[PATCH iproute2 0/2] ip: ipstats: Fix statistics split across netlink messages

[PATCH iproute2 0/2] ip: ipstats: Fix statistics split across netlink messages

[Alexander Zubkov]

When dumping statistics, the kernel may split the statistics of a single
interface across several netlink messages: if an attribute does not fit
into the current skb, rtnl_fill_statsinfo() keeps the partial message and
the dump is resumed at the same ifindex. "ip stats" formats every message
on its own, so such an interface is reported twice and part of its
statistics is lost. With "group offload subgroup l3_stats", which requests
two attributes that the kernel emits one by one, this is easy to hit on a
box with many netdevices:

    109: vlan859: group offload subgroup l3_stats on used on

    109: vlan859: group offload subgroup l3_stats

The first record holds IFLA_OFFLOAD_XSTATS_HW_S_INFO, the second one
holds IFLA_OFFLOAD_XSTATS_L3_STATS, and the counters are never shown.

Patch 1 reassembles such messages before formatting them. Patch 2 is an
unrelated hardening cleanup and can be dropped.

The merge itself was also exercised out of tree against the two message
shapes the kernel can split, offload xstats cut between HW_S_INFO and
L3_STATS, and bridge per-VLAN xstats cut between two BRIDGE_XSTATS_VLAN
entries, plus a layout that must be refused rather than merged.

I faced the issue on kernel 7.1.5 / iproute2-7.0.0, mlxsw switch, with
~120 netdevices. The split was visible in an strace as two RTM_NEWSTATS
messages with the same ifindex, carrying IFLA_OFFLOAD_XSTATS_HW_S_INFO
and IFLA_OFFLOAD_XSTATS_L3_STATS respectively. The further research,
patches and supporting texts was prepared with the help of an AI
assistant. I reviewed and tested the patches against iproute2-7.1.0, with
the proposed patches the interface is correctly reported once, with its
counters. Although the patches makes sense to me, I have little
experience with netlink handling.


Alexander Zubkov (2):
  ip: ipstats: Merge statistics split across several netlink messages
  ip: ipstats: Do not hide HW statistics when hw_stats_info is missing

 ip/ipstats.c | 285 ++++++++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 280 insertions(+), 5 deletions(-)

-- 
2.55.0

[PATCH 1/2] ip: ipstats: Merge statistics split across several netlink messages

[Alexander Zubkov]

A netlink dump can split the statistics of a single interface across
several messages. When an attribute does not fit into the current skb,
rtnl_fill_statsinfo() keeps the partial message, records how far it got
in idxattr/prividx, and the dump is resumed at the same ifindex, with
the remaining attributes emitted in the next message.

"ip stats show group offload subgroup l3_stats" requests both
IFLA_OFFLOAD_XSTATS_HW_S_INFO and IFLA_OFFLOAD_XSTATS_L3_STATS, and
rtnl_offload_xstats_fill() emits them one by one, so a dump over a
sufficient number of netdevices regularly ends up cut in between the
two. Each message is currently formatted on its own, which yields two
bogus records instead of one, and, because ipstats_show_hw_stats() bails
out when the HW_S_INFO attribute is missing, the counters carried by the
second message are dropped:

    109: vlan859: group offload subgroup l3_stats on used on

    109: vlan859: group offload subgroup l3_stats

Postpone formatting until the whole object has been received: keep the
message around, merge into it any immediately following message that
carries the same ifindex, and format the result once a message for
another ifindex arrives or the dump ends. At most one interface worth of
statistics is buffered at a time.

Merging the two messages means merging the nests that were open when the
first one was cut short and that got reopened in the second one. Netlink
does not mark nests reliably here -- nla_nest_start_noflag() is used --
so they are recognized by the message layout instead: a reopened nest
appears exactly once in either message, and at the outer level is known
to be a nest by ipstats_stat_ifla_max[]. That knowledge only reaches as
deep as ipstats.c parses the message, so merging stops at the group
nests and their contents are appended in the order they arrived, which
is correct both for a deeper nest reopened between two of its children
and for an array of same-type attributes, as bridge per-VLAN xstats are.

A layout that does not fit the above cannot be told apart from an array
of same-type attributes, and reassembling it would corrupt the result.
Such a message pair is refused with -EOPNOTSUPP and the two halves are
formatted separately, as before this patch. Failures to allocate or to
build the merged message are not papered over that way and terminate the
dump.

Fixes: 54d82b0699a0 ("ip: Add a new family of commands, "stats"")
Assisted-by: Claude:claude-opus-5
Signed-off-by: Alexander Zubkov <green@qrator.net>
---
 ip/ipstats.c | 280 ++++++++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 276 insertions(+), 4 deletions(-)

diff --git a/ip/ipstats.c b/ip/ipstats.c
index 8a2ac85e..c531885f 100644
--- a/ip/ipstats.c
+++ b/ip/ipstats.c
@@ -850,12 +850,225 @@ ipstats_show_one(int ifindex, struct ipstats_stat_enabled *enabled)
 	return err;
 }
 
