On Wed, Feb 21, 2024 at 4:26 PM Markus Armbruster <arm...@redhat.com> wrote:
> Yong Huang <yong.hu...@smartx.com> writes:
>
> > On Wed, Feb 21, 2024 at 2:43 PM Markus Armbruster <arm...@redhat.com>
> wrote:
> >
> >> Hyman Huang <yong.hu...@smartx.com> writes:
> >>
> >> > Add comment in detail for commit 433957bb7f (qapi:
> >> > Make parameter 'file' optional for
> >> > BlockdevCreateOptionsLUKS).
> >> >
> >> > Signed-off-by: Hyman Huang <yong.hu...@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;
}
}
>
> >> With 3., @size is recorded in the detached LUKS header, and the @file
> >> image is resized as with 1.
> >>
> >
> > Same reason as above, @size is not recorded and @file image is
> > resized to @size exactly.
> >
> >
> >>
> >> Correct?
> >>
> >>
> > Thanks for the comment,
> >
> > Yong
>
>
--
Best regards
