Re: [Xen-devel] [PATCH v2 4/6] libxl: allow creation of domains with a specified or random domid

[Jason Andryuk <jandryuk@xxxxxxxxx>]

On Thu, Jan 9, 2020 at 6:50 AM Paul Durrant <pdurrant@xxxxxxxxxx> wrote:
>
> This patch adds a 'domid' field to libxl_domain_create_info and then
> modifies do_domain_create() to use that value if it is valid. Any valid
> domid will be checked against the retired domid list before being passed
> to libxl__domain_make().
> If the domid value is invalid then Xen will choose the domid, as before,
> unless the value is the new special RANDOM_DOMID value added to the API.
> This value instructs libxl__domain_make() to select a random domid value,
> check it for validity, verify it does not match a retired domain, and then
> pass it to Xen's XEN_DOMCTL_createdomain operation. If Xen determines that
> it co-incides with an existing domain, a new random value will be
> selected and the operation will be re-tried.
>
> NOTE: libxl__logv() is also modified to only log valid domid values in
>       messages rather than any domid, valid or otherwise, that is not
>       INVALID_DOMID.
>
> Signed-off-by: Paul Durrant <pdurrant@xxxxxxxxxx>
> ---
> Cc: Ian Jackson <ian.jackson@xxxxxxxxxxxxx>
> Cc: Wei Liu <wl@xxxxxxx>
> Cc: Anthony PERARD <anthony.perard@xxxxxxxxxx>
>
> v2:
>  - Re-worked to use a value from libxl_domain_create_info
> ---
>  tools/libxl/libxl.h          |  9 +++++++++
>  tools/libxl/libxl_create.c   | 32 +++++++++++++++++++++++++++++++-
>  tools/libxl/libxl_internal.c |  2 +-
>  tools/libxl/libxl_types.idl  |  1 +
>  4 files changed, 42 insertions(+), 2 deletions(-)
>

<snip>

> diff --git a/tools/libxl/libxl_create.c b/tools/libxl/libxl_create.c
> index 1835a5502c..ee76dee364 100644
> --- a/tools/libxl/libxl_create.c
> +++ b/tools/libxl/libxl_create.c
> @@ -600,9 +600,39 @@ int libxl__domain_make(libxl__gc *gc, 
> libxl_domain_config *d_config,
>              goto out;
>          }
>
> -        ret = xc_domain_create(ctx->xch, domid, &create);
> +        if (libxl_domid_valid_guest(info->domid)) {
> +            *domid = info->domid;
> +
> +            if (libxl__is_retired_domid(gc, *domid)) {
> +                LOGED(ERROR, *domid, "domain id is retired");
> +                rc = ERROR_FAIL;
> +                goto out;
> +            }
> +        } else if (info->domid == RANDOM_DOMID) {
> +            *domid = 0; /* Zero-out initial value */
> +        }
> +
> +        for (;;) {
> +            if (info->domid == RANDOM_DOMID) {
> +                /* Randomize lower order bytes */
> +                ret = libxl__random_bytes(gc, (void *)domid,
> +                                          sizeof(uint16_t));

Casting to void * assumes little endian.  Using a temporary uint16_t
would avoid that assumption.  Also, masking down to 0x7fff would clear
the top bit which is never valid.

Regards,
Jason

> +                if (ret < 0)
> +                    break;
> +
> +                if (!libxl_domid_valid_guest(*domid) ||
> +                    libxl__is_retired_domid(gc, *domid))
> +                    continue;
> +            }
> +
> +            ret = xc_domain_create(ctx->xch, domid, &create);
> +            if (ret == 0 || errno != EEXIST || info->domid != RANDOM_DOMID)
> +                break;
> +        }
> +
>          if (ret < 0) {
>              LOGED(ERROR, *domid, "domain creation fail");
> +            *domid = INVALID_DOMID;
>              rc = ERROR_FAIL;
>              goto out;
>          }

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