-static int ipstats_dump_one(struct nlmsghdr *n, void *arg)
+/* A netlink dump can split the statistics of one interface across several
+ * messages: when an attribute does not fit into the current skb, the kernel
+ * ends the message and resumes the dump at the same ifindex, emitting the
+ * remaining attributes in the next message. See rtnl_fill_statsinfo() and
+ * rtnl_offload_xstats_fill() in the kernel.
+ *
+ * Reassembling the two messages means merging the nests that were open when
+ * the first message was cut short and got reopened in the second one. Netlink
+ * does not mark nests reliably -- nla_nest_start_noflag() is used here -- so
+ * they are recognized by the message layout instead: a nest that was reopened
+ * appears exactly once in either message, and, at the outer level, is known to
+ * be a nest by ipstats_stat_ifla_max[].
+ *
+ * That knowledge only reaches as deep as ipstats.c parses the message, so
+ * merging stops at the group nests, and their contents are appended in the
+ * order they arrived. Appending is the right thing to do whether a deeper nest
+ * was reopened between two of its children or the children are just an array
+ * of same-type attributes, as bridge per-VLAN xstats are. It does the wrong
+ * thing only if a kernel splits a nest that ipstats.c does not know how to
+ * show either, in which case the merge is not the first thing that needs
+ * updating.
+ */
+
+enum {
+	/* Merge the outer level and the group nests, append below that. */
+	IPSTATS_MERGE_MAX_DEPTH = 1,
+};
+
+static struct rtattr *ipstats_rta_find(struct rtattr *rtas, int len,
+				       unsigned short type, unsigned int *count)
+{
+	struct rtattr *found = NULL;
+	struct rtattr *rta;
+
+	*count = 0;
+	for (rta = rtas; RTA_OK(rta, len); rta = RTA_NEXT(rta, len)) {
+		if (rta->rta_type != type)
+			continue;
+		if (found == NULL)
+			found = rta;
+		(*count)++;
+	}
+
+	return found;
+}
+
+/* Whether the two messages can be reassembled at this attribute, and how.
+ * MERGE means RTA and OTHER are the two halves of one nest that the first
+ * message left open. APPEND means the attribute belongs to one message only
+ * and is copied over as it is. REFUSE means the type is present in both
+ * messages in a layout that cannot be told apart from an array of same-type
+ * attributes, so reassembling it would corrupt the result.
+ */
+enum ipstats_merge_action {
+	IPSTATS_MERGE_APPEND,
+	IPSTATS_MERGE_MERGE,
+	IPSTATS_MERGE_REFUSE,
+};
+
+static enum ipstats_merge_action
+ipstats_merge_action(const struct rtattr *rta, unsigned int nrta,
+		     const struct rtattr *other, unsigned int nother, int depth)
+{
+	if (rta == NULL || other == NULL)
+		return IPSTATS_MERGE_APPEND;
+
+	if (nrta != 1 || nother != 1)
+		return IPSTATS_MERGE_REFUSE;
+
+	if (depth == 0) {
+		unsigned int type = rta->rta_type;
+
+		if (type >= ARRAY_SIZE(ipstats_stat_ifla_max) ||
+		    ipstats_stat_ifla_max[type] == 0)
+			return IPSTATS_MERGE_REFUSE;
+	}
+
+	return IPSTATS_MERGE_MERGE;
+}
+
+static int ipstats_copy_rtas(struct nlmsghdr *n, int maxlen,
+			     struct rtattr *rtas, int len)
+{
+	struct rtattr *rta;
+
+	for (rta = rtas; RTA_OK(rta, len); rta = RTA_NEXT(rta, len)) {
+		if (addattr_l(n, maxlen, rta->rta_type,
+			      RTA_DATA(rta), RTA_PAYLOAD(rta)))
+			return -EMSGSIZE;
+	}
+
+	return 0;
+}
+
+static int ipstats_merge_rtas(struct nlmsghdr *n, int maxlen,
+			      struct rtattr *a, int alen,
+			      struct rtattr *b, int blen, int depth)
+{
+	struct rtattr *rta;
+	int rem;
+	int err;
+
+	for (rta = a, rem = alen; RTA_OK(rta, rem); rta = RTA_NEXT(rta, rem)) {
+		unsigned int nrta, nother;
+		struct rtattr *other;
+		struct rtattr *nest;
+
+		other = ipstats_rta_find(b, blen, rta->rta_type, &nother);
+		ipstats_rta_find(a, alen, rta->rta_type, &nrta);
+
+		switch (ipstats_merge_action(rta, nrta, other, nother, depth)) {
+		case IPSTATS_MERGE_REFUSE:
+			return -EOPNOTSUPP;
+		case IPSTATS_MERGE_APPEND:
+			if (addattr_l(n, maxlen, rta->rta_type,
+				      RTA_DATA(rta), RTA_PAYLOAD(rta)))
+				return -EMSGSIZE;
+			continue;
+		case IPSTATS_MERGE_MERGE:
+			break;
+		}
+
+		nest = addattr_nest(n, maxlen, rta->rta_type);
+		if (nest == NULL)
+			return -EMSGSIZE;
+
+		if (depth < IPSTATS_MERGE_MAX_DEPTH) {
+			err = ipstats_merge_rtas(n, maxlen,
+						 RTA_DATA(rta), RTA_PAYLOAD(rta),
+						 RTA_DATA(other),
+						 RTA_PAYLOAD(other), depth + 1);
+		} else {
+			err = ipstats_copy_rtas(n, maxlen, RTA_DATA(rta),
+						RTA_PAYLOAD(rta));
+			if (!err)
+				err = ipstats_copy_rtas(n, maxlen,
+							RTA_DATA(other),
+							RTA_PAYLOAD(other));
+		}
+		if (err)
+			return err;
+
+		addattr_nest_end(n, nest);
+	}
+
+	for (rta = b, rem = blen; RTA_OK(rta, rem); rta = RTA_NEXT(rta, rem)) {
+		unsigned int nrta, nother;
+		struct rtattr *other;
+
+		other = ipstats_rta_find(a, alen, rta->rta_type, &nother);
+		ipstats_rta_find(b, blen, rta->rta_type, &nrta);
+
+		/* Whatever is present in both messages has been dealt with
+		 * in the loop above.
+		 */
+		if (ipstats_merge_action(rta, nrta, other, nother,
+					 depth) != IPSTATS_MERGE_APPEND)
+			continue;
+
+		if (addattr_l(n, maxlen, rta->rta_type,
+			      RTA_DATA(rta), RTA_PAYLOAD(rta)))
+			return -EMSGSIZE;
+	}
+
+	return 0;
+}
+
+static int ipstats_ifsm_len(const struct nlmsghdr *n)
+{
+	return n->nlmsg_len - NLMSG_LENGTH(sizeof(struct if_stats_msg));
+}
+
+static struct nlmsghdr *ipstats_msg_merge(const struct nlmsghdr *a,
+					  const struct nlmsghdr *b, int *err)
+{
+	int hdrlen = NLMSG_LENGTH(sizeof(struct if_stats_msg));
+	int maxlen = a->nlmsg_len + b->nlmsg_len;
+	struct nlmsghdr *n;
+
+	n = calloc(1, maxlen);
+	if (n == NULL) {
+		fprintf(stderr, "Error merging netlink answer: %s\n",
+			strerror(errno));
+		*err = -errno;
+		return NULL;
+	}
+
+	memcpy(n, a, hdrlen);
+	n->nlmsg_len = hdrlen;
+
+	*err = ipstats_merge_rtas(n, maxlen,
+				  IFLA_STATS_RTA(NLMSG_DATA(a)),
+				  ipstats_ifsm_len(a),
+				  IFLA_STATS_RTA(NLMSG_DATA(b)),
+				  ipstats_ifsm_len(b), 0);
+	if (*err) {
+		free(n);
+		return NULL;
+	}
+
+	return n;
+}
+
+struct ipstats_dump_ctx {
+	struct ipstats_stat_enabled *enabled;
+	struct nlmsghdr *pending;
+};
+
+static int ipstats_dump_pending(struct ipstats_dump_ctx *ctx)
 {
-	struct ipstats_stat_enabled *enabled = arg;
+	struct nlmsghdr *n = ctx->pending;
 	int rc;
 
-	rc = ipstats_process_ifsm(stdout, n, enabled);
+	if (n == NULL)
+		return 0;
+
+	ctx->pending = NULL;
+	rc = ipstats_process_ifsm(stdout, n, ctx->enabled);
+	free(n);
 	if (rc)
 		return rc;
 
@@ -863,8 +1076,63 @@ static int ipstats_dump_one(struct nlmsghdr *n, void *arg)
 	return 0;
 }
 
+static int ipstats_dump_one(struct nlmsghdr *n, void *arg)
+{
+	struct ipstats_dump_ctx *ctx = arg;
+	struct nlmsghdr *merged;
+	struct nlmsghdr *copy;
+	int rc;
+
+	if (ipstats_ifsm_len(n) < 0) {
+		fprintf(stderr, "BUG: wrong nlmsg len %d\n", n->nlmsg_len);
+		return -EINVAL;
+	}
+
+	if (ctx->pending != NULL) {
+		const struct if_stats_msg *pending = NLMSG_DATA(ctx->pending);
+		const struct if_stats_msg *ifsm = NLMSG_DATA(n);
+
+		if (pending->ifindex == ifsm->ifindex) {
+			merged = ipstats_msg_merge(ctx->pending, n, &rc);
+			if (merged != NULL) {
+				free(ctx->pending);
+				ctx->pending = merged;
+				return 0;
+			}
+			if (rc != -EOPNOTSUPP)
+				return rc;
+
+			/* An attribute layout that cannot be reassembled:
+			 * show the two halves separately, like older
+			 * versions did, rather than dropping either.
+			 */
+			fprintf(stderr,
+				"Cannot merge statistics of ifindex %d split across messages\n",
+				ifsm->ifindex);
+		}
+
+		rc = ipstats_dump_pending(ctx);
+		if (rc)
+			return rc;
+	}
+
+	copy = malloc(n->nlmsg_len);
+	if (copy == NULL) {
+		fprintf(stderr, "Error saving netlink answer: %s\n",
+			strerror(errno));
+		return -ENOMEM;
+	}
+
+	memcpy(copy, n, n->nlmsg_len);
+	ctx->pending = copy;
+	return 0;
+}
+
 static int ipstats_dump(struct ipstats_stat_enabled *enabled)
 {
+	struct ipstats_dump_ctx ctx = {
+		.enabled = enabled,
+	};
 	int rc = 0;
 
 	if (rtnl_statsdump_req_filter(&rth, PF_UNSPEC, 0,
@@ -874,11 +1142,15 @@ static int ipstats_dump(struct ipstats_stat_enabled *enabled)
 		return -2;
 	}
 
-	if (rtnl_dump_filter(&rth, ipstats_dump_one, enabled) < 0) {
+	if (rtnl_dump_filter(&rth, ipstats_dump_one, &ctx) < 0) {
 		fprintf(stderr, "Dump terminated\n");
 		rc = -2;
 	}
 
+	if (rc == 0)
+		rc = ipstats_dump_pending(&ctx);
+
+	free(ctx.pending);
 	return rc;
 }
 
-- 
2.55.0

[PATCH 2/2] ip: ipstats: Do not hide HW statistics when hw_stats_info is missing

[Alexander Zubkov]

ipstats_show_hw_stats() returns as soon as the
IFLA_OFFLOAD_XSTATS_HW_S_INFO attribute is absent, silently dropping the
counters that the message does carry. __ipstats_show_hw_stats() already
copes with a NULL info attribute, so let it, and skip the record only
when neither attribute is present.

