resized to that size: if it's shorter, it's grown, 
> > >> >> if
> > >> >> it's longer, it's truncated.
> > >> >>
> > >> >
> > >> > Yes
> > >> >
> > >> >
> > >> >> With 2., @size is merely recorded in the detached LUKS header.
> > >> >>
> > >> >
> > >> > In LUKS1 specification, payload data size is not contained in the 
> > >> > header,
> > >> > so in this case, @size is not recorded in the detached LUKS header.
> > >> > The creation logic just does the LUKS header formatting only.
> > >>
> > >> Is @size unused then?
> > >>
> > >
> > > IIUC, yes. Creation logic will ignore the @size. See the following code
> > > in function block_crypto_co_create_luks:
> > >
> > >     if (luks_opts->header) {
> > >         /* LUKS volume with detached header */
> > >         hdr_bs = bdrv_co_open_blockdev_ref(luks_opts->header, errp);
> > >         if (hdr_bs == NULL) {
> > >             return -EIO;
> > >         }
> > >
> > >         cflags |= QCRYPTO_BLOCK_CREATE_DETACHED;
> > >
> > >         /* Format the LUKS header node, here just ignore the size
> > >           * and passed zero to block_crypto_co_create_generic */
> > >         ret = block_crypto_co_create_generic(hdr_bs, 0, &create_opts,
> > >                                              PREALLOC_MODE_OFF, cflags, 
> > > errp);
> > >         if (ret < 0) {
> > >             goto fail;
> > >         }
> > >
> > >         /* Format the LUKS payload node */
> > >         if (luks_opts->file) {
> > >             ret = block_crypto_co_format_luks_payload(luks_opts, errp);
> > >             if (ret < 0) {
> > >                 goto fail;
> > >             }
> > >         }
> > 
> > @size is a required argument, but silently ignored when @header is
> > present and @file is absent (2. Create just a detached LUKS header).
> > Feels awkward.
> > 
> > Should @size be optional, absent when and only when @header is present
> > and @file is absent?
> > 
> > Kevin or Hanna, got an opinion?
> 
> What is the use case for creating a header without a corresponding
> image?

eg you're creating a detached LUKS header that will be used in
combination with a block device, or an NBD export, or some other
storage that was pre-created in some manner, and onto which you
now want to store LUKS encrypted data.


With regards,
Daniel
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