[PATCH bpf-next v3 1/2] libbpf: add request buffer type for netlink messages

[PATCH bpf-next v3 1/2] libbpf: add request buffer type for netlink messages

[Kumar Kartikeya Dwivedi]

Coverity complains about OOB writes to nlmsghdr. There is no OOB as we
write to the trailing buffer, but static analyzers and compilers may
rightfully be confused as the nlmsghdr pointer has subobject provenance
(and hence subobject bounds).

Fix this by using an explicit request structure containing the nlmsghdr,
struct tcmsg/ifinfomsg, and attribute buffer.

Also switch nh_tail (renamed to req_tail) to cast req * to char * so
that it can be understood as arithmetic on pointer to the representation
array (hence having same bound as request structure), which should
further appease analyzers.

As a bonus, callers don't have to pass sizeof(req) all the time now, as
size is implicitly obtained using the pointer. While at it, also reduce
the size of attribute buffer to 128 bytes (132 for ifinfomsg using
functions due to the padding).

Summary of problem:
  Even though C standard allows interconvertibility of pointer to first
  member and pointer to struct, for the purposes of alias analysis it
  would still consider the first as having pointer value "pointer to T"
  where T is type of first member hence having subobject bounds,
  allowing analyzers within reason to complain when object is accessed
  beyond the size of pointed to object.

  The only exception to this rule may be when a char * is formed to a
  member subobject. It is not possible for the compiler to be able to
  tell the intent of the programmer that it is a pointer to member
  object or the underlying representation array of the containing
  object, so such diagnosis is suppressed.

Fixes: 715c5ce454a6 ("libbpf: Add low level TC-BPF management API")
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
---
Changelog:
v2 -> v3:
 * Remove confusing _pad[4] member and description (Andrii)
 * Convert to void * cast everywhere (next patch) (Andrii)
 * Fix typos in commit message (Andrii)
v1 -> v2:
 * Add short summary instead of links about the underlying issue (Daniel)
 * Rename struct to libbpf_nla_req (Daniel)
---
 tools/lib/bpf/netlink.c | 107 +++++++++++++++-------------------------
 tools/lib/bpf/nlattr.h  |  34 ++++++++-----
 2 files changed, 62 insertions(+), 79 deletions(-)

diff --git a/tools/lib/bpf/netlink.c b/tools/lib/bpf/netlink.c
index cf9381f03b16..a17470045455 100644
--- a/tools/lib/bpf/netlink.c
+++ b/tools/lib/bpf/netlink.c
@@ -154,7 +154,7 @@ static int libbpf_netlink_recv(int sock, __u32 nl_pid, int seq,
 	return ret;
 }

-static int libbpf_netlink_send_recv(struct nlmsghdr *nh,
+static int libbpf_netlink_send_recv(struct libbpf_nla_req *req,
 				    __dump_nlmsg_t parse_msg,
 				    libbpf_dump_nlmsg_t parse_attr,
 				    void *cookie)
@@ -166,15 +166,15 @@ static int libbpf_netlink_send_recv(struct nlmsghdr *nh,
 	if (sock < 0)
 		return sock;

-	nh->nlmsg_pid = 0;
-	nh->nlmsg_seq = time(NULL);
+	req->nh.nlmsg_pid = 0;
+	req->nh.nlmsg_seq = time(NULL);

-	if (send(sock, nh, nh->nlmsg_len, 0) < 0) {
+	if (send(sock, req, req->nh.nlmsg_len, 0) < 0) {
 		ret = -errno;
 		goto out;
 	}

-	ret = libbpf_netlink_recv(sock, nl_pid, nh->nlmsg_seq,
+	ret = libbpf_netlink_recv(sock, nl_pid, req->nh.nlmsg_seq,
 				  parse_msg, parse_attr, cookie);
 out:
 	libbpf_netlink_close(sock);
@@ -186,11 +186,7 @@ static int __bpf_set_link_xdp_fd_replace(int ifindex, int fd, int old_fd,
 {
 	struct nlattr *nla;
 	int ret;
-	struct {
-		struct nlmsghdr  nh;
-		struct ifinfomsg ifinfo;
-		char             attrbuf[64];
-	} req;
+	struct libbpf_nla_req req;

 	memset(&req, 0, sizeof(req));
 	req.nh.nlmsg_len      = NLMSG_LENGTH(sizeof(struct ifinfomsg));
@@ -199,27 +195,26 @@ static int __bpf_set_link_xdp_fd_replace(int ifindex, int fd, int old_fd,
 	req.ifinfo.ifi_family = AF_UNSPEC;
 	req.ifinfo.ifi_index  = ifindex;