There is no known way to reach this through "ip stats": the l3_stats
descriptor always requests HW_S_INFO next to the counters, and since the
previous patch the two are reassembled even when the kernel splits them
across messages. This is a hardening measure so that a message carrying
just the counters is displayed rather than silently discarded.

Assisted-by: Claude:claude-opus-5
Signed-off-by: Alexander Zubkov <green@qrator.net>
---
 ip/ipstats.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/ip/ipstats.c b/ip/ipstats.c
index c531885f..54b8267d 100644
--- a/ip/ipstats.c
+++ b/ip/ipstats.c
@@ -472,13 +472,16 @@ static int ipstats_show_hw_stats(struct ipstats_stat_show_attrs *attrs,
 	int err = 0;
 
 	at_hwsi = ipstats_stat_show_get_attr(attrs, group, hw_s_info, &err);
-	if (at_hwsi == NULL)
+	if (at_hwsi == NULL && err != 0)
 		return err;
 
 	at_stats = ipstats_stat_show_get_attr(attrs, group, hw_stats, &err);
 	if (at_stats == NULL && err != 0)
 		return err;
 
+	if (at_hwsi == NULL && at_stats == NULL)
+		return 0;
+
 	return __ipstats_show_hw_stats(at_hwsi, at_stats, idx);
 }
 
-- 
2.55.0

Re: [PATCH 1/2] ip: ipstats: Merge statistics split across several netlink messages

[Stephen Hemminger]

On Sun, 30 Aug 2026 20:19:48 +0200
Alexander Zubkov <green@qrator.net> wrote:

> A netlink dump can split the statistics of a single interface across
> several messages. When an attribute does not fit into the current skb,
> rtnl_fill_statsinfo() keeps the partial message, records how far it got
> in idxattr/prividx, and the dump is resumed at the same ifindex, with
> the remaining attributes emitted in the next message.
> 
> "ip stats show group offload subgroup l3_stats" requests both
> IFLA_OFFLOAD_XSTATS_HW_S_INFO and IFLA_OFFLOAD_XSTATS_L3_STATS, and
> rtnl_offload_xstats_fill() emits them one by one, so a dump over a
> sufficient number of netdevices regularly ends up cut in between the
> two. Each message is currently formatted on its own, which yields two
> bogus records instead of one, and, because ipstats_show_hw_stats() bails
> out when the HW_S_INFO attribute is missing, the counters carried by the
> second message are dropped:
> 
>     109: vlan859: group offload subgroup l3_stats on used on
> 
>     109: vlan859: group offload subgroup l3_stats
> 
> Postpone formatting until the whole object has been received: keep the
> message around, merge into it any immediately following message that
> carries the same ifindex, and format the result once a message for
> another ifindex arrives or the dump ends. At most one interface worth of
> statistics is buffered at a time.
> 
> Merging the two messages means merging the nests that were open when the
> first one was cut short and that got reopened in the second one. Netlink
> does not mark nests reliably here -- nla_nest_start_noflag() is used --
> so they are recognized by the message layout instead: a reopened nest
> appears exactly once in either message, and at the outer level is known
> to be a nest by ipstats_stat_ifla_max[]. That knowledge only reaches as
> deep as ipstats.c parses the message, so merging stops at the group
> nests and their contents are appended in the order they arrived, which
> is correct both for a deeper nest reopened between two of its children
> and for an array of same-type attributes, as bridge per-VLAN xstats are.
> 
> A layout that does not fit the above cannot be told apart from an array
> of same-type attributes, and reassembling it would corrupt the result.
> Such a message pair is refused with -EOPNOTSUPP and the two halves are
> formatted separately, as before this patch. Failures to allocate or to
> build the merged message are not papered over that way and terminate the
> dump.
> 
> Fixes: 54d82b0699a0 ("ip: Add a new family of commands, "stats"")
> Assisted-by: Claude:claude-opus-5
> Signed-off-by: Alexander Zubkov <green@qrator.net>
> ---
>  ip/ipstats.c | 280 ++++++++++++++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 276 insertions(+), 4 deletions(-)
> 
> diff --git a/ip/ipstats.c b/ip/ipstats.c
> index 8a2ac85e..c531885f 100644
> --- a/ip/ipstats.c
> +++ b/ip/ipstats.c
> @@ -850,12 +850,225 @@ ipstats_show_one(int ifindex, struct ipstats_stat_enabled *enabled)
>  	return err;
>  }
>  
> -static int ipstats_dump_one(struct nlmsghdr *n, void *arg)
> +/* A netlink dump can split the statistics of one interface across several
> + * messages: when an attribute does not fit into the current skb, the kernel
> + * ends the message and resumes the dump at the same ifindex, emitting the
> + * remaining attributes in the next message. See rtnl_fill_statsinfo() and
> + * rtnl_offload_xstats_fill() in the kernel.
> + *
> + * Reassembling the two messages means merging the nests that were open when
> + * the first message was cut short and got reopened in the second one. Netlink
> + * does not mark nests reliably -- nla_nest_start_noflag() is used here -- so
> + * they are recognized by the message layout instead: a nest that was reopened
> + * appears exactly once in either message, and, at the outer level, is known to
> + * be a nest by ipstats_stat_ifla_max[].
> + *
> + * That knowledge only reaches as deep as ipstats.c parses the message, so
> + * merging stops at the group nests, and their contents are appended in the
> + * order they arrived. Appending is the right thing to do whether a deeper nest
> + * was reopened between two of its children or the children are just an array
> + * of same-type attributes, as bridge per-VLAN xstats are. It does the wrong
> + * thing only if a kernel splits a nest that ipstats.c does not know how to
> + * show either, in which case the merge is not the first thing that needs
> + * updating.
> + */
> +

The patch makes sense but please stop with the AI text spew in comments and
commit message. This is garbage. If you insist on using AI to write stuff,
then tell it to be terse and succinct. We don't need three paragraphs here.
One sentence will do fine.

Try again

Re: [PATCH iproute2 0/2] ip: ipstats: Fix statistics split across netlink messages

[Stephen Hemminger]

On Sun, 30 Aug 2026 20:19:47 +0200
Alexander Zubkov <green@qrator.net> wrote:

