netlink: suppress sending NLMSG_ERROR if NLMSG_DONE is already sent
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
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@ -24,7 +24,7 @@
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.\"
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.\" $FreeBSD$
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.\"
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.Dd November 1, 2022
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.Dd November 30, 2022
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.Dt NETLINK 4
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.Os
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.Sh NAME
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@ -212,11 +212,16 @@ If the
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socket option is not set, the remainder of the original message will follow.
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If the
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.Dv NETLINK_EXT_ACK
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socket option is set, kernel may add a
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socket option is set, the kernel may add a
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.Dv NLMSGERR_ATTR_MSG
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string TLV with the textual error description, optionally followed by the
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.Dv NLMSGERR_ATTR_OFFS
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TLV, indicating the offset from the message start that triggered an error.
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If the operation reply is a multipart message, then no
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.Dv NLMSG_ERROR
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reply is generated, only a
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.Dv NLMSG_DONE
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message, closing multipart sequence.
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.Pp
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.Dv NLMSG_DONE
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indicates the end of the message group: typically, the end of the dump.
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@ -405,8 +405,9 @@ nl_receive_message(struct nlmsghdr *hdr, int remaining_length,
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nl_handler_f handler = nl_handlers[nlp->nl_proto].cb;
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int error = 0;
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NL_LOG(LOG_DEBUG2, "msg len: %d type: %d", hdr->nlmsg_len,
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hdr->nlmsg_type);
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NLP_LOG(LOG_DEBUG2, nlp, "msg len: %u type: %d: flags: 0x%X seq: %u pid: %u",
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hdr->nlmsg_len, hdr->nlmsg_type, hdr->nlmsg_flags, hdr->nlmsg_seq,
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hdr->nlmsg_pid);
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if (__predict_false(hdr->nlmsg_len > remaining_length)) {
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NLP_LOG(LOG_DEBUG, nlp, "message is not entirely present: want %d got %d",
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@ -439,9 +440,10 @@ nl_receive_message(struct nlmsghdr *hdr, int remaining_length,
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NL_LOG(LOG_DEBUG2, "retcode: %d", error);
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}
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if ((hdr->nlmsg_flags & NLM_F_ACK) || (error != 0 && error != EINTR)) {
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if (!npt->nw->suppress_ack) {
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NL_LOG(LOG_DEBUG3, "ack");
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nlmsg_ack(nlp, error, hdr, npt);
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NL_LOG(LOG_DEBUG3, "done");
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}
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}
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return (0);
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@ -455,6 +457,7 @@ npt_clear(struct nl_pstate *npt)
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npt->err_msg = NULL;
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npt->err_off = 0;
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npt->hdr = NULL;
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npt->nw->suppress_ack = false;
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}
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/*
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@ -600,6 +600,9 @@ nlmsg_end(struct nl_writer *nw)
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}
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nw->hdr->nlmsg_len = (uint32_t)(nw->data + nw->offset - (char *)nw->hdr);
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NL_LOG(LOG_DEBUG2, "wrote msg len: %u type: %d: flags: 0x%X seq: %u pid: %u",
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nw->hdr->nlmsg_len, nw->hdr->nlmsg_type, nw->hdr->nlmsg_flags,
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nw->hdr->nlmsg_seq, nw->hdr->nlmsg_pid);
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nw->hdr = NULL;
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nw->num_messages++;
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return (true);
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@ -681,6 +684,7 @@ nlmsg_end_dump(struct nl_writer *nw, int error, struct nlmsghdr *hdr)
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nw->offset, perror);
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*perror = error;
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nlmsg_end(nw);
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nw->suppress_ack = true;
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return (true);
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}
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@ -55,6 +55,7 @@ struct nl_writer {
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uint8_t writer_target; /* NS_WRITER_TARGET_* */
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bool ignore_limit; /* If true, ignores RCVBUF limit */
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bool enomem; /* True if ENOMEM occured */
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bool suppress_ack; /* If true, don't send NLMSG_ERR */
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};
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#define NS_WRITER_TARGET_SOCKET 0
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#define NS_WRITER_TARGET_GROUP 1
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