-	nla = nlattr_begin_nested(&req.nh, sizeof(req), IFLA_XDP);
+	nla = nlattr_begin_nested(&req, IFLA_XDP);
 	if (!nla)
 		return -EMSGSIZE;
-	ret = nlattr_add(&req.nh, sizeof(req), IFLA_XDP_FD, &fd, sizeof(fd));
+	ret = nlattr_add(&req, IFLA_XDP_FD, &fd, sizeof(fd));
 	if (ret < 0)
 		return ret;
 	if (flags) {
-		ret = nlattr_add(&req.nh, sizeof(req), IFLA_XDP_FLAGS, &flags,
-				 sizeof(flags));
+		ret = nlattr_add(&req, IFLA_XDP_FLAGS, &flags, sizeof(flags));
 		if (ret < 0)
 			return ret;
 	}
 	if (flags & XDP_FLAGS_REPLACE) {
-		ret = nlattr_add(&req.nh, sizeof(req), IFLA_XDP_EXPECTED_FD,
-				 &old_fd, sizeof(old_fd));
+		ret = nlattr_add(&req, IFLA_XDP_EXPECTED_FD, &old_fd,
+				 sizeof(old_fd));
 		if (ret < 0)
 			return ret;
 	}
-	nlattr_end_nested(&req.nh, nla);
+	nlattr_end_nested(&req, nla);

-	return libbpf_netlink_send_recv(&req.nh, NULL, NULL, NULL);
+	return libbpf_netlink_send_recv(&req, NULL, NULL, NULL);
 }

 int bpf_set_link_xdp_fd_opts(int ifindex, int fd, __u32 flags,
@@ -314,14 +309,11 @@ int bpf_get_link_xdp_info(int ifindex, struct xdp_link_info *info,
 	struct xdp_id_md xdp_id = {};
 	__u32 mask;
 	int ret;
-	struct {
-		struct nlmsghdr  nh;
-		struct ifinfomsg ifm;
-	} req = {
+	struct libbpf_nla_req req = {
 		.nh.nlmsg_len   = NLMSG_LENGTH(sizeof(struct ifinfomsg)),
 		.nh.nlmsg_type  = RTM_GETLINK,
 		.nh.nlmsg_flags = NLM_F_DUMP | NLM_F_REQUEST,
-		.ifm.ifi_family = AF_PACKET,
+		.ifinfo.ifi_family = AF_PACKET,
 	};

 	if (flags & ~XDP_FLAGS_MASK || !info_size)
@@ -336,7 +328,7 @@ int bpf_get_link_xdp_info(int ifindex, struct xdp_link_info *info,
 	xdp_id.ifindex = ifindex;
 	xdp_id.flags = flags;

-	ret = libbpf_netlink_send_recv(&req.nh, __dump_link_nlmsg,
+	ret = libbpf_netlink_send_recv(&req, __dump_link_nlmsg,
 				       get_xdp_info, &xdp_id);
 	if (!ret) {
 		size_t sz = min(info_size, sizeof(xdp_id.info));
@@ -376,15 +368,14 @@ int bpf_get_link_xdp_id(int ifindex, __u32 *prog_id, __u32 flags)
 	return libbpf_err(ret);
 }

-typedef int (*qdisc_config_t)(struct nlmsghdr *nh, struct tcmsg *t,
-			      size_t maxsz);
+typedef int (*qdisc_config_t)(struct libbpf_nla_req *req);

-static int clsact_config(struct nlmsghdr *nh, struct tcmsg *t, size_t maxsz)
+static int clsact_config(struct libbpf_nla_req *req)
 {
-	t->tcm_parent = TC_H_CLSACT;
-	t->tcm_handle = TC_H_MAKE(TC_H_CLSACT, 0);
+	req->tc.tcm_parent = TC_H_CLSACT;
+	req->tc.tcm_handle = TC_H_MAKE(TC_H_CLSACT, 0);