> When dumping statistics, the kernel may split the statistics of a single
> interface across several netlink messages: if an attribute does not fit
> into the current skb, rtnl_fill_statsinfo() keeps the partial message and
> the dump is resumed at the same ifindex. "ip stats" formats every message
> on its own, so such an interface is reported twice and part of its
> statistics is lost. With "group offload subgroup l3_stats", which requests
> two attributes that the kernel emits one by one, this is easy to hit on a
> box with many netdevices:
> 
>     109: vlan859: group offload subgroup l3_stats on used on
> 
>     109: vlan859: group offload subgroup l3_stats
> 
> The first record holds IFLA_OFFLOAD_XSTATS_HW_S_INFO, the second one
> holds IFLA_OFFLOAD_XSTATS_L3_STATS, and the counters are never shown.
> 
> Patch 1 reassembles such messages before formatting them. Patch 2 is an
> unrelated hardening cleanup and can be dropped.
> 
> The merge itself was also exercised out of tree against the two message
> shapes the kernel can split, offload xstats cut between HW_S_INFO and
> L3_STATS, and bridge per-VLAN xstats cut between two BRIDGE_XSTATS_VLAN
> entries, plus a layout that must be refused rather than merged.
> 
> I faced the issue on kernel 7.1.5 / iproute2-7.0.0, mlxsw switch, with
> ~120 netdevices. The split was visible in an strace as two RTM_NEWSTATS
> messages with the same ifindex, carrying IFLA_OFFLOAD_XSTATS_HW_S_INFO
> and IFLA_OFFLOAD_XSTATS_L3_STATS respectively. The further research,
> patches and supporting texts was prepared with the help of an AI
> assistant. I reviewed and tested the patches against iproute2-7.1.0, with
> the proposed patches the interface is correctly reported once, with its
> counters. Although the patches makes sense to me, I have little
> experience with netlink handling.
> 
> 
> Alexander Zubkov (2):
>   ip: ipstats: Merge statistics split across several netlink messages
>   ip: ipstats: Do not hide HW statistics when hw_stats_info is missing
> 
>  ip/ipstats.c | 285 ++++++++++++++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 280 insertions(+), 5 deletions(-)
> 


Please write patches for humans not AI.
Tell Claude to be concise and terse and follow patterns of other code
in the same project.

Re: [PATCH 1/2] ip: ipstats: Merge statistics split across several netlink messages

[Petr Machata]

Alexander Zubkov <green@qrator.net> writes:

> A netlink dump can split the statistics of a single interface across
> several messages. When an attribute does not fit into the current skb,
> rtnl_fill_statsinfo() keeps the partial message, records how far it got
> in idxattr/prividx, and the dump is resumed at the same ifindex, with
> the remaining attributes emitted in the next message.
>
> "ip stats show group offload subgroup l3_stats" requests both

The reproducer I had to use was

    ip stats show group offload | grep 'subgroup l3_stats' | less

No amount of interfaces (tried up to 4K VLAN netdevices) triggered the
issue when I requested 'group offload subgroup l3_stats'.

I guess I should have enabled some of those L3 counters :)

> IFLA_OFFLOAD_XSTATS_HW_S_INFO and IFLA_OFFLOAD_XSTATS_L3_STATS, and
> rtnl_offload_xstats_fill() emits them one by one, so a dump over a
> sufficient number of netdevices regularly ends up cut in between the
> two. Each message is currently formatted on its own, which yields two
> bogus records instead of one, and, because ipstats_show_hw_stats() bails
> out when the HW_S_INFO attribute is missing, the counters carried by the
> second message are dropped:
>
>     109: vlan859: group offload subgroup l3_stats on used on
>
>     109: vlan859: group offload subgroup l3_stats
>
> Postpone formatting until the whole object has been received: keep the
> message around, merge into it any immediately following message that
> carries the same ifindex, and format the result once a message for
> another ifindex arrives or the dump ends. At most one interface worth of
> statistics is buffered at a time.
>
> Merging the two messages means merging the nests that were open when the
> first one was cut short and that got reopened in the second one. Netlink
> does not mark nests reliably here -- nla_nest_start_noflag() is used --
> so they are recognized by the message layout instead: a reopened nest
> appears exactly once in either message, and at the outer level is known
> to be a nest by ipstats_stat_ifla_max[]. That knowledge only reaches as
> deep as ipstats.c parses the message, so merging stops at the group
> nests and their contents are appended in the order they arrived, which
> is correct both for a deeper nest reopened between two of its children
> and for an array of same-type attributes, as bridge per-VLAN xstats are.
>
> A layout that does not fit the above cannot be told apart from an array
> of same-type attributes, and reassembling it would corrupt the result.
> Such a message pair is refused with -EOPNOTSUPP and the two halves are
> formatted separately, as before this patch. Failures to allocate or to
> build the merged message are not papered over that way and terminate the
> dump.
>
> Fixes: 54d82b0699a0 ("ip: Add a new family of commands, "stats"")
> Assisted-by: Claude:claude-opus-5
> Signed-off-by: Alexander Zubkov <green@qrator.net>

The patch looks broadly OK to me, including the commit message, which I
see Stephen isn't too fond of. I agree the comments are way too verbose.
Some more commentary below.

> ---
>  ip/ipstats.c | 280 ++++++++++++++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 276 insertions(+), 4 deletions(-)
>
> diff --git a/ip/ipstats.c b/ip/ipstats.c
> index 8a2ac85e..c531885f 100644
> --- a/ip/ipstats.c
> +++ b/ip/ipstats.c
> @@ -850,12 +850,225 @@ ipstats_show_one(int ifindex, struct ipstats_stat_enabled *enabled)
>  	return err;
>  }
>  
> -static int ipstats_dump_one(struct nlmsghdr *n, void *arg)

So the following comment is super long and nobody will read it. You need
to look into the code to figure out what's what anyway, and the comment
does not really help with that. So I'd keep this:

> +/* A netlink dump can split the statistics of one interface across several
> + * messages:

and drop all this:

>               when an attribute does not fit into the current skb, the kernel
> + * ends the message and resumes the dump at the same ifindex, emitting the
> + * remaining attributes in the next message. See rtnl_fill_statsinfo() and
> + * rtnl_offload_xstats_fill() in the kernel.
> + *
> + * Reassembling the two messages means merging the nests that were open when
> + * the first message was cut short and got reopened in the second one. Netlink
> + * does not mark nests reliably -- nla_nest_start_noflag() is used here -- so
> + * they are recognized by the message layout instead: a nest that was reopened
> + * appears exactly once in either message, and, at the outer level, is known to
> + * be a nest by ipstats_stat_ifla_max[].
> + *
> + * That knowledge only reaches as deep as ipstats.c parses the message, so
> + * merging stops at the group nests, and their contents are appended in the
> + * order they arrived. Appending is the right thing to do whether a deeper nest
> + * was reopened between two of its children or the children are just an array
> + * of same-type attributes, as bridge per-VLAN xstats are. It does the wrong
> + * thing only if a kernel splits a nest that ipstats.c does not know how to
> + * show either, in which case the merge is not the first thing that needs
> + * updating.
> + */

Writing is kinda Claude's thing, so it does it a lot, it comments
everything like crazy, inlines whole functions instead of reusing, etc.
I find myself having to moderate its code a lot because of this.

> +
> +enum {
> +	/* Merge the outer level and the group nests, append below that. */
> +	IPSTATS_MERGE_MAX_DEPTH = 1,
> +};
> +
> +static struct rtattr *ipstats_rta_find(struct rtattr *rtas, int len,
> +				       unsigned short type, unsigned int *count)
> +{
> +	struct rtattr *found = NULL;
> +	struct rtattr *rta;
> +
> +	*count = 0;
> +	for (rta = rtas; RTA_OK(rta, len); rta = RTA_NEXT(rta, len)) {
> +		if (rta->rta_type != type)
> +			continue;
> +		if (found == NULL)
> +			found = rta;
> +		(*count)++;
> +	}
> +
> +	return found;
> +}
> +
> +/* Whether the two messages can be reassembled at this attribute, and how.
> + * MERGE means RTA and OTHER are the two halves of one nest that the first
> + * message left open. APPEND means the attribute belongs to one message only
> + * and is copied over as it is. REFUSE means the type is present in both
> + * messages in a layout that cannot be told apart from an array of same-type
> + * attributes, so reassembling it would corrupt the result.
> + */

Drop the comment. It's clear from the code and the names what it does.

