[RPC] Topic: Issues and Testing Regarding SELinx AVC Cache Modification

[Canfeng Guo]

When calling avc_insert to add nodes to the avc cache, they are inserted into
the head of the hash chain. Similarly, avc_calim_node removes nodes from
the head of the same chain. so, SElinux will delete the latest added cache
infromation.

I question whether the deletion logic proposed in the patch is more appropriate
than the current implementation, or whether alternative mechanisms such as
LRU caching are beneficial.

In my testing environment, I applied the above patch when avc_cache.solt and
cache_threshold were both set to 512 by default. I only have over 280 nodes
in my cache, and the longest observation length of the AVC cache linked list
is only 7 entries. Considering this small size, the cost of traversing the
list is minimal, and such modifications may not incur additional costs.

However, I don't know how to design a test case to verify its cost.
And I cannot prove that this patch is beneficial.

I attempted to simulate a more demanding scenario by increasing the cache_threshold
to 2048 in order to establish a longer linked list of AVC caches, but
I was unable to generate more than 2048 AVC records, possibly due to the need
for a highly complex environment with numerous different SID interactions.

Therefore, I have two questions:
The necessity of modification:
     Considering its potential impact on the cache performance of SELinx AVC,
     is it worth investing effort into this modification?, i think that in most cases,
     this modification is not necessart.
Verification method:
     If making such modifications is reasonable, how can I effectively
     measure its impact on system performance?

Signed-off-by: Canfeng Guo <guocanfeng@xxxxxxxxxxxxx>
---
 security/selinux/avc.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/security/selinux/avc.c b/security/selinux/avc.c
index 32eb67fb3e42..9999028660c9 100644
--- a/security/selinux/avc.c
+++ b/security/selinux/avc.c
@@ -477,6 +477,9 @@ static inline int avc_reclaim_node(void)
 
 		rcu_read_lock();
 		hlist_for_each_entry(node, head, list) {
+			while(node->next){
+				node = node->next;
+			}
 			avc_node_delete(node);
 			avc_cache_stats_incr(reclaims);
 			ecx++;
-- 
2.20.1