-	return nlattr_add(nh, maxsz, TCA_KIND, "clsact", sizeof("clsact"));
+	return nlattr_add(req, TCA_KIND, "clsact", sizeof("clsact"));
 }

 static int attach_point_to_config(struct bpf_tc_hook *hook,
@@ -431,11 +422,7 @@ static int tc_qdisc_modify(struct bpf_tc_hook *hook, int cmd, int flags)
 {
 	qdisc_config_t config;
 	int ret;
-	struct {
-		struct nlmsghdr nh;
-		struct tcmsg tc;
-		char buf[256];
-	} req;
+	struct libbpf_nla_req req;

 	ret = attach_point_to_config(hook, &config);
 	if (ret < 0)
@@ -448,11 +435,11 @@ static int tc_qdisc_modify(struct bpf_tc_hook *hook, int cmd, int flags)
 	req.tc.tcm_family  = AF_UNSPEC;
 	req.tc.tcm_ifindex = OPTS_GET(hook, ifindex, 0);

-	ret = config(&req.nh, &req.tc, sizeof(req));
+	ret = config(&req);
 	if (ret < 0)
 		return ret;

-	return libbpf_netlink_send_recv(&req.nh, NULL, NULL, NULL);
+	return libbpf_netlink_send_recv(&req, NULL, NULL, NULL);
 }

 static int tc_qdisc_create_excl(struct bpf_tc_hook *hook)
@@ -544,7 +531,7 @@ static int get_tc_info(struct nlmsghdr *nh, libbpf_dump_nlmsg_t fn,
 	return __get_tc_info(cookie, tc, tb, nh->nlmsg_flags & NLM_F_ECHO);
 }

-static int tc_add_fd_and_name(struct nlmsghdr *nh, size_t maxsz, int fd)
+static int tc_add_fd_and_name(struct libbpf_nla_req *req, int fd)
 {
 	struct bpf_prog_info info = {};
 	__u32 info_len = sizeof(info);
@@ -555,7 +542,7 @@ static int tc_add_fd_and_name(struct nlmsghdr *nh, size_t maxsz, int fd)
 	if (ret < 0)
 		return ret;

-	ret = nlattr_add(nh, maxsz, TCA_BPF_FD, &fd, sizeof(fd));
+	ret = nlattr_add(req, TCA_BPF_FD, &fd, sizeof(fd));
 	if (ret < 0)
 		return ret;
 	len = snprintf(name, sizeof(name), "%s:[%u]", info.name, info.id);
@@ -563,7 +550,7 @@ static int tc_add_fd_and_name(struct nlmsghdr *nh, size_t maxsz, int fd)
 		return -errno;
 	if (len >= sizeof(name))
 		return -ENAMETOOLONG;
-	return nlattr_add(nh, maxsz, TCA_BPF_NAME, name, len + 1);
+	return nlattr_add(req, TCA_BPF_NAME, name, len + 1);
 }

 int bpf_tc_attach(const struct bpf_tc_hook *hook, struct bpf_tc_opts *opts)
@@ -571,12 +558,8 @@ int bpf_tc_attach(const struct bpf_tc_hook *hook, struct bpf_tc_opts *opts)
 	__u32 protocol, bpf_flags, handle, priority, parent, prog_id, flags;
 	int ret, ifindex, attach_point, prog_fd;
 	struct bpf_cb_ctx info = {};
+	struct libbpf_nla_req req;
 	struct nlattr *nla;
-	struct {
-		struct nlmsghdr nh;
-		struct tcmsg tc;
-		char buf[256];
-	} req;

 	if (!hook || !opts ||
 	    !OPTS_VALID(hook, bpf_tc_hook) ||
@@ -618,25 +601,24 @@ int bpf_tc_attach(const struct bpf_tc_hook *hook, struct bpf_tc_opts *opts)
 		return libbpf_err(ret);
 	req.tc.tcm_parent = parent;