> +enum ipstats_merge_action {
> +	IPSTATS_MERGE_APPEND,
> +	IPSTATS_MERGE_MERGE,
> +	IPSTATS_MERGE_REFUSE,
> +};
> +
> +static enum ipstats_merge_action
> +ipstats_merge_action(const struct rtattr *rta, unsigned int nrta,
> +		     const struct rtattr *other, unsigned int nother, int depth)
> +{
> +	if (rta == NULL || other == NULL)
> +		return IPSTATS_MERGE_APPEND;

This is one useful tidbit from the wall of text above, and it should be
here, not there:

	/* If we have >1 hit for an attribute, it's an array of
         * same-type attributes. Bail out. */
> +	if (nrta != 1 || nother != 1)
> +		return IPSTATS_MERGE_REFUSE;
> +
> +	if (depth == 0) {

Hmm, I suspect this depth == 0 and the above IPSTATS_MERGE_MAX_DEPTH == 1
are correlated? We can't validate attribute type in ipstats_merge_action
at depth > 0, so it's unnecessary to recurse into ipstats_merge_rtas()
beyond and and we can optimize this case by just copying all the
attributes right away through ipstats_copy_rtas.

If this just returned an IPSTATS_MERGE_APPEND in an else branch, then we
wouldn't need ipstats_copy_rtas(), IPSTATS_MERGE_MAX_DEPTH, and the
related special casing.

> +		unsigned int type = rta->rta_type;
> +
> +		if (type >= ARRAY_SIZE(ipstats_stat_ifla_max) ||
> +		    ipstats_stat_ifla_max[type] == 0)
> +			return IPSTATS_MERGE_REFUSE;
> +	}
(	  else {
		return IPSTATS_MERGE_APPEND;
	})

Maybe the branch needs to be elsewhere for actual array calls. I took a
code modified like this for a spin, and got the same output, but I
wasn't trying to cover all counter types, so I might be missing
something.

Plus the optimized copy saves the find calls, which is not nothing.

But in any case I don't like how we end up with this
IPSTATS_MERGE_MAX_DEPTH pretending to be all configurable when in fact
the only value that makes sense is 1.

> +
> +	return IPSTATS_MERGE_MERGE;
> +}
> +
> +static int ipstats_copy_rtas(struct nlmsghdr *n, int maxlen,
> +			     struct rtattr *rtas, int len)
> +{
> +	struct rtattr *rta;
> +
> +	for (rta = rtas; RTA_OK(rta, len); rta = RTA_NEXT(rta, len)) {
> +		if (addattr_l(n, maxlen, rta->rta_type,
> +			      RTA_DATA(rta), RTA_PAYLOAD(rta)))
> +			return -EMSGSIZE;
> +	}
> +
> +	return 0;
> +}
> +
> +static int ipstats_merge_rtas(struct nlmsghdr *n, int maxlen,
> +			      struct rtattr *a, int alen,
> +			      struct rtattr *b, int blen, int depth)
> +{
> +	struct rtattr *rta;
> +	int rem;
> +	int err;
> +
> +	for (rta = a, rem = alen; RTA_OK(rta, rem); rta = RTA_NEXT(rta, rem)) {
> +		unsigned int nrta, nother;
> +		struct rtattr *other;
> +		struct rtattr *nest;
> +
> +		other = ipstats_rta_find(b, blen, rta->rta_type, &nother);
> +		ipstats_rta_find(a, alen, rta->rta_type, &nrta);
> +
> +		switch (ipstats_merge_action(rta, nrta, other, nother, depth)) {
> +		case IPSTATS_MERGE_REFUSE:
> +			return -EOPNOTSUPP;
> +		case IPSTATS_MERGE_APPEND:
> +			if (addattr_l(n, maxlen, rta->rta_type,
> +				      RTA_DATA(rta), RTA_PAYLOAD(rta)))
> +				return -EMSGSIZE;
> +			continue;
> +		case IPSTATS_MERGE_MERGE:
> +			break;
> +		}
> +
> +		nest = addattr_nest(n, maxlen, rta->rta_type);
> +		if (nest == NULL)
> +			return -EMSGSIZE;
> +
> +		if (depth < IPSTATS_MERGE_MAX_DEPTH) {
> +			err = ipstats_merge_rtas(n, maxlen,
> +						 RTA_DATA(rta), RTA_PAYLOAD(rta),
> +						 RTA_DATA(other),
> +						 RTA_PAYLOAD(other), depth + 1);
> +		} else {
> +			err = ipstats_copy_rtas(n, maxlen, RTA_DATA(rta),
> +						RTA_PAYLOAD(rta));
> +			if (!err)
> +				err = ipstats_copy_rtas(n, maxlen,
> +							RTA_DATA(other),
> +							RTA_PAYLOAD(other));
> +		}
> +		if (err)
> +			return err;
> +
> +		addattr_nest_end(n, nest);
> +	}
> +
> +	for (rta = b, rem = blen; RTA_OK(rta, rem); rta = RTA_NEXT(rta, rem)) {
> +		unsigned int nrta, nother;
> +		struct rtattr *other;
> +
> +		other = ipstats_rta_find(a, alen, rta->rta_type, &nother);
> +		ipstats_rta_find(b, blen, rta->rta_type, &nrta);
> +
> +		/* Whatever is present in both messages has been dealt with
> +		 * in the loop above.
> +		 */
> +		if (ipstats_merge_action(rta, nrta, other, nother,
> +					 depth) != IPSTATS_MERGE_APPEND)
> +			continue;
> +
> +		if (addattr_l(n, maxlen, rta->rta_type,
> +			      RTA_DATA(rta), RTA_PAYLOAD(rta)))
> +			return -EMSGSIZE;
> +	}
> +
> +	return 0;
> +}
> +
> +static int ipstats_ifsm_len(const struct nlmsghdr *n)
> +{
> +	return n->nlmsg_len - NLMSG_LENGTH(sizeof(struct if_stats_msg));
> +}
> +
> +static struct nlmsghdr *ipstats_msg_merge(const struct nlmsghdr *a,
> +					  const struct nlmsghdr *b, int *err)
> +{
> +	int hdrlen = NLMSG_LENGTH(sizeof(struct if_stats_msg));
> +	int maxlen = a->nlmsg_len + b->nlmsg_len;
> +	struct nlmsghdr *n;
> +
> +	n = calloc(1, maxlen);
> +	if (n == NULL) {
> +		fprintf(stderr, "Error merging netlink answer: %s\n",
> +			strerror(errno));
> +		*err = -errno;
> +		return NULL;
> +	}
> +
> +	memcpy(n, a, hdrlen);
> +	n->nlmsg_len = hdrlen;
> +
> +	*err = ipstats_merge_rtas(n, maxlen,
> +				  IFLA_STATS_RTA(NLMSG_DATA(a)),
> +				  ipstats_ifsm_len(a),
> +				  IFLA_STATS_RTA(NLMSG_DATA(b)),
> +				  ipstats_ifsm_len(b), 0);
> +	if (*err) {
> +		free(n);
> +		return NULL;
> +	}
> +
> +	return n;
> +}
> +
> +struct ipstats_dump_ctx {
> +	struct ipstats_stat_enabled *enabled;
> +	struct nlmsghdr *pending;
> +};
> +
> +static int ipstats_dump_pending(struct ipstats_dump_ctx *ctx)
>  {
> -	struct ipstats_stat_enabled *enabled = arg;
> +	struct nlmsghdr *n = ctx->pending;
>  	int rc;
>  
> -	rc = ipstats_process_ifsm(stdout, n, enabled);
> +	if (n == NULL)
> +		return 0;
> +
> +	ctx->pending = NULL;
> +	rc = ipstats_process_ifsm(stdout, n, ctx->enabled);
> +	free(n);
>  	if (rc)
>  		return rc;
>  
> @@ -863,8 +1076,63 @@ static int ipstats_dump_one(struct nlmsghdr *n, void *arg)
>  	return 0;
>  }
>  
> +static int ipstats_dump_one(struct nlmsghdr *n, void *arg)
> +{
> +	struct ipstats_dump_ctx *ctx = arg;
> +	struct nlmsghdr *merged;
> +	struct nlmsghdr *copy;
> +	int rc;
> +
> +	if (ipstats_ifsm_len(n) < 0) {
> +		fprintf(stderr, "BUG: wrong nlmsg len %d\n", n->nlmsg_len);
> +		return -EINVAL;
> +	}
> +
> +	if (ctx->pending != NULL) {
> +		const struct if_stats_msg *pending = NLMSG_DATA(ctx->pending);
> +		const struct if_stats_msg *ifsm = NLMSG_DATA(n);
> +
> +		if (pending->ifindex == ifsm->ifindex) {
> +			merged = ipstats_msg_merge(ctx->pending, n, &rc);
> +			if (merged != NULL) {
> +				free(ctx->pending);
> +				ctx->pending = merged;
> +				return 0;
> +			}
> +			if (rc != -EOPNOTSUPP)
> +				return rc;
> +
> +			/* An attribute layout that cannot be reassembled:
> +			 * show the two halves separately, like older
> +			 * versions did, rather than dropping either.
> +			 */
> +			fprintf(stderr,
> +				"Cannot merge statistics of ifindex %d split across messages\n",
> +				ifsm->ifindex);
> +		}
> +
> +		rc = ipstats_dump_pending(ctx);
> +		if (rc)
> +			return rc;
> +	}
> +
> +	copy = malloc(n->nlmsg_len);
> +	if (copy == NULL) {
> +		fprintf(stderr, "Error saving netlink answer: %s\n",
> +			strerror(errno));
> +		return -ENOMEM;
> +	}
> +
> +	memcpy(copy, n, n->nlmsg_len);
> +	ctx->pending = copy;
> +	return 0;
> +}
> +
>  static int ipstats_dump(struct ipstats_stat_enabled *enabled)
>  {
> +	struct ipstats_dump_ctx ctx = {
> +		.enabled = enabled,
> +	};
>  	int rc = 0;
>  
>  	if (rtnl_statsdump_req_filter(&rth, PF_UNSPEC, 0,
> @@ -874,11 +1142,15 @@ static int ipstats_dump(struct ipstats_stat_enabled *enabled)
>  		return -2;
>  	}
>  
> -	if (rtnl_dump_filter(&rth, ipstats_dump_one, enabled) < 0) {
> +	if (rtnl_dump_filter(&rth, ipstats_dump_one, &ctx) < 0) {
>  		fprintf(stderr, "Dump terminated\n");
>  		rc = -2;
>  	}
>  
> +	if (rc == 0)
> +		rc = ipstats_dump_pending(&ctx);
> +
> +	free(ctx.pending);
>  	return rc;
>  }

