block: Support detached LUKS header creation using qemu-img

Even though a LUKS header might be created with cryptsetup,
qemu-img should be enhanced to accommodate it as well.

Add the 'detached-header' option to specify the creation of
a detached LUKS header. This is how it is used:
$ qemu-img create --object secret,id=sec0,data=abc123 -f luks
> -o cipher-alg=aes-256,cipher-mode=xts -o key-secret=sec0
> -o detached-header=true header.luks

Using qemu-img or cryptsetup tools to query information of
an LUKS header image as follows:

Assume a detached LUKS header image has been created by:
$ dd if=/dev/zero of=test-header.img bs=1M count=32
$ dd if=/dev/zero of=test-payload.img bs=1M count=1000
$ cryptsetup luksFormat --header test-header.img test-payload.img
> --force-password --type luks1

Header image information could be queried using cryptsetup:
$ cryptsetup luksDump test-header.img

or qemu-img:
$ qemu-img info 'json:{"driver":"luks","file":{"filename":
> "test-payload.img"},"header":{"filename":"test-header.img"}}'

When using qemu-img, keep in mind that the entire disk
information specified by the JSON-format string above must be
supplied on the commandline; if not, an overlay check will reveal
a problem with the LUKS volume check logic.

Signed-off-by: Hyman Huang <yong.huang@smartx.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
[changed to pass 'cflags' to block_crypto_co_create_generic]
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>

1 files changed, 4 insertions, 1 deletions

diff --git a/block.c b/block.c
index 30afdcbba6..1ed9214f66 100644
--- a/block.c
+++ b/block.c
@@ -7357,7 +7357,10 @@ void bdrv_img_create(const char *filename, const char *fmt,
         goto out;
     }
 
-    if (size == -1) {
+    /* Parameter 'size' is not needed for detached LUKS header */
+    if (size == -1 &&
+        !(!strcmp(fmt, "luks") &&
+          qemu_opt_get_bool(opts, "detached-header", false))) {
         error_setg(errp, "Image creation needs a size parameter");
         goto out;
     }