path: root/net
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authorRadu Rendec <radu.rendec@ines.ro>2007-11-07 01:20:12 -0800
committerDavid S. Miller <davem@sunset.davemloft.net>2007-11-07 04:11:45 -0800
commit543821c6f5dea5221426eaf1eac98b100249c7ac (patch)
treea2664eca43694b3f8e7326179038b32fd13de356 /net
parent[NET]: Removing duplicit #includes (diff)
[PKT_SCHED] CLS_U32: Fix endianness problem with u32 classifier hash masks.
While trying to implement u32 hashes in my shaping machine I ran into a possible bug in the u32 hash/bucket computing algorithm (net/sched/cls_u32.c). The problem occurs only with hash masks that extend over the octet boundary, on little endian machines (where htonl() actually does something). Let's say that I would like to use 0x3fc0 as the hash mask. This means 8 contiguous "1" bits starting at b6. With such a mask, the expected (and logical) behavior is to hash any address in, for instance, in bucket 0, then any address in in bucket 1, then in bucket 2 and so on. This is exactly what would happen on a big endian machine, but on little endian machines, what would actually happen with current implementation is 0x3fc0 being reversed (into 0xc03f0000) by htonl() in the userspace tool and then applied to 192.168.x.x in the u32 classifier. When shifting right by 16 bits (rank of first "1" bit in the reversed mask) and applying the divisor mask (0xff for divisor 256), what would actually remain is 0x3f applied on the "168" octet of the address. One could say is this can be easily worked around by taking endianness into account in userspace and supplying an appropriate mask (0xfc03) that would be turned into contiguous "1" bits when reversed (0x03fc0000). But the actual problem is the network address (inside the packet) not being converted to host order, but used as a host-order value when computing the bucket. Let's say the network address is written as n31 n30 ... n0, with n0 being the least significant bit. When used directly (without any conversion) on a little endian machine, it becomes n7 ... n0 n8 ..n15 etc in the machine's registers. Thus bits n7 and n8 would no longer be adjacent and and would no longer be consecutive. The fix is to apply ntohl() on the hmask before computing fshift, and in u32_hash_fold() convert the packet data to host order before shifting down by fshift. With helpful feedback from Jamal Hadi Salim and Jarek Poplawski. Signed-off-by: David S. Miller <davem@davemloft.net>
Diffstat (limited to 'net')
1 files changed, 2 insertions, 2 deletions
diff --git a/net/sched/cls_u32.c b/net/sched/cls_u32.c
index 9e98c6e567dd..53171029439f 100644
--- a/net/sched/cls_u32.c
+++ b/net/sched/cls_u32.c
@@ -91,7 +91,7 @@ static struct tc_u_common *u32_list;
static __inline__ unsigned u32_hash_fold(u32 key, struct tc_u32_sel *sel, u8 fshift)
- unsigned h = (key & sel->hmask)>>fshift;
+ unsigned h = ntohl(key & sel->hmask)>>fshift;
return h;
@@ -615,7 +615,7 @@ static int u32_change(struct tcf_proto *tp, unsigned long base, u32 handle,
n->handle = handle;
u8 i = 0;
- u32 mask = s->hmask;
+ u32 mask = ntohl(s->hmask);
if (mask) {
while (!(mask & 1)) {