Re: [PATCH 2/2] ip: ipstats: Do not hide HW statistics when hw_stats_info is missing

[Petr Machata]

Alexander Zubkov <green@qrator.net> writes:

> ipstats_show_hw_stats() returns as soon as the
> IFLA_OFFLOAD_XSTATS_HW_S_INFO attribute is absent, silently dropping the
> counters that the message does carry. __ipstats_show_hw_stats() already
> copes with a NULL info attribute, so let it, and skip the record only
> when neither attribute is present.
>
> There is no known way to reach this through "ip stats": the l3_stats
> descriptor always requests HW_S_INFO next to the counters, and since the
> previous patch the two are reassembled even when the kernel splits them
> across messages. This is a hardening measure so that a message carrying
> just the counters is displayed rather than silently discarded.
>
> Assisted-by: Claude:claude-opus-5
> Signed-off-by: Alexander Zubkov <green@qrator.net>
> ---
>  ip/ipstats.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
>
> diff --git a/ip/ipstats.c b/ip/ipstats.c
> index c531885f..54b8267d 100644
> --- a/ip/ipstats.c
> +++ b/ip/ipstats.c
> @@ -472,13 +472,16 @@ static int ipstats_show_hw_stats(struct ipstats_stat_show_attrs *attrs,
>  	int err = 0;
>  
>  	at_hwsi = ipstats_stat_show_get_attr(attrs, group, hw_s_info, &err);
> -	if (at_hwsi == NULL)
> +	if (at_hwsi == NULL && err != 0)
>  		return err;

I am pretty sure this is deliberate. The logic is, if we do not even get
hw_s_info, then surely we will not get hw_stats either, because the
kernel is supposed to provide information about the statistics together
with the statistics themselves.

Now this did not take into account that messages might be split. But
with your patch, this is will not be an issue anymore either.

On second though, it is not the task of iproute2 to policy the kernel,
and when life gives you hw_stats but no hw_s_info, you make hw_stats'ade
or whatever. So I actually think the patch is OK.

So,
Reviewed-by: Petr Machata <me@pmachata.org>

>  
>  	at_stats = ipstats_stat_show_get_attr(attrs, group, hw_stats, &err);
>  	if (at_stats == NULL && err != 0)
>  		return err;
>  
> +	if (at_hwsi == NULL && at_stats == NULL)
> +		return 0;
> +
>  	return __ipstats_show_hw_stats(at_hwsi, at_stats, idx);
>  }

Re: [PATCH 1/2] ip: ipstats: Merge statistics split across several netlink messages

[Alexander Zubkov]

Hi guys,

Thanks for the feedback. I wasn't sure how much explanation is ok
here, that is why I passed it as is. I agree that AI agents are way
too verbose. I'll try to get/make a more concise version. I'll need to
post v2 patches here, right?

On Tue, Sep 1, 2026 at 5:12 PM Petr Machata <petrm@nvidia.com> wrote:
>
> Alexander Zubkov <green@qrator.net> writes:
>
> > A netlink dump can split the statistics of a single interface across
> > several messages. When an attribute does not fit into the current skb,
> > rtnl_fill_statsinfo() keeps the partial message, records how far it got
> > in idxattr/prividx, and the dump is resumed at the same ifindex, with
> > the remaining attributes emitted in the next message.
> >
> > "ip stats show group offload subgroup l3_stats" requests both
>
> The reproducer I had to use was
>
>     ip stats show group offload | grep 'subgroup l3_stats' | less
>
> No amount of interfaces (tried up to 4K VLAN netdevices) triggered the
> issue when I requested 'group offload subgroup l3_stats'.
>
> I guess I should have enabled some of those L3 counters :)

Probably yes. Because I do not have a lot of interfaces, 119 to be
precise, and first 90 of them do not have l3_stats enabled. And all of
them fit into 2 netlink reads, problem interface is number 109. So I
do not think much of them is required to hit the issue, but some bad
luck so that one of them gets fragmented.

