Re: [PATCH] netlink: align attributes on 64-bits
From: Ben Hutchings <hidden>
Date: 2012-12-14 15:49:26
On Fri, 2012-12-14 at 14:16 +0100, Nicolas Dichtel wrote:
On 64 bits arch, we must ensure that attributes are always aligned on 64-bits boundary. We do that by adding attributes of type 0, size 4 (alignment on 32-bits is already done) when needed. Attribute type 0 should be available and unused in all netlink families.
[...]
quoted hunk ↗ jump to hunk
--- a/lib/nlattr.c +++ b/lib/nlattr.c@@ -450,9 +450,18 @@ EXPORT_SYMBOL(__nla_put_nohdr); */ int nla_put(struct sk_buff *skb, int attrtype, int attrlen, const void *data) { - if (unlikely(skb_tailroom(skb) < nla_total_size(attrlen))) + int align = IS_ALIGNED((unsigned long)skb_tail_pointer(skb), sizeof(void *)) ? 0 : 4;
The assumption here is that nothing needs to be aligned to a greater width than that of a pointer. However, for most 32-bit architectures (i386 being an exception) the C ABI requires 64-bit alignment for 64-bit types. There may be cases where a mostly 32-bit processor really requires 64-bit alignment, e.g. to load or save a pair of registers. Ben.
+ if (unlikely(skb_tailroom(skb) < nla_total_size(attrlen) + align))
return -EMSGSIZE;
+ if (align) {
+ /* Goal is to add an attribute with size 4. We know that
+ * NLA_HDRLEN is 4, hence payload is 0.
+ */
+ __nla_reserve(skb, 0, 0);
+ }
+
__nla_put(skb, attrtype, attrlen, data);
return 0;
}-- Ben Hutchings, Staff Engineer, Solarflare Not speaking for my employer; that's the marketing department's job. They asked us to note that Solarflare product names are trademarked.