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netns: get more entropy from net_hash_mix() 

struct net are effectively allocated from order-1 pages on x86,
with one object per slab, meaning that the 13 low order bits
of their addresses are zero.

Once shifted by L1_CACHE_SHIFT, this leaves 7 zero-bits,
meaning that net_hash_mix() does not help spreading
objects on various hash tables.

For example, TCP listen table has 32 buckets, meaning that
all netns use the same bucket for port 80 or port 443.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: Maciej Żenczykowski <maze@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
hifive-unleashed-5.1
Eric Dumazet 2018-06-22 16:27:47 -07:00 committed by David S. Miller
parent 6c1f0a1ffb
commit 5424ea2739
1 changed files with 1 additions and 6 deletions
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7
include/net/netns/hash.h
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@ -9,12 +9,7 @@ struct net;
static inline u32 net_hash_mix(const struct net *net)
{
#ifdef CONFIG_NET_NS
/*
* shift this right to eliminate bits, that are
* always zeroed
*/
return (u32)(((unsigned long)net) >> L1_CACHE_SHIFT);
return (u32)(((unsigned long)net) >> ilog2(sizeof(*net)));
#else
return 0;
#endif