[xen master] xen/events: fix get_global_virq_handler() usage without hardware domain

[patchbot@xxxxxxx]

commit fe5757f25de6062732c088ae54e2180236632552
Author:     Juergen Gross <jgross@xxxxxxxx>
AuthorDate: Thu Mar 6 17:23:36 2025 +0100
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Thu Mar 6 17:23:36 2025 +0100

    xen/events: fix get_global_virq_handler() usage without hardware domain
    
    Some use cases of get_global_virq_handler() didn't account for the
    case of running without hardware domain.
    
    Fix that by testing get_global_virq_handler() returning NULL where
    needed (e.g. when directly dereferencing the result).
    
    Fixes: 980822c5edd1 ("xen/events: allow setting of global virq handler only 
for unbound virqs")
    Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
    Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
---
 xen/common/event_channel.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/xen/common/event_channel.c b/xen/common/event_channel.c
index 4ee6b6b4ce..c68aa97135 100644
--- a/xen/common/event_channel.c
+++ b/xen/common/event_channel.c
@@ -1036,7 +1036,9 @@ int set_global_virq_handler(struct domain *d, uint32_t 
virq)
     {
         old = global_virq_handlers[virq];
         hdl = get_global_virq_handler(virq);
-        if ( hdl != d )
+        if ( !hdl )
+            global_virq_handlers[virq] = d;
+        else if ( hdl != d )
         {
             read_lock(&hdl->event_lock);
 
@@ -1091,7 +1093,7 @@ struct domain *lock_dom_exc_handler(void)
     struct domain *d;
 
     d = get_global_virq_handler(VIRQ_DOM_EXC);
-    if ( unlikely(!get_domain(d)) )
+    if ( unlikely(!d) || unlikely(!get_domain(d)) )
         return NULL;
 
     read_lock(&d->event_lock);
@@ -1101,6 +1103,9 @@ struct domain *lock_dom_exc_handler(void)
 
 void unlock_dom_exc_handler(struct domain *d)
 {
+    if ( likely(!d) )
+        return;
+
     read_unlock(&d->event_lock);
 
     put_domain(d);
--
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