Re: [PATCHv2] sctp: Fix port hash table size computation

On jeu., 2016-02-18 at 16:10 -0500, Neil Horman wrote:

Dmitry Vyukov noted recently that the sctp_port_hashtable had an error in
its size computation, observing that the current method never guaranteed
that the hashsize (measured in number of entries) would be a power of two,
which the input hash function for that table requires.  The root cause of
the problem is that two values need to be computed (one, the allocation
order of the storage requries, as passed to __get_free_pages, and two the
number of entries for the hash table).  Both need to be ^2, but for
different reasons, and the existing code is simply computing one order
value, and using it as the basis for both, which is wrong (i.e. it assumes
that ((1<<order)*PAGE_SIZE)/sizeof(bucket) is still ^2 when its not).

Looks complicated for a stable submission.

What about reusing existing trick instead ?

diff --git a/net/sctp/protocol.c b/net/sctp/protocol.c
index ab0d538..3e4e11b 100644
--- a/net/sctp/protocol.c
+++ b/net/sctp/protocol.c

@@ -1434,6 +1434,10 @@ static __init int sctp_init(void)
 	do {
 		sctp_port_hashsize = (1UL << order) * PAGE_SIZE /
 					sizeof(struct sctp_bind_hashbucket);
+
+		while (sctp_port_hashsize & (sctp_port_hashsize - 1))
+			sctp_port_hashsize--;
+
 	if ((sctp_port_hashsize > (64 * 1024)) && order > 0)
 		continue;
 	sctp_port_hashtable = (struct sctp_bind_hashbucket *)