>
> > IFLA_OFFLOAD_XSTATS_HW_S_INFO and IFLA_OFFLOAD_XSTATS_L3_STATS, and
> > rtnl_offload_xstats_fill() emits them one by one, so a dump over a
> > sufficient number of netdevices regularly ends up cut in between the
> > two. Each message is currently formatted on its own, which yields two
> > bogus records instead of one, and, because ipstats_show_hw_stats() bails
> > out when the HW_S_INFO attribute is missing, the counters carried by the
> > second message are dropped:
> >
> >     109: vlan859: group offload subgroup l3_stats on used on
> >
> >     109: vlan859: group offload subgroup l3_stats
> >
> > Postpone formatting until the whole object has been received: keep the
> > message around, merge into it any immediately following message that
> > carries the same ifindex, and format the result once a message for
> > another ifindex arrives or the dump ends. At most one interface worth of
> > statistics is buffered at a time.
> >
> > Merging the two messages means merging the nests that were open when the
> > first one was cut short and that got reopened in the second one. Netlink
> > does not mark nests reliably here -- nla_nest_start_noflag() is used --
> > so they are recognized by the message layout instead: a reopened nest
> > appears exactly once in either message, and at the outer level is known
> > to be a nest by ipstats_stat_ifla_max[]. That knowledge only reaches as
> > deep as ipstats.c parses the message, so merging stops at the group
> > nests and their contents are appended in the order they arrived, which
> > is correct both for a deeper nest reopened between two of its children
> > and for an array of same-type attributes, as bridge per-VLAN xstats are.
> >
> > A layout that does not fit the above cannot be told apart from an array
> > of same-type attributes, and reassembling it would corrupt the result.
> > Such a message pair is refused with -EOPNOTSUPP and the two halves are
> > formatted separately, as before this patch. Failures to allocate or to
> > build the merged message are not papered over that way and terminate the
> > dump.
> >
> > Fixes: 54d82b0699a0 ("ip: Add a new family of commands, "stats"")
> > Assisted-by: Claude:claude-opus-5
> > Signed-off-by: Alexander Zubkov <green@qrator.net>
>
> The patch looks broadly OK to me, including the commit message, which I
> see Stephen isn't too fond of. I agree the comments are way too verbose.
> Some more commentary below.
>
> > ---
> >  ip/ipstats.c | 280 ++++++++++++++++++++++++++++++++++++++++++++++++++-
> >  1 file changed, 276 insertions(+), 4 deletions(-)
> >
> > diff --git a/ip/ipstats.c b/ip/ipstats.c
> > index 8a2ac85e..c531885f 100644
> > --- a/ip/ipstats.c
> > +++ b/ip/ipstats.c
> > @@ -850,12 +850,225 @@ ipstats_show_one(int ifindex, struct ipstats_stat_enabled *enabled)
> >       return err;
> >  }
> >
> > -static int ipstats_dump_one(struct nlmsghdr *n, void *arg)
>
> So the following comment is super long and nobody will read it. You need
> to look into the code to figure out what's what anyway, and the comment
> does not really help with that. So I'd keep this:
>
> > +/* A netlink dump can split the statistics of one interface across several
> > + * messages:
>
> and drop all this:
>
> >               when an attribute does not fit into the current skb, the kernel
> > + * ends the message and resumes the dump at the same ifindex, emitting the
> > + * remaining attributes in the next message. See rtnl_fill_statsinfo() and
> > + * rtnl_offload_xstats_fill() in the kernel.
> > + *
> > + * Reassembling the two messages means merging the nests that were open when
> > + * the first message was cut short and got reopened in the second one. Netlink
> > + * does not mark nests reliably -- nla_nest_start_noflag() is used here -- so
> > + * they are recognized by the message layout instead: a nest that was reopened
> > + * appears exactly once in either message, and, at the outer level, is known to
> > + * be a nest by ipstats_stat_ifla_max[].
> > + *
> > + * That knowledge only reaches as deep as ipstats.c parses the message, so
> > + * merging stops at the group nests, and their contents are appended in the
> > + * order they arrived. Appending is the right thing to do whether a deeper nest
> > + * was reopened between two of its children or the children are just an array
> > + * of same-type attributes, as bridge per-VLAN xstats are. It does the wrong
> > + * thing only if a kernel splits a nest that ipstats.c does not know how to
> > + * show either, in which case the merge is not the first thing that needs
> > + * updating.
> > + */
>
> Writing is kinda Claude's thing, so it does it a lot, it comments
> everything like crazy, inlines whole functions instead of reusing, etc.
> I find myself having to moderate its code a lot because of this.
>
> > +
> > +enum {
> > +     /* Merge the outer level and the group nests, append below that. */
> > +     IPSTATS_MERGE_MAX_DEPTH = 1,
> > +};
> > +
> > +static struct rtattr *ipstats_rta_find(struct rtattr *rtas, int len,
> > +                                    unsigned short type, unsigned int *count)
> > +{
> > +     struct rtattr *found = NULL;
> > +     struct rtattr *rta;
> > +
> > +     *count = 0;
> > +     for (rta = rtas; RTA_OK(rta, len); rta = RTA_NEXT(rta, len)) {
> > +             if (rta->rta_type != type)
> > +                     continue;
> > +             if (found == NULL)
> > +                     found = rta;
> > +             (*count)++;
> > +     }
> > +
> > +     return found;
> > +}
> > +
> > +/* Whether the two messages can be reassembled at this attribute, and how.
> > + * MERGE means RTA and OTHER are the two halves of one nest that the first
> > + * message left open. APPEND means the attribute belongs to one message only
> > + * and is copied over as it is. REFUSE means the type is present in both
> > + * messages in a layout that cannot be told apart from an array of same-type
> > + * attributes, so reassembling it would corrupt the result.
> > + */
>
> Drop the comment. It's clear from the code and the names what it does.
>
> > +enum ipstats_merge_action {
> > +     IPSTATS_MERGE_APPEND,
> > +     IPSTATS_MERGE_MERGE,
> > +     IPSTATS_MERGE_REFUSE,
> > +};
> > +
> > +static enum ipstats_merge_action
> > +ipstats_merge_action(const struct rtattr *rta, unsigned int nrta,
> > +                  const struct rtattr *other, unsigned int nother, int depth)
> > +{
> > +     if (rta == NULL || other == NULL)
> > +             return IPSTATS_MERGE_APPEND;
>
> This is one useful tidbit from the wall of text above, and it should be
> here, not there:
>
>         /* If we have >1 hit for an attribute, it's an array of
>          * same-type attributes. Bail out. */
> > +     if (nrta != 1 || nother != 1)
> > +             return IPSTATS_MERGE_REFUSE;
> > +
> > +     if (depth == 0) {
>
> Hmm, I suspect this depth == 0 and the above IPSTATS_MERGE_MAX_DEPTH == 1
> are correlated? We can't validate attribute type in ipstats_merge_action
> at depth > 0, so it's unnecessary to recurse into ipstats_merge_rtas()
> beyond and and we can optimize this case by just copying all the
> attributes right away through ipstats_copy_rtas.
>
> If this just returned an IPSTATS_MERGE_APPEND in an else branch, then we
> wouldn't need ipstats_copy_rtas(), IPSTATS_MERGE_MAX_DEPTH, and the
> related special casing.

I can only tell now, that it first was merging attributes the other
way, than after some of my questions it found out that it would not
work for bridge vlan attributes, because there are some other sort of
merging is needed and produced this construction. But I do not
understood it much. I would need to invest some time to understand
those attributes and things. Probably it would be better anyway, I'll
try to figure it out.