-	ret = nlattr_add(&req.nh, sizeof(req), TCA_KIND, "bpf", sizeof("bpf"));
+	ret = nlattr_add(&req, TCA_KIND, "bpf", sizeof("bpf"));
 	if (ret < 0)
 		return libbpf_err(ret);
-	nla = nlattr_begin_nested(&req.nh, sizeof(req), TCA_OPTIONS);
+	nla = nlattr_begin_nested(&req, TCA_OPTIONS);
 	if (!nla)
 		return libbpf_err(-EMSGSIZE);
-	ret = tc_add_fd_and_name(&req.nh, sizeof(req), prog_fd);
+	ret = tc_add_fd_and_name(&req, prog_fd);
 	if (ret < 0)
 		return libbpf_err(ret);
 	bpf_flags = TCA_BPF_FLAG_ACT_DIRECT;
-	ret = nlattr_add(&req.nh, sizeof(req), TCA_BPF_FLAGS, &bpf_flags,
-			 sizeof(bpf_flags));
+	ret = nlattr_add(&req, TCA_BPF_FLAGS, &bpf_flags, sizeof(bpf_flags));
 	if (ret < 0)
 		return libbpf_err(ret);
-	nlattr_end_nested(&req.nh, nla);
+	nlattr_end_nested(&req, nla);

 	info.opts = opts;

-	ret = libbpf_netlink_send_recv(&req.nh, get_tc_info, NULL, &info);
+	ret = libbpf_netlink_send_recv(&req, get_tc_info, NULL, &info);
 	if (ret < 0)
 		return libbpf_err(ret);
 	if (!info.processed)
@@ -650,11 +632,7 @@ static int __bpf_tc_detach(const struct bpf_tc_hook *hook,
 {
 	__u32 protocol = 0, handle, priority, parent, prog_id, flags;
 	int ret, ifindex, attach_point, prog_fd;
-	struct {
-		struct nlmsghdr nh;
-		struct tcmsg tc;
-		char buf[256];
-	} req;
+	struct libbpf_nla_req req;

 	if (!hook ||
 	    !OPTS_VALID(hook, bpf_tc_hook) ||
@@ -701,13 +679,12 @@ static int __bpf_tc_detach(const struct bpf_tc_hook *hook,
 	req.tc.tcm_parent = parent;

 	if (!flush) {
-		ret = nlattr_add(&req.nh, sizeof(req), TCA_KIND,
-				 "bpf", sizeof("bpf"));
+		ret = nlattr_add(&req, TCA_KIND, "bpf", sizeof("bpf"));
 		if (ret < 0)
 			return ret;
 	}

-	return libbpf_netlink_send_recv(&req.nh, NULL, NULL, NULL);
+	return libbpf_netlink_send_recv(&req, NULL, NULL, NULL);
 }

 int bpf_tc_detach(const struct bpf_tc_hook *hook,
@@ -727,11 +704,7 @@ int bpf_tc_query(const struct bpf_tc_hook *hook, struct bpf_tc_opts *opts)
 	__u32 protocol, handle, priority, parent, prog_id, flags;
 	int ret, ifindex, attach_point, prog_fd;
 	struct bpf_cb_ctx info = {};
-	struct {
-		struct nlmsghdr nh;
-		struct tcmsg tc;
-		char buf[256];
-	} req;
+	struct libbpf_nla_req req;

 	if (!hook || !opts ||
 	    !OPTS_VALID(hook, bpf_tc_hook) ||
@@ -770,13 +743,13 @@ int bpf_tc_query(const struct bpf_tc_hook *hook, struct bpf_tc_opts *opts)
 		return libbpf_err(ret);
 	req.tc.tcm_parent = parent;

-	ret = nlattr_add(&req.nh, sizeof(req), TCA_KIND, "bpf", sizeof("bpf"));
+	ret = nlattr_add(&req, TCA_KIND, "bpf", sizeof("bpf"));
 	if (ret < 0)
 		return libbpf_err(ret);

 	info.opts = opts;

