libbpf: add error reporting in XDP

Parse netlink ext attribute to get the error message returned by
the card. Code is partially take from libnl.

We add netlink.h to the uapi include of tools. And we need to
avoid include of userspace netlink header to have a successful
build of sample so nlattr.h has a define to avoid
the inclusion. Using a direct define could have been an issue
as NLMSGERR_ATTR_MAX can change in the future.

We also define SOL_NETLINK if not defined to avoid to have to
copy socket.h for a fixed value.

Signed-off-by: Eric Leblond <[e-mail]>
Acked-by: Alexei Starovoitov <[e-mail]>
Signed-off-by: Alexei Starovoitov <[e-mail]>

samples/bpf/Makefile

···
 hostprogs-y += syscall_tp
 
 # Libbpf dependencies
-LIBBPF := ../../tools/lib/bpf/bpf.o
+LIBBPF := ../../tools/lib/bpf/bpf.o ../../tools/lib/bpf/nlattr.o
 CGROUP_HELPERS := ../../tools/testing/selftests/bpf/cgroup_helpers.o
 
 test_lru_dist-objs := test_lru_dist.o $(LIBBPF)

tools/lib/bpf/Build

···
-libbpf-y := libbpf.o bpf.o
+libbpf-y := libbpf.o bpf.o nlattr.o

tools/lib/bpf/bpf.c

···
 #include <sys/socket.h>
 #include <errno.h>
 
+#ifndef SOL_NETLINK
+#define SOL_NETLINK 270
+#endif
+
 /*
  * When building perf, unistd.h is overridden. __NR_bpf is
  * required to be defined explicitly.
···
 	struct nlmsghdr *nh;
 	struct nlmsgerr *err;
 	socklen_t addrlen;
+	int one = 1;
 
 	memset(&sa, 0, sizeof(sa));
 	sa.nl_family = AF_NETLINK;
···
 	sock = socket(AF_NETLINK, SOCK_RAW, NETLINK_ROUTE);
 	if (sock < 0) {
 		return -errno;
+	}
+
+	if (setsockopt(sock, SOL_NETLINK, NETLINK_EXT_ACK,
+		       &one, sizeof(one)) < 0) {
+		fprintf(stderr, "Netlink error reporting not supported\n");
 	}
 
 	if (bind(sock, (struct sockaddr *)&sa, sizeof(sa)) < 0) {
···
 			if (!err->error)
 				continue;
 			ret = err->error;
+			nla_dump_errormsg(nh);
 			goto cleanup;
 		case NLMSG_DONE:
 			break;

