Re: [PATCH v3 5/7] xen: add new domctl get_changed_domain

[Jan Beulich <jbeulich@xxxxxxxx>]

On 13.12.2024 17:24, Juergen Gross wrote:
> --- a/xen/common/domain.c
> +++ b/xen/common/domain.c
> @@ -193,6 +193,57 @@ static void domain_changed_state(const struct domain *d)
>      spin_unlock(&dom_state_changed_lock);
>  }
>  
> +static void set_domain_state_info(struct xen_domctl_get_domain_state *info,
> +                                  const struct domain *d)
> +{
> +    info->state = XEN_DOMCTL_GETDOMSTATE_STATE_EXIST;
> +    if ( d->is_shut_down )
> +        info->state |= XEN_DOMCTL_GETDOMSTATE_STATE_SHUTDOWN;
> +    if ( d->is_dying == DOMDYING_dying )
> +        info->state |= XEN_DOMCTL_GETDOMSTATE_STATE_DYING;
> +    if ( d->is_dying == DOMDYING_dead )
> +        info->state |= XEN_DOMCTL_GETDOMSTATE_STATE_DEAD;
> +    info->unique_id = d->unique_id;
> +}
> +
> +int get_domain_state(struct xen_domctl_get_domain_state *info, struct domain 
> *d,
> +                     domid_t *domid)
> +{
> +    unsigned int dom;
> +
> +    if ( info->pad0 || info->pad1 )
> +        return -EINVAL;
> +
> +    if ( d )
> +    {
> +        set_domain_state_info(info, d);
> +
> +        return 0;
> +    }
> +
> +    while ( (dom = find_first_bit(dom_state_changed, DOMID_MASK + 1)) <
> +            DOMID_FIRST_RESERVED )
> +    {
> +        if ( test_and_clear_bit(dom, dom_state_changed) )

For these two accesses to dom_state_changed don't you need to hold the
lock patch 4 introduces? Also didn't you say you'd constrain the new
sub-op to the sole domain having VIRQ_DOM_EXEC bound (which, ftaod,
isn't enough to eliminate the race)?

> +        {
> +            *domid = dom;
> +
> +            d = rcu_lock_domain_by_id(dom);
> +
> +            if ( d )
> +            {
> +                set_domain_state_info(info, d);
> +
> +                rcu_unlock_domain(d);
> +            }

Oh, on the implicit "else" is where the original memset() would come into
play: You want to make sure at least ->state, but perhaps also ->unique_id
are cleared (rather than demanding the caller to clear them ahead of making
the call).

Jan