[Libguestfs] [PATCH nbdkit] luks: Don't advertise zero support

[Laszlo Ersek]

On 06/10/22 13:47, Richard W.M. Jones wrote:
> Fix an assertion failure in nbdkit:
> 
> $ qemu-img create -f luks --object secret,data=LETMEPASS,id=sec0 -o key-secret=sec0 encrypted.img 100M
> Formatting 'encrypted.img', fmt=luks size=104857600 key-secret=sec0
> $ rm -f data.img
> $ truncate -s 100M data.img
> $ nbdkit file encrypted.img --filter=luks passphrase=LETMEPASS --run 'nbdcopy data.img $nbd' -v
> ...
> nbdkit: file.10: debug: luks: zero count=104857600 offset=0 may_trim=1 fua=0 fast=0
> nbdkit: backend.c:718: backend_zero: Assertion `c->can_zero > NBDKIT_ZERO_NONE' failed.
> 
> It's always wrong for filters to try to return NBDKIT_ZERO_EMULATE, at
> least with the current implementation of the server.  This is because
> emulation is only done at the plugin layer (see server/plugins.c:
> plugin_zero), and not at the filter layer.
> 
> We could adjust nbdkit's filter layer (server/filter.c:filter_zero) to
> do similar emulation, but an easier solution here is not to advertise
> it to the client at all.
> 
> In future we should adjust the LUKS plugin so it does whatever the
> kernel does for trimming/zeroing.
> ---
>  filters/luks/luks.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/filters/luks/luks.c b/filters/luks/luks.c
> index 8ad3f4ec..9c1a3389 100644
> --- a/filters/luks/luks.c
> +++ b/filters/luks/luks.c
> @@ -189,7 +189,7 @@ luks_can_trim (nbdkit_next *next, void *handle)
>  static int
>  luks_can_zero (nbdkit_next *next, void *handle)
>  {
> -  return NBDKIT_ZERO_EMULATE;
> +  return NBDKIT_ZERO_NONE;
>  }
>  
>  static int
> 

so this is withdrawn now, right?