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Re: [PATCH] xen/sched: fix sched_move_domain() for domain without vcpus

On 06.09.21 13:23, Jan Beulich wrote:
On 06.09.2021 13:18, Andrew Cooper wrote:
On 06/09/2021 12:14, Andrew Cooper wrote:
On 06/09/2021 12:00, Juergen Gross wrote:
In case a domain is created with a cpupool other than Pool-0 specified
it will be moved to that cpupool before any vcpus are allocated.

This will lead to a NULL pointer dereference in sched_move_domain().

Fix that by tolerating vcpus not being allocated yet.

Fixes: 70fadc41635b9b6 ("xen/cpupool: support moving domain between cpupools with 
different granularity")
Reported-by: Bertrand Marquis <bertrand.marquis@xxxxxxx>
Reviewed-by: Bertrand Marquis <bertrand.marquis@xxxxxxx>
Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
  xen/common/sched/core.c | 4 ++++
  1 file changed, 4 insertions(+)

diff --git a/xen/common/sched/core.c b/xen/common/sched/core.c
index 8d178baf3d..79c9100680 100644
--- a/xen/common/sched/core.c
+++ b/xen/common/sched/core.c
@@ -671,6 +671,10 @@ int sched_move_domain(struct domain *d, struct cpupool *c)
for ( unit_idx = 0; unit_idx < n_units; unit_idx++ )
+        /* Special case for move at domain creation time. */
+        if ( !d->vcpu[unit_idx * gran] )
+            break;
          unit = sched_alloc_unit_mem();
          if ( unit )
I think the logic would be clearer if you wrap the entire for loop in if
( d->max_vcpus ).

And of course, this is wrong.  Turns out the domain_has_vcpus()
predicate still hasn't been committed, but d->vcpu[0] is the correct

Which in turn might want to be done by setting n_units to zero when
d->vcpus[0] is NULL?

Yes, this would be possible.

OTOH my variant is more robust in case not all vcpus are allocated,
but I guess this will explode somewhere else anyway.

In case I don't get any other comment today I'll change the patch to set
n_units to 0 if d->vcpus[0] is NULL.


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