unimore/kernel-hacking-2024-linux-steffo
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netlink: add support for ext_ack missing attributes

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There is currently no way to report via extack in a structured way
that an attribute is missing. This leads to families resorting to
string messages.

Add a pair of attributes - @offset and @type for machine-readable
way of reporting missing attributes. The @offset points to the
nest which should have contained the attribute, @type is the
expected nla_type. The offset will be skipped if the attribute
is missing at the message level rather than inside a nest.

User space should be able to figure out which attribute enum
(AKA attribute space AKA attribute set) the nest pointed to by
@offset is using.

Reviewed-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
This commit is contained in:
Jakub Kicinski 2022-08-25 20:09:31 -07:00 committed by Paolo Abeni
parent 0c95cea24f
commit 690252f19f
4 changed files with 36 additions and 2 deletions
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7
Documentation/userspace-api/netlink/intro.rst
View file

@ -359,8 +359,8 @@ compatibility this feature has to be explicitly enabled by setting
the ``NETLINK_EXT_ACK`` setsockopt() to ``1``.
Types of extended ack attributes are defined in enum nlmsgerr_attrs.
The two most commonly used attributes are ``NLMSGERR_ATTR_MSG``
and ``NLMSGERR_ATTR_OFFS``.
The most commonly used attributes are ``NLMSGERR_ATTR_MSG``,
``NLMSGERR_ATTR_OFFS`` and ``NLMSGERR_ATTR_MISS_*``.
``NLMSGERR_ATTR_MSG`` carries a message in English describing
the encountered problem. These messages are far more detailed
@ -368,6 +368,9 @@ than what can be expressed thru standard UNIX error codes.
``NLMSGERR_ATTR_OFFS`` points to the attribute which caused the problem.
``NLMSGERR_ATTR_MISS_TYPE`` and ``NLMSGERR_ATTR_MISS_NEST``
inform about a missing attribute.
Extended ACKs can be reported on errors as well as in case of success.
The latter should be treated as a warning.

13
include/linux/netlink.h
View file

@ -71,6 +71,8 @@ netlink_kernel_create(struct net *net, int unit, struct netlink_kernel_cfg *cfg)
* %NL_SET_ERR_MSG
* @bad_attr: attribute with error
* @policy: policy for a bad attribute
* @miss_type: attribute type which was missing
* @miss_nest: nest missing an attribute (%NULL if missing top level attr)
* @cookie: cookie data to return to userspace (for success)
* @cookie_len: actual cookie data length
*/
@ -78,6 +80,8 @@ struct netlink_ext_ack {
const char *_msg;
const struct nlattr *bad_attr;
const struct nla_policy *policy;
const struct nlattr *miss_nest;
u16 miss_type;
u8 cookie[NETLINK_MAX_COOKIE_LEN];
u8 cookie_len;
};
@ -126,6 +130,15 @@ struct netlink_ext_ack {
#define NL_SET_ERR_MSG_ATTR(extack, attr, msg) \
NL_SET_ERR_MSG_ATTR_POL(extack, attr, NULL, msg)
#define NL_SET_ERR_ATTR_MISS(extack, nest, type) do { \
struct netlink_ext_ack *__extack = (extack); \
\
if (__extack) { \
__extack->miss_nest = (nest); \
__extack->miss_type = (type); \
} \
} while (0)
static inline void nl_set_extack_cookie_u64(struct netlink_ext_ack *extack,
u64 cookie)
{

6
include/uapi/linux/netlink.h
View file

@ -140,6 +140,10 @@ struct nlmsgerr {
* be used - in the success case - to identify a created
* object or operation or similar (binary)
* @NLMSGERR_ATTR_POLICY: policy for a rejected attribute
* @NLMSGERR_ATTR_MISS_TYPE: type of a missing required attribute,
* %NLMSGERR_ATTR_MISS_NEST will not be present if the attribute was
* missing at the message level
* @NLMSGERR_ATTR_MISS_NEST: offset of the nest where attribute was missing
* @__NLMSGERR_ATTR_MAX: number of attributes
* @NLMSGERR_ATTR_MAX: highest attribute number
*/
@ -149,6 +153,8 @@ enum nlmsgerr_attrs {
NLMSGERR_ATTR_OFFS,
NLMSGERR_ATTR_COOKIE,
NLMSGERR_ATTR_POLICY,
NLMSGERR_ATTR_MISS_TYPE,
NLMSGERR_ATTR_MISS_NEST,
__NLMSGERR_ATTR_MAX,
NLMSGERR_ATTR_MAX = __NLMSGERR_ATTR_MAX - 1

12
net/netlink/af_netlink.c
View file

@ -2423,6 +2423,10 @@ netlink_ack_tlv_len(struct netlink_sock *nlk, int err,
tlvlen += nla_total_size(sizeof(u32));
if (extack->policy)
tlvlen += netlink_policy_dump_attr_size_estimate(extack->policy);
if (extack->miss_type)
tlvlen += nla_total_size(sizeof(u32));
if (extack->miss_nest)
tlvlen += nla_total_size(sizeof(u32));
return tlvlen;
}
@ -2449,6 +2453,14 @@ netlink_ack_tlv_fill(struct sk_buff *in_skb, struct sk_buff *skb,
if (extack->policy)
netlink_policy_dump_write_attr(skb, extack->policy,
NLMSGERR_ATTR_POLICY);
if (extack->miss_type)
WARN_ON(nla_put_u32(skb, NLMSGERR_ATTR_MISS_TYPE,
extack->miss_type));
if (extack->miss_nest &&
!WARN_ON((u8 *)extack->miss_nest < in_skb->data ||
(u8 *)extack->miss_nest > in_skb->data + in_skb->len))
WARN_ON(nla_put_u32(skb, NLMSGERR_ATTR_MISS_NEST,
(u8 *)extack->miss_nest - (u8 *)nlh));
}
void netlink_ack(struct sk_buff *in_skb, struct nlmsghdr *nlh, int err,