tools/lib/bpf/nlattr.c

···
+// SPDX-License-Identifier: LGPL-2.1
+
+/*
+ * NETLINK      Netlink attributes
+ *
+ *	This library is free software; you can redistribute it and/or
+ *	modify it under the terms of the GNU Lesser General Public
+ *	License as published by the Free Software Foundation version 2.1
+ *	of the License.
+ *
+ * Copyright (c) 2003-2013 Thomas Graf <[e-mail]>
+ */
+
+#include <errno.h>
+#include "nlattr.h"
+#include <linux/rtnetlink.h>
+#include <string.h>
+#include <stdio.h>
+
+static uint16_t nla_attr_minlen[NLA_TYPE_MAX+1] = {
+	[NLA_U8]	= sizeof(uint8_t),
+	[NLA_U16]	= sizeof(uint16_t),
+	[NLA_U32]	= sizeof(uint32_t),
+	[NLA_U64]	= sizeof(uint64_t),
+	[NLA_STRING]	= 1,
+	[NLA_FLAG]	= 0,
+};
+
+static int nla_len(const struct nlattr *nla)
+{
+	return nla->nla_len - NLA_HDRLEN;
+}
+
+static struct nlattr *nla_next(const struct nlattr *nla, int *remaining)
+{
+	int totlen = NLA_ALIGN(nla->nla_len);
+
+	*remaining -= totlen;
+	return (struct nlattr *) ((char *) nla + totlen);
+}
+
+static int nla_ok(const struct nlattr *nla, int remaining)
+{
+	return remaining >= sizeof(*nla) &&
+	       nla->nla_len >= sizeof(*nla) &&
+	       nla->nla_len <= remaining;
+}
+
+static void *nla_data(const struct nlattr *nla)
+{
+	return (char *) nla + NLA_HDRLEN;
+}
+
+static int nla_type(const struct nlattr *nla)
+{
+	return nla->nla_type & NLA_TYPE_MASK;
+}
+
+static int validate_nla(struct nlattr *nla, int maxtype,
+			struct nla_policy *policy)
+{
+	struct nla_policy *pt;
+	unsigned int minlen = 0;
+	int type = nla_type(nla);
+
+	if (type < 0 || type > maxtype)
+		return 0;
+
+	pt = &policy[type];
+
+	if (pt->type > NLA_TYPE_MAX)
+		return 0;
+
+	if (pt->minlen)
+		minlen = pt->minlen;
+	else if (pt->type != NLA_UNSPEC)
+		minlen = nla_attr_minlen[pt->type];
+
+	if (nla_len(nla) < minlen)
+		return -1;
+
+	if (pt->maxlen && nla_len(nla) > pt->maxlen)
+		return -1;
+
+	if (pt->type == NLA_STRING) {
+		char *data = nla_data(nla);
+		if (data[nla_len(nla) - 1] != '\0')
+			return -1;
+	}
+
+	return 0;
+}
+
+static inline int nlmsg_len(const struct nlmsghdr *nlh)
+{
+	return nlh->nlmsg_len - NLMSG_HDRLEN;
+}
+
+/**
+ * Create attribute index based on a stream of attributes.
+ * @arg tb		Index array to be filled (maxtype+1 elements).
+ * @arg maxtype		Maximum attribute type expected and accepted.
+ * @arg head		Head of attribute stream.
+ * @arg len		Length of attribute stream.
+ * @arg policy		Attribute validation policy.
+ *
+ * Iterates over the stream of attributes and stores a pointer to each
+ * attribute in the index array using the attribute type as index to
+ * the array. Attribute with a type greater than the maximum type
+ * specified will be silently ignored in order to maintain backwards
+ * compatibility. If \a policy is not NULL, the attribute will be
+ * validated using the specified policy.
+ *
+ * @see nla_validate
+ * @return 0 on success or a negative error code.
+ */
+static int nla_parse(struct nlattr *tb[], int maxtype, struct nlattr *head, int len,
+		     struct nla_policy *policy)
+{
+	struct nlattr *nla;
+	int rem, err;
+
+	memset(tb, 0, sizeof(struct nlattr *) * (maxtype + 1));
+
+	nla_for_each_attr(nla, head, len, rem) {
+		int type = nla_type(nla);
+
+		if (type > maxtype)
+			continue;
+
+		if (policy) {
+			err = validate_nla(nla, maxtype, policy);
+			if (err < 0)
+				goto errout;
+		}
+
+		if (tb[type])
+			fprintf(stderr, "Attribute of type %#x found multiple times in message, "
+				  "previous attribute is being ignored.\n", type);
+
+		tb[type] = nla;
+	}
+
+	err = 0;
+errout:
+	return err;
+}
+
+/* dump netlink extended ack error message */
+int nla_dump_errormsg(struct nlmsghdr *nlh)
+{
+	struct nla_policy extack_policy[NLMSGERR_ATTR_MAX + 1] = {
+		[NLMSGERR_ATTR_MSG]	= { .type = NLA_STRING },
+		[NLMSGERR_ATTR_OFFS]	= { .type = NLA_U32 },
+	};
+	struct nlattr *tb[NLMSGERR_ATTR_MAX + 1], *attr;
+	struct nlmsgerr *err;
+	char *errmsg = NULL;
+	int hlen, alen;
+
+	/* no TLVs, nothing to do here */
+	if (!(nlh->nlmsg_flags & NLM_F_ACK_TLVS))
+		return 0;
+
+	err = (struct nlmsgerr *)NLMSG_DATA(nlh);
+	hlen = sizeof(*err);
+
+	/* if NLM_F_CAPPED is set then the inner err msg was capped */
+	if (!(nlh->nlmsg_flags & NLM_F_CAPPED))
+		hlen += nlmsg_len(&err->msg);
+
+	attr = (struct nlattr *) ((void *) err + hlen);
+	alen = nlh->nlmsg_len - hlen;
+
+	if (nla_parse(tb, NLMSGERR_ATTR_MAX, attr, alen, extack_policy) != 0) {
+		fprintf(stderr,
+			"Failed to parse extended error attributes\n");
+		return 0;
+	}
+
+	if (tb[NLMSGERR_ATTR_MSG])
+		errmsg = (char *) nla_data(tb[NLMSGERR_ATTR_MSG]);
+
+	fprintf(stderr, "Kernel error message: %s\n", errmsg);
+
+	return 0;
+}

tools/lib/bpf/nlattr.h

···
+/* SPDX-License-Identifier: LGPL-2.1 */
+
+/*
+ * NETLINK      Netlink attributes
+ *
+ *	This library is free software; you can redistribute it and/or
+ *	modify it under the terms of the GNU Lesser General Public
+ *	License as published by the Free Software Foundation version 2.1
+ *	of the License.
+ *
+ * Copyright (c) 2003-2013 Thomas Graf <[e-mail]>
+ */
+
+#ifndef __NLATTR_H
+#define __NLATTR_H
+
+#include <stdint.h>
+#include <linux/netlink.h>
+/* avoid multiple definition of netlink features */
+#define __LINUX_NETLINK_H
+
+/**
+ * Standard attribute types to specify validation policy
+ */
+enum {
+	NLA_UNSPEC,	/**< Unspecified type, binary data chunk */
+	NLA_U8,		/**< 8 bit integer */
+	NLA_U16,	/**< 16 bit integer */
+	NLA_U32,	/**< 32 bit integer */
+	NLA_U64,	/**< 64 bit integer */
+	NLA_STRING,	/**< NUL terminated character string */
+	NLA_FLAG,	/**< Flag */
+	NLA_MSECS,	/**< Micro seconds (64bit) */
+	NLA_NESTED,	/**< Nested attributes */
+	__NLA_TYPE_MAX,
+};
+
+#define NLA_TYPE_MAX (__NLA_TYPE_MAX - 1)
+
+/**
+ * @ingroup attr
+ * Attribute validation policy.
+ *
+ * See section @core_doc{core_attr_parse,Attribute Parsing} for more details.
+ */
+struct nla_policy {
+	/** Type of attribute or NLA_UNSPEC */
+	uint16_t	type;
+
+	/** Minimal length of payload required */
+	uint16_t	minlen;
+
+	/** Maximal length of payload allowed */
+	uint16_t	maxlen;
+};
+
+/**
+ * @ingroup attr
+ * Iterate over a stream of attributes
+ * @arg pos	loop counter, set to current attribute
+ * @arg head	head of attribute stream
+ * @arg len	length of attribute stream
+ * @arg rem	initialized to len, holds bytes currently remaining in stream
+ */
+#define nla_for_each_attr(pos, head, len, rem) \
+	for (pos = head, rem = len; \
+	     nla_ok(pos, rem); \
+	     pos = nla_next(pos, &(rem)))
+
+int nla_dump_errormsg(struct nlmsghdr *nlh);
+
+#endif /* __NLATTR_H */