>
> > +             unsigned int type = rta->rta_type;
> > +
> > +             if (type >= ARRAY_SIZE(ipstats_stat_ifla_max) ||
> > +                 ipstats_stat_ifla_max[type] == 0)
> > +                     return IPSTATS_MERGE_REFUSE;
> > +     }
> (         else {
>                 return IPSTATS_MERGE_APPEND;
>         })
>
> Maybe the branch needs to be elsewhere for actual array calls. I took a
> code modified like this for a spin, and got the same output, but I
> wasn't trying to cover all counter types, so I might be missing
> something.
>
> Plus the optimized copy saves the find calls, which is not nothing.
>
> But in any case I don't like how we end up with this
> IPSTATS_MERGE_MAX_DEPTH pretending to be all configurable when in fact
> the only value that makes sense is 1.
>
> > +
> > +     return IPSTATS_MERGE_MERGE;
> > +}
> > +
> > +static int ipstats_copy_rtas(struct nlmsghdr *n, int maxlen,
> > +                          struct rtattr *rtas, int len)
> > +{
> > +     struct rtattr *rta;
> > +
> > +     for (rta = rtas; RTA_OK(rta, len); rta = RTA_NEXT(rta, len)) {
> > +             if (addattr_l(n, maxlen, rta->rta_type,
> > +                           RTA_DATA(rta), RTA_PAYLOAD(rta)))
> > +                     return -EMSGSIZE;
> > +     }
> > +
> > +     return 0;
> > +}
> > +
> > +static int ipstats_merge_rtas(struct nlmsghdr *n, int maxlen,
> > +                           struct rtattr *a, int alen,
> > +                           struct rtattr *b, int blen, int depth)
> > +{
> > +     struct rtattr *rta;
> > +     int rem;
> > +     int err;
> > +
> > +     for (rta = a, rem = alen; RTA_OK(rta, rem); rta = RTA_NEXT(rta, rem)) {
> > +             unsigned int nrta, nother;
> > +             struct rtattr *other;
> > +             struct rtattr *nest;
> > +
> > +             other = ipstats_rta_find(b, blen, rta->rta_type, &nother);
> > +             ipstats_rta_find(a, alen, rta->rta_type, &nrta);
> > +
> > +             switch (ipstats_merge_action(rta, nrta, other, nother, depth)) {
> > +             case IPSTATS_MERGE_REFUSE:
> > +                     return -EOPNOTSUPP;
> > +             case IPSTATS_MERGE_APPEND:
> > +                     if (addattr_l(n, maxlen, rta->rta_type,
> > +                                   RTA_DATA(rta), RTA_PAYLOAD(rta)))
> > +                             return -EMSGSIZE;
> > +                     continue;
> > +             case IPSTATS_MERGE_MERGE:
> > +                     break;
> > +             }
> > +
> > +             nest = addattr_nest(n, maxlen, rta->rta_type);
> > +             if (nest == NULL)
> > +                     return -EMSGSIZE;
> > +
> > +             if (depth < IPSTATS_MERGE_MAX_DEPTH) {
> > +                     err = ipstats_merge_rtas(n, maxlen,
> > +                                              RTA_DATA(rta), RTA_PAYLOAD(rta),
> > +                                              RTA_DATA(other),
> > +                                              RTA_PAYLOAD(other), depth + 1);
> > +             } else {
> > +                     err = ipstats_copy_rtas(n, maxlen, RTA_DATA(rta),
> > +                                             RTA_PAYLOAD(rta));
> > +                     if (!err)
> > +                             err = ipstats_copy_rtas(n, maxlen,
> > +                                                     RTA_DATA(other),
> > +                                                     RTA_PAYLOAD(other));
> > +             }
> > +             if (err)
> > +                     return err;
> > +
> > +             addattr_nest_end(n, nest);
> > +     }
> > +
> > +     for (rta = b, rem = blen; RTA_OK(rta, rem); rta = RTA_NEXT(rta, rem)) {
> > +             unsigned int nrta, nother;
> > +             struct rtattr *other;
> > +
> > +             other = ipstats_rta_find(a, alen, rta->rta_type, &nother);
> > +             ipstats_rta_find(b, blen, rta->rta_type, &nrta);
> > +
> > +             /* Whatever is present in both messages has been dealt with
> > +              * in the loop above.
> > +              */
> > +             if (ipstats_merge_action(rta, nrta, other, nother,
> > +                                      depth) != IPSTATS_MERGE_APPEND)
> > +                     continue;
> > +
> > +             if (addattr_l(n, maxlen, rta->rta_type,
> > +                           RTA_DATA(rta), RTA_PAYLOAD(rta)))
> > +                     return -EMSGSIZE;
> > +     }
> > +
> > +     return 0;
> > +}
> > +
> > +static int ipstats_ifsm_len(const struct nlmsghdr *n)
> > +{
> > +     return n->nlmsg_len - NLMSG_LENGTH(sizeof(struct if_stats_msg));
> > +}
> > +
> > +static struct nlmsghdr *ipstats_msg_merge(const struct nlmsghdr *a,
> > +                                       const struct nlmsghdr *b, int *err)
> > +{
> > +     int hdrlen = NLMSG_LENGTH(sizeof(struct if_stats_msg));
> > +     int maxlen = a->nlmsg_len + b->nlmsg_len;
> > +     struct nlmsghdr *n;
> > +
> > +     n = calloc(1, maxlen);
> > +     if (n == NULL) {
> > +             fprintf(stderr, "Error merging netlink answer: %s\n",
> > +                     strerror(errno));
> > +             *err = -errno;
> > +             return NULL;
> > +     }
> > +
> > +     memcpy(n, a, hdrlen);
> > +     n->nlmsg_len = hdrlen;
> > +
> > +     *err = ipstats_merge_rtas(n, maxlen,
> > +                               IFLA_STATS_RTA(NLMSG_DATA(a)),
> > +                               ipstats_ifsm_len(a),
> > +                               IFLA_STATS_RTA(NLMSG_DATA(b)),
> > +                               ipstats_ifsm_len(b), 0);
> > +     if (*err) {
> > +             free(n);
> > +             return NULL;
> > +     }
> > +
> > +     return n;
> > +}
> > +
> > +struct ipstats_dump_ctx {
> > +     struct ipstats_stat_enabled *enabled;
> > +     struct nlmsghdr *pending;
> > +};
> > +
> > +static int ipstats_dump_pending(struct ipstats_dump_ctx *ctx)
> >  {
> > -     struct ipstats_stat_enabled *enabled = arg;
> > +     struct nlmsghdr *n = ctx->pending;
> >       int rc;
> >
> > -     rc = ipstats_process_ifsm(stdout, n, enabled);
> > +     if (n == NULL)
> > +             return 0;
> > +
> > +     ctx->pending = NULL;
> > +     rc = ipstats_process_ifsm(stdout, n, ctx->enabled);
> > +     free(n);
> >       if (rc)
> >               return rc;
> >
> > @@ -863,8 +1076,63 @@ static int ipstats_dump_one(struct nlmsghdr *n, void *arg)
> >       return 0;
> >  }
> >
> > +static int ipstats_dump_one(struct nlmsghdr *n, void *arg)
> > +{
> > +     struct ipstats_dump_ctx *ctx = arg;
> > +     struct nlmsghdr *merged;
> > +     struct nlmsghdr *copy;
> > +     int rc;
> > +
> > +     if (ipstats_ifsm_len(n) < 0) {
> > +             fprintf(stderr, "BUG: wrong nlmsg len %d\n", n->nlmsg_len);
> > +             return -EINVAL;
> > +     }
> > +
> > +     if (ctx->pending != NULL) {
> > +             const struct if_stats_msg *pending = NLMSG_DATA(ctx->pending);
> > +             const struct if_stats_msg *ifsm = NLMSG_DATA(n);
> > +
> > +             if (pending->ifindex == ifsm->ifindex) {
> > +                     merged = ipstats_msg_merge(ctx->pending, n, &rc);
> > +                     if (merged != NULL) {
> > +                             free(ctx->pending);
> > +                             ctx->pending = merged;
> > +                             return 0;
> > +                     }
> > +                     if (rc != -EOPNOTSUPP)
> > +                             return rc;
> > +
> > +                     /* An attribute layout that cannot be reassembled:
> > +                      * show the two halves separately, like older
> > +                      * versions did, rather than dropping either.
> > +                      */
> > +                     fprintf(stderr,
> > +                             "Cannot merge statistics of ifindex %d split across messages\n",
> > +                             ifsm->ifindex);
> > +             }
> > +
> > +             rc = ipstats_dump_pending(ctx);
> > +             if (rc)
> > +                     return rc;
> > +     }
> > +
> > +     copy = malloc(n->nlmsg_len);
> > +     if (copy == NULL) {
> > +             fprintf(stderr, "Error saving netlink answer: %s\n",
> > +                     strerror(errno));
> > +             return -ENOMEM;
> > +     }
> > +
> > +     memcpy(copy, n, n->nlmsg_len);
> > +     ctx->pending = copy;
> > +     return 0;
> > +}
> > +
> >  static int ipstats_dump(struct ipstats_stat_enabled *enabled)
> >  {
> > +     struct ipstats_dump_ctx ctx = {
> > +             .enabled = enabled,
> > +     };
> >       int rc = 0;
> >
> >       if (rtnl_statsdump_req_filter(&rth, PF_UNSPEC, 0,
> > @@ -874,11 +1142,15 @@ static int ipstats_dump(struct ipstats_stat_enabled *enabled)
> >               return -2;
> >       }
> >
> > -     if (rtnl_dump_filter(&rth, ipstats_dump_one, enabled) < 0) {
> > +     if (rtnl_dump_filter(&rth, ipstats_dump_one, &ctx) < 0) {
> >               fprintf(stderr, "Dump terminated\n");
> >               rc = -2;
> >       }
> >
> > +     if (rc == 0)
> > +             rc = ipstats_dump_pending(&ctx);
> > +
> > +     free(ctx.pending);
> >       return rc;
> >  }