Re: [PATCH bpf-next 2/5] libbpf: add helpers for preparing netlink attributes

On Thu, Mar 25, 2021 at 5:01 AM Kumar Kartikeya Dwivedi memxor@gmail.com wrote:

This change introduces a few helpers to wrap open coded attribute preparation in netlink.c.

Every nested attribute's closure must happen using the helper end_nlattr_nested, which sets its length properly. NLA_F_NESTED is enforeced using begin_nlattr_nested helper. Other simple attributes can be added directly.

The maxsz parameter corresponds to the size of the request structure which is being filled in, so for instance with req being:

struct { struct nlmsghdr nh; struct tcmsg t; char buf[4096]; } req;

Then, maxsz should be sizeof(req).

This change also converts the open coded attribute preparation with the helpers. Note that the only failure the internal call to add_nlattr could result in the nested helper would be -EMSGSIZE, hence that is what we return to our caller.

Reviewed-by: Toke Høiland-Jørgensen toke@redhat.com Signed-off-by: Kumar Kartikeya Dwivedi memxor@gmail.com

---

tools/lib/bpf/netlink.c | 37 +++++++++++++++-------------------- tools/lib/bpf/nlattr.h | 43 +++++++++++++++++++++++++++++++++++++++++ 2 files changed, 59 insertions(+), 21 deletions(-)

diff --git a/tools/lib/bpf/netlink.c b/tools/lib/bpf/netlink.c index 4dd73de00b6f..f448c29de76d 100644 --- a/tools/lib/bpf/netlink.c +++ b/tools/lib/bpf/netlink.c @@ -135,7 +135,7 @@ static int __bpf_set_link_xdp_fd_replace(int ifindex, int fd, int old_fd, __u32 flags) { int sock, seq = 0, ret;

•   struct nlattr *nla, *nla_xdp;
  

•   struct nlattr *nla;
    struct {
            struct nlmsghdr  nh;
            struct ifinfomsg ifinfo;
  

@@ -157,36 +157,31 @@ static int __bpf_set_link_xdp_fd_replace(int ifindex, int fd, int old_fd, req.ifinfo.ifi_index = ifindex;

    /* started nested attribute for XDP */

•   nla = (struct nlattr *)(((char *)&req)
  

•                           + NLMSG_ALIGN(req.nh.nlmsg_len));
  

•   nla->nla_type = NLA_F_NESTED | IFLA_XDP;
  

•   nla->nla_len = NLA_HDRLEN;
  

•   nla = begin_nlattr_nested(&req.nh, sizeof(req), IFLA_XDP);
  

•   if (!nla) {
  

•           ret = -EMSGSIZE;
  

•           goto cleanup;
  

•   }
  
    /* add XDP fd */
  

•   nla_xdp = (struct nlattr *)((char *)nla + nla->nla_len);
  

•   nla_xdp->nla_type = IFLA_XDP_FD;
  

•   nla_xdp->nla_len = NLA_HDRLEN + sizeof(int);
  

•   memcpy((char *)nla_xdp + NLA_HDRLEN, &fd, sizeof(fd));
  

•   nla->nla_len += nla_xdp->nla_len;
  

•   ret = add_nlattr(&req.nh, sizeof(req), IFLA_XDP_FD, &fd, sizeof(fd));
  

•   if (ret < 0)
  

•           goto cleanup;
  
    /* if user passed in any flags, add those too */
    if (flags) {
  

•           nla_xdp = (struct nlattr *)((char *)nla + nla->nla_len);
  

•           nla_xdp->nla_type = IFLA_XDP_FLAGS;
  

•           nla_xdp->nla_len = NLA_HDRLEN + sizeof(flags);
  

•           memcpy((char *)nla_xdp + NLA_HDRLEN, &flags, sizeof(flags));
  

•           nla->nla_len += nla_xdp->nla_len;
  

•           ret = add_nlattr(&req.nh, sizeof(req), IFLA_XDP_FLAGS, &flags, sizeof(flags));
  

•           if (ret < 0)
  

