Re: [Xen-devel] [PATCH v4 10/12] livepatch: Handle arbitrary size names with the list operation

[Jan Beulich <jbeulich@xxxxxxxx>]

On 28.09.2019 17:13, Pawel Wieczorkiewicz wrote:
> --- a/xen/include/public/sysctl.h
> +++ b/xen/include/public/sysctl.h
> @@ -925,10 +925,11 @@ struct xen_sysctl_livepatch_get {
>   *
>   * If the hypercall returns an positive number, it is the number (up to `nr`)
>   * of the payloads returned, along with `nr` updated with the number of 
> remaining
> - * payloads, `version` updated (it may be the same across hypercalls. If it
> - * varies the data is stale and further calls could fail). The `status`,
> - * `name`, and `len`' are updated at their designed index value (`idx`) with
> - * the returned value of data.
> + * payloads, `version` updated (it may be the same across hypercalls. If it 
> varies
> + * the data is stale and further calls could fail) and the name_total_size
> + * containing total size of transferred data for the array.
> + * The `status`, `name`, `len` are updated at their designed index value 
> (`idx`)
> + * with the returned value of data.
>   *
>   * If the hypercall returns E2BIG the `nr` is too big and should be
>   * lowered. The upper limit of `nr` is left to the implemention.
> @@ -951,11 +952,13 @@ struct xen_sysctl_livepatch_list {
>                                                 amount of payloads and 
> version.
>                                                 OUT: How many payloads left. 
> */
>      uint32_t pad;                           /* IN: Must be zero. */
> +    uint32_t name_total_size;               /* OUT: Total size of all 
> transfer names */
>      XEN_GUEST_HANDLE_64(xen_livepatch_status_t) status;  /* OUT. Must have 
> enough
>                                                 space allocate for nr of 
> them. */
>      XEN_GUEST_HANDLE_64(char) name;         /* OUT: Array of names. Each 
> member
> -                                               MUST XEN_LIVEPATCH_NAME_SIZE 
> in size.
> -                                               Must have nr of them. */
> +                                               may have an arbitrary length 
> up to
> +                                               XEN_LIVEPATCH_NAME_SIZE 
> bytes. Must have
> +                                               nr of them. */
>      XEN_GUEST_HANDLE_64(uint32) len;        /* OUT: Array of lengths of 
> name's.
>                                                 Must have nr of them. */
>  };

Non-binary-compatible changes require an interface version bump.
I wonder though why you don't use the "pad" field; in fact the
way you make the change you'd have to introduce a 2nd padding
field, to make padding explicit _and_ check it's zero on input
(for future extensibility _without_ having to bump the interface
version).

Jan

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