-	ret = libbpf_netlink_send_recv(&req.nh, get_tc_info, NULL, &info);
+	ret = libbpf_netlink_send_recv(&req, get_tc_info, NULL, &info);
 	if (ret < 0)
 		return libbpf_err(ret);
 	if (!info.processed)
diff --git a/tools/lib/bpf/nlattr.h b/tools/lib/bpf/nlattr.h
index 3c780ab6d022..76cbfeb21955 100644
--- a/tools/lib/bpf/nlattr.h
+++ b/tools/lib/bpf/nlattr.h
@@ -13,6 +13,7 @@
 #include <string.h>
 #include <errno.h>
 #include <linux/netlink.h>
+#include <linux/rtnetlink.h>

 /* avoid multiple definition of netlink features */
 #define __LINUX_NETLINK_H
@@ -52,6 +53,15 @@ struct libbpf_nla_policy {
 	uint16_t	maxlen;
 };

+struct libbpf_nla_req {
+	struct nlmsghdr nh;
+	union {
+		struct ifinfomsg ifinfo;
+		struct tcmsg tc;
+	};
+	char buf[128];
+};
+
 /**
  * @ingroup attr
  * Iterate over a stream of attributes
@@ -111,44 +121,44 @@ static inline struct nlattr *nla_data(struct nlattr *nla)
 	return (struct nlattr *)((char *)nla + NLA_HDRLEN);
 }

-static inline struct nlattr *nh_tail(struct nlmsghdr *nh)
+static inline struct nlattr *req_tail(struct libbpf_nla_req *req)
 {
-	return (struct nlattr *)((char *)nh + NLMSG_ALIGN(nh->nlmsg_len));
+	return (struct nlattr *)((char *)req + NLMSG_ALIGN(req->nh.nlmsg_len));
 }

-static inline int nlattr_add(struct nlmsghdr *nh, size_t maxsz, int type,
+static inline int nlattr_add(struct libbpf_nla_req *req, int type,
 			     const void *data, int len)
 {
 	struct nlattr *nla;

-	if (NLMSG_ALIGN(nh->nlmsg_len) + NLA_ALIGN(NLA_HDRLEN + len) > maxsz)
+	if (NLMSG_ALIGN(req->nh.nlmsg_len) + NLA_ALIGN(NLA_HDRLEN + len) > sizeof(*req))
 		return -EMSGSIZE;
 	if (!!data != !!len)
 		return -EINVAL;

-	nla = nh_tail(nh);
+	nla = req_tail(req);
 	nla->nla_type = type;
 	nla->nla_len = NLA_HDRLEN + len;
 	if (data)
 		memcpy(nla_data(nla), data, len);
-	nh->nlmsg_len = NLMSG_ALIGN(nh->nlmsg_len) + NLA_ALIGN(nla->nla_len);
+	req->nh.nlmsg_len = NLMSG_ALIGN(req->nh.nlmsg_len) + NLA_ALIGN(nla->nla_len);
 	return 0;
 }

-static inline struct nlattr *nlattr_begin_nested(struct nlmsghdr *nh,
-						 size_t maxsz, int type)
+static inline struct nlattr *nlattr_begin_nested(struct libbpf_nla_req *req, int type)
 {
 	struct nlattr *tail;

-	tail = nh_tail(nh);
-	if (nlattr_add(nh, maxsz, type | NLA_F_NESTED, NULL, 0))
+	tail = req_tail(req);
+	if (nlattr_add(req, type | NLA_F_NESTED, NULL, 0))
 		return NULL;
 	return tail;
 }

-static inline void nlattr_end_nested(struct nlmsghdr *nh, struct nlattr *tail)
+static inline void nlattr_end_nested(struct libbpf_nla_req *req,
+				     struct nlattr *tail)
 {
-	tail->nla_len = (char *)nh_tail(nh) - (char *)tail;
+	tail->nla_len = (char *)req_tail(req) - (char *)tail;
 }

 #endif /* __LIBBPF_NLATTR_H */
--
2.31.1

[PATCH bpf-next v3 2/2] libbpf: netlink: switch to void * casting

[Kumar Kartikeya Dwivedi]

Netlink helpers I added in 8bbb77b7c7a2 ("libbpf: Add various netlink
helpers") used char * casts everywhere, and there were a few more that
existed from before.