•                   goto cleanup;
    }
  
    if (flags & XDP_FLAGS_REPLACE) {
  

•           nla_xdp = (struct nlattr *)((char *)nla + nla->nla_len);
  

•           nla_xdp->nla_type = IFLA_XDP_EXPECTED_FD;
  

•           nla_xdp->nla_len = NLA_HDRLEN + sizeof(old_fd);
  

•           memcpy((char *)nla_xdp + NLA_HDRLEN, &old_fd, sizeof(old_fd));
  

•           nla->nla_len += nla_xdp->nla_len;
  

•           ret = add_nlattr(&req.nh, sizeof(req), IFLA_XDP_EXPECTED_FD, &flags, sizeof(flags));
  

•           if (ret < 0)
  

•                   goto cleanup;
    }
  
  

•   req.nh.nlmsg_len += NLA_ALIGN(nla->nla_len);
  

•   end_nlattr_nested(&req.nh, nla);
  
    if (send(sock, &req, req.nh.nlmsg_len, 0) < 0) {
            ret = -errno;
  

diff --git a/tools/lib/bpf/nlattr.h b/tools/lib/bpf/nlattr.h index 6cc3ac91690f..463a53bf3022 100644 --- a/tools/lib/bpf/nlattr.h +++ b/tools/lib/bpf/nlattr.h @@ -10,7 +10,10 @@ #define __LIBBPF_NLATTR_H

#include <stdint.h> +#include <string.h> +#include <errno.h> #include <linux/netlink.h>

/* avoid multiple definition of netlink features */ #define __LINUX_NETLINK_H

@@ -103,4 +106,44 @@ int libbpf_nla_parse_nested(struct nlattr *tb[], int maxtype,

int libbpf_nla_dump_errormsg(struct nlmsghdr *nlh);

+/* Helpers for preparing/consuming attributes */

+#define NLA_DATA(nla) ((struct nlattr *)((char *)(nla) + NLA_HDRLEN))

`((char *)nh + NLMSG_ALIGN(nh->nlmsg_len))` seems to be another popular one (three occurrences in this file), maybe extract that one as well?

And can you please use functions, not macros? This way you can specify what types you expect, as one of the benefits.

+static inline int add_nlattr(struct nlmsghdr *nh, size_t maxsz, int type,

•                        const void *data, int len)
  

+{

•   struct nlattr *nla;
  

•   if (NLMSG_ALIGN(nh->nlmsg_len) + NLA_ALIGN(NLA_HDRLEN + len) > maxsz)
  

•           return -EMSGSIZE;
  

•   if ((!data && len) || (data && !len))
  

•           return -EINVAL;
  

•   nla = (struct nlattr *)((char *)nh + NLMSG_ALIGN(nh->nlmsg_len));
  

•   nla->nla_type = type;
  

•   nla->nla_len = NLA_HDRLEN + len;
  

•   if (data)
  

•           memcpy((char *)nla + NLA_HDRLEN, data, len);
  

•   nh->nlmsg_len = NLMSG_ALIGN(nh->nlmsg_len) + NLA_ALIGN(nla->nla_len);
  

•   return 0;
  

+}

+static inline struct nlattr *begin_nlattr_nested(struct nlmsghdr *nh, size_t maxsz,

•                                          int type)
  

+{

•   struct nlattr *tail;
  

•   tail = (struct nlattr *)((char *)nh + NLMSG_ALIGN(nh->nlmsg_len));
  

•   if (add_nlattr(nh, maxsz, type | NLA_F_NESTED, NULL, 0))
  

•           return NULL;
  

•   return tail;
  

+}

+static inline void end_nlattr_nested(struct nlmsghdr *nh, struct nlattr *tail)

I don't know much about their use (yet, I feel like I'm about to learn :( ), but would nlattr_add, nlattr_begin_nested/nlattr_start_nested, nlattr_end_nested make sense and be a bit more in line with overall object_action naming pattern?

+{

•   tail->nla_len = ((char *)nh + NLMSG_ALIGN(nh->nlmsg_len)) - (char *)(tail);
  

+}

#endif /* __LIBBPF_NLATTR_H */

2.30.2