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Date: Wed, 19 Dec 2012 12:22:36 +0100
From: Nicolas Dichtel <nicolas.dichtel@...nd.com>
To: Thomas Graf <tgraf@...g.ch>
CC: bhutchings@...arflare.com, netdev@...r.kernel.org,
davem@...emloft.net, David.Laight@...LAB.COM
Subject: Re: [PATCH v2] netlink: align attributes on 64-bits
Le 18/12/2012 13:57, Thomas Graf a écrit :
> On 12/17/12 at 05:49pm, Nicolas Dichtel wrote:
>> @@ -492,6 +492,15 @@ static inline struct nlmsghdr *nlmsg_put_answer(struct sk_buff *skb,
>> */
>> static inline struct sk_buff *nlmsg_new(size_t payload, gfp_t flags)
>> {
>> + /* Because attributes may be aligned on 64-bits boundary with fake
>> + * attribute (type 0, size 4 (attributes are 32-bits align by default)),
>> + * an exact payload size cannot be calculated. Hence, we need to reserve
>> + * more space for these attributes.
>> + * 128 is arbitrary: it allows to align up to 32 attributes.
>> + */
>> + if (payload < NLMSG_DEFAULT_SIZE)
>> + payload = min(payload + 128, (size_t)NLMSG_DEFAULT_SIZE);
>
> This is doomed to fail eventually. A netlink message may carry
> hundreds of attributes eventually. See my suggestion below.
>
>> diff --git a/lib/nlattr.c b/lib/nlattr.c
>> index 18eca78..7440a80 100644
>> --- 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), 8) ? 0 : 4;
>
> This does not look right. In order for the attribute data to be
> aligned properly you would need to check skb_tail_pointer(skb) +
> NLA_HDRLEN for proper alignment or you end up aligning the
> attribute header.
>
> How about we change nla_total_size() to return the size with
> needed padding taken into account. That should fix the message
> size caluclation problem and we only need to reserve room for
> the initial padding to align the very first attribute.
>
> Below is an untested patch that does this. What do you think?
>
> diff --git a/include/net/netlink.h b/include/net/netlink.h
> index 9690b0f..7ce8e76 100644
> --- a/include/net/netlink.h
> +++ b/include/net/netlink.h
> @@ -114,7 +114,6 @@
> * Attribute Length Calculations:
> * nla_attr_size(payload) length of attribute w/o padding
> * nla_total_size(payload) length of attribute w/ padding
> - * nla_padlen(payload) length of padding
> *
> * Attribute Payload Access:
> * nla_data(nla) head of attribute payload
> @@ -492,6 +491,14 @@ static inline struct nlmsghdr *nlmsg_put_answer(struct sk_buff *skb,
> */
> static inline struct sk_buff *nlmsg_new(size_t payload, gfp_t flags)
> {
> + /* If an exact size if specified, reserve some additional space to
> + * align the first attribute, all subsequent attributes should have
> + * padding accounted for.
> + */
> + if (payload != NLMSG_DEFAULT_SIZE)
> + payload = min_t(size_t, payload + NLA_ATTR_ALIGN,
> + NLMSG_DEFAULT_SIZE);
> +
> return alloc_skb(nlmsg_total_size(payload), flags);
> }
>
> @@ -653,16 +660,12 @@ static inline int nla_attr_size(int payload)
> */
> static inline int nla_total_size(int payload)
> {
> - return NLA_ALIGN(nla_attr_size(payload));
> -}
> + size_t len = NLA_ALIGN(nla_attr_size(payload));
>
> -/**
> - * nla_padlen - length of padding at the tail of attribute
> - * @payload: length of payload
> - */
> -static inline int nla_padlen(int payload)
> -{
> - return nla_total_size(payload) - nla_attr_size(payload);
> + if (!IS_ALIGNED(len, NLA_ATTR_ALIGN))
> + len = ALIGN(len + NLA_HDRLEN, NLA_ATTR_ALIGN);
> +
> + return len;
> }
>
> /**
> diff --git a/include/uapi/linux/netlink.h b/include/uapi/linux/netlink.h
> index 78d5b8a..1856729 100644
> --- a/include/uapi/linux/netlink.h
> +++ b/include/uapi/linux/netlink.h
> @@ -149,5 +149,10 @@ struct nlattr {
> #define NLA_ALIGN(len) (((len) + NLA_ALIGNTO - 1) & ~(NLA_ALIGNTO - 1))
> #define NLA_HDRLEN ((int) NLA_ALIGN(sizeof(struct nlattr)))
>
> +/* Padding attribute type, added automatically to align attributes,
> + * must be ignored by readers. */
> +#define NLA_PADDING 0
> +#define NLA_ATTR_ALIGN 8
> +
>
> #endif /* _UAPI__LINUX_NETLINK_H */
> diff --git a/lib/nlattr.c b/lib/nlattr.c
> index 18eca78..b09473c 100644
> --- a/lib/nlattr.c
> +++ b/lib/nlattr.c
> @@ -322,18 +322,36 @@ int nla_strcmp(const struct nlattr *nla, const char *str)
> * Adds a netlink attribute header to a socket buffer and reserves
> * room for the payload but does not copy it.
> *
> + * May add a padding attribute of type NLA_PADDING before the
> + * real attribute to ensure proper alignment.
> + *
> * The caller is responsible to ensure that the skb provides enough
> * tailroom for the attribute header and payload.
> */
> struct nlattr *__nla_reserve(struct sk_buff *skb, int attrtype, int attrlen)
> {
> struct nlattr *nla;
> + size_t offset;
> +
> + offset = (size_t) skb_tail_pointer(skb);
> + if (!IS_ALIGNED(offset + NLA_HDRLEN, NLA_ATTR_ALIGN)) {
> + struct nlattr *pad;
> + size_t padlen;
> +
> + padlen = nla_total_size(offset) - offset - NLA_HDRLEN;
Here padlen will return 4, which is wrong: padlen + NLA_HDRLEN = 8, alignment is
the same than before. Here is a proposal fix:
diff --git a/lib/nlattr.c b/lib/nlattr.c
index e4f0329..1556313 100644
--- a/lib/nlattr.c
+++ b/lib/nlattr.c
@@ -338,7 +338,10 @@ struct nlattr *__nla_reserve(struct sk_buff *skb, int
attrtype, int attrlen)
struct nlattr *pad;
size_t padlen;
- padlen = nla_total_size(offset) - offset - NLA_HDRLEN;
+ /* We need to remove NLA_HDRLEN two times: one time for the
+ * attribute hdr and one time for the pad attribute hdr.
+ */
+ padlen = nla_total_size(offset) - offset - 2 * NLA_HDRLEN;
pad = (struct nlattr *) skb_put(skb, nla_attr_size(padlen));
pad->nla_type = 0;
pad->nla_len = nla_attr_size(padlen);
With this patch, it seems goods. attribute are always aligned on 8 bytes. Also
I did not notice any problem with size calculation (I try some ip link, ip xfrm,
ip [m]route).
Do you want to make more tests? Or will your repost the full patch?
I can do it if you don't have time.
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