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netlink: suppress sending NLMSG_ERROR if NLMSG_DONE is already sent

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Netlink has a confirmation/error reporting mechanism for the sent
messages. Kernel explicitly acks each messages if requested (NLM_F_ACK)
 or if message processing results in an error.
Similarly, for multipart messages - typically dumps, where each message
 represents a single object like an interface or a route - another
 message, NLMSG_DONE is used to indicate the end of dump and the
 resulting status.
As a result, successfull dump ends with both NLMSG_DONE and NLMSG_ERROR
 messages.
RFC 3549 does not say anything specific about such case.
Linux adopted an optimisation which suppresses NLMSG_ERROR message
 when NLMSG_DONE is already sent. Certain libraries/applications like
 libnl depends on such behavior.

Suppress sending NLMSG_ERROR if NLMSG_DONE is already sent, by
 setting newly-added 'suppress_ack' flag in the writer and checking
 this flag when generating ack.

This change restores libnl compatibility.

Before:
```
~ nl-link-list
Error: Unable to allocate link cache: Message sequence number mismatch
````

After:
```
~ nl-link-list
vtnet0 ether 52:54:00:14:e3:19 <broadcast,multicast,up,running>
lo0 ieee1394 <loopback,multicast,up,running>
```

Reviewed by:	bapt,pauamma
Tested by:	bapt
Differential Revision: https://reviews.freebsd.org/D37565
This commit is contained in:
Alexander V. Chernikov 2022-11-30 12:15:23 +00:00
parent 821549a9df
commit f4d3aa7490
4 changed files with 20 additions and 7 deletions
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9
share/man/man4/netlink.4
View File

@ -24,7 +24,7 @@
.\"
.\" $FreeBSD$
.\"
.Dd November 1, 2022
.Dd November 30, 2022
.Dt NETLINK 4
.Os
.Sh NAME
@ -212,11 +212,16 @@ If the
socket option is not set, the remainder of the original message will follow.
If the
.Dv NETLINK_EXT_ACK
socket option is set, kernel may add a
socket option is set, the kernel may add a
.Dv NLMSGERR_ATTR_MSG
string TLV with the textual error description, optionally followed by the
.Dv NLMSGERR_ATTR_OFFS
TLV, indicating the offset from the message start that triggered an error.
If the operation reply is a multipart message, then no
.Dv NLMSG_ERROR
reply is generated, only a
.Dv NLMSG_DONE
message, closing multipart sequence.
.Pp
.Dv NLMSG_DONE
indicates the end of the message group: typically, the end of the dump.

13
sys/netlink/netlink_io.c
View File

@ -405,8 +405,9 @@ nl_receive_message(struct nlmsghdr *hdr, int remaining_length,
nl_handler_f handler = nl_handlers[nlp->nl_proto].cb;
int error = 0;
NL_LOG(LOG_DEBUG2, "msg len: %d type: %d", hdr->nlmsg_len,
hdr->nlmsg_type);
NLP_LOG(LOG_DEBUG2, nlp, "msg len: %u type: %d: flags: 0x%X seq: %u pid: %u",
hdr->nlmsg_len, hdr->nlmsg_type, hdr->nlmsg_flags, hdr->nlmsg_seq,
hdr->nlmsg_pid);
if (__predict_false(hdr->nlmsg_len > remaining_length)) {
NLP_LOG(LOG_DEBUG, nlp, "message is not entirely present: want %d got %d",
@ -439,9 +440,10 @@ nl_receive_message(struct nlmsghdr *hdr, int remaining_length,
NL_LOG(LOG_DEBUG2, "retcode: %d", error);
}
if ((hdr->nlmsg_flags & NLM_F_ACK) || (error != 0 && error != EINTR)) {
NL_LOG(LOG_DEBUG3, "ack");
nlmsg_ack(nlp, error, hdr, npt);
NL_LOG(LOG_DEBUG3, "done");
if (!npt->nw->suppress_ack) {
NL_LOG(LOG_DEBUG3, "ack");
nlmsg_ack(nlp, error, hdr, npt);
}
}
return (0);
@ -455,6 +457,7 @@ npt_clear(struct nl_pstate *npt)
npt->err_msg = NULL;
npt->err_off = 0;
npt->hdr = NULL;
npt->nw->suppress_ack = false;
}
/*

4
sys/netlink/netlink_message_writer.c
View File

@ -600,6 +600,9 @@ nlmsg_end(struct nl_writer *nw)
}
nw->hdr->nlmsg_len = (uint32_t)(nw->data + nw->offset - (char *)nw->hdr);
NL_LOG(LOG_DEBUG2, "wrote msg len: %u type: %d: flags: 0x%X seq: %u pid: %u",
nw->hdr->nlmsg_len, nw->hdr->nlmsg_type, nw->hdr->nlmsg_flags,
nw->hdr->nlmsg_seq, nw->hdr->nlmsg_pid);
nw->hdr = NULL;
nw->num_messages++;
return (true);
@ -681,6 +684,7 @@ nlmsg_end_dump(struct nl_writer *nw, int error, struct nlmsghdr *hdr)
nw->offset, perror);
*perror = error;
nlmsg_end(nw);
nw->suppress_ack = true;
return (true);
}

1
sys/netlink/netlink_message_writer.h
View File

@ -55,6 +55,7 @@ struct nl_writer {
uint8_t writer_target; /* NS_WRITER_TARGET_* */
bool ignore_limit; /* If true, ignores RCVBUF limit */
bool enomem; /* True if ENOMEM occured */
bool suppress_ack; /* If true, don't send NLMSG_ERR */
};
#define NS_WRITER_TARGET_SOCKET 0
#define NS_WRITER_TARGET_GROUP 1