Convert all of them to void * cast, as it is treated equivalently by
clang/gcc for the purposes of pointer arithmetic and to follow the
convention elsewhere in the kernel/libbpf.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
---
 tools/lib/bpf/netlink.c | 2 +-
 tools/lib/bpf/nlattr.c  | 2 +-
 tools/lib/bpf/nlattr.h  | 8 ++++----
 3 files changed, 6 insertions(+), 6 deletions(-)

diff --git a/tools/lib/bpf/netlink.c b/tools/lib/bpf/netlink.c
index a17470045455..7b9821d0d70a 100644
--- a/tools/lib/bpf/netlink.c
+++ b/tools/lib/bpf/netlink.c
@@ -524,7 +524,7 @@ static int get_tc_info(struct nlmsghdr *nh, libbpf_dump_nlmsg_t fn,
 	struct nlattr *tb[TCA_MAX + 1];
 
 	libbpf_nla_parse(tb, TCA_MAX,
-			 (struct nlattr *)((char *)tc + NLMSG_ALIGN(sizeof(*tc))),
+			 (struct nlattr *)((void *)tc + NLMSG_ALIGN(sizeof(*tc))),
 			 NLMSG_PAYLOAD(nh, sizeof(*tc)), NULL);
 	if (!tb[TCA_KIND])
 		return NL_CONT;
diff --git a/tools/lib/bpf/nlattr.c b/tools/lib/bpf/nlattr.c
index b607fa9852b1..f57e77a6e40f 100644
--- a/tools/lib/bpf/nlattr.c
+++ b/tools/lib/bpf/nlattr.c
@@ -27,7 +27,7 @@ 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);
+	return (struct nlattr *)((void *)nla + totlen);
 }
 
 static int nla_ok(const struct nlattr *nla, int remaining)
diff --git a/tools/lib/bpf/nlattr.h b/tools/lib/bpf/nlattr.h
index 76cbfeb21955..4d15ae2ff812 100644
--- a/tools/lib/bpf/nlattr.h
+++ b/tools/lib/bpf/nlattr.h
@@ -81,7 +81,7 @@ struct libbpf_nla_req {
  */
 static inline void *libbpf_nla_data(const struct nlattr *nla)
 {
-	return (char *) nla + NLA_HDRLEN;
+	return (void *)nla + NLA_HDRLEN;
 }
 
 static inline uint8_t libbpf_nla_getattr_u8(const struct nlattr *nla)
@@ -118,12 +118,12 @@ int libbpf_nla_dump_errormsg(struct nlmsghdr *nlh);
 
 static inline struct nlattr *nla_data(struct nlattr *nla)
 {
-	return (struct nlattr *)((char *)nla + NLA_HDRLEN);
+	return (struct nlattr *)((void *)nla + NLA_HDRLEN);
 }
 
 static inline struct nlattr *req_tail(struct libbpf_nla_req *req)
 {
-	return (struct nlattr *)((char *)req + NLMSG_ALIGN(req->nh.nlmsg_len));
+	return (struct nlattr *)((void *)req + NLMSG_ALIGN(req->nh.nlmsg_len));
 }
 
 static inline int nlattr_add(struct libbpf_nla_req *req, int type,
@@ -158,7 +158,7 @@ static inline struct nlattr *nlattr_begin_nested(struct libbpf_nla_req *req, int
 static inline void nlattr_end_nested(struct libbpf_nla_req *req,
 				     struct nlattr *tail)
 {
-	tail->nla_len = (char *)req_tail(req) - (char *)tail;
+	tail->nla_len = (void *)req_tail(req) - (void *)tail;
 }
 
 #endif /* __LIBBPF_NLATTR_H */
-- 
2.31.1

Re: [PATCH bpf-next v3 1/2] libbpf: add request buffer type for netlink messages

[Daniel Borkmann]

On 6/19/21 6:14 AM, Kumar Kartikeya Dwivedi wrote:
> Coverity complains about OOB writes to nlmsghdr. There is no OOB as we
> write to the trailing buffer, but static analyzers and compilers may
> rightfully be confused as the nlmsghdr pointer has subobject provenance
> (and hence subobject bounds).
[...]

Both applied, thanks!