libbpf: Update a bpf_link with another struct_ops.

Introduce bpf_link__update_map(), which allows to atomically update
underlying struct_ops implementation for given struct_ops BPF link.

Also add old_map_fd to struct bpf_link_update_opts to handle
BPF_F_REPLACE feature.

Signed-off-by: Kui-Feng Lee <kuifeng@meta.com>
Link: https://lore.kernel.org/r/20230323032405.3735486-7-kuifeng@meta.com
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>

diff --git a/tools/lib/bpf/bpf.c b/tools/lib/bpf/bpf.c
index e750b6f..7670359 100644 (file)
--- a/tools/lib/bpf/bpf.c
+++ b/tools/lib/bpf/bpf.c
@@ -794,11 +794,17 @@ int bpf_link_update(int link_fd, int new_prog_fd,
        if (!OPTS_VALID(opts, bpf_link_update_opts))
                return libbpf_err(-EINVAL);
 
+       if (OPTS_GET(opts, old_prog_fd, 0) && OPTS_GET(opts, old_map_fd, 0))
+               return libbpf_err(-EINVAL);
+
        memset(&attr, 0, attr_sz);
        attr.link_update.link_fd = link_fd;
        attr.link_update.new_prog_fd = new_prog_fd;
        attr.link_update.flags = OPTS_GET(opts, flags, 0);
-       attr.link_update.old_prog_fd = OPTS_GET(opts, old_prog_fd, 0);
+       if (OPTS_GET(opts, old_prog_fd, 0))
+               attr.link_update.old_prog_fd = OPTS_GET(opts, old_prog_fd, 0);
+       else if (OPTS_GET(opts, old_map_fd, 0))
+               attr.link_update.old_map_fd = OPTS_GET(opts, old_map_fd, 0);
 
        ret = sys_bpf(BPF_LINK_UPDATE, &attr, attr_sz);
        return libbpf_err_errno(ret);

diff --git a/tools/lib/bpf/bpf.h b/tools/lib/bpf/bpf.h
index f0f7863..b073e73 100644 (file)
--- a/tools/lib/bpf/bpf.h
+++ b/tools/lib/bpf/bpf.h
@@ -336,8 +336,9 @@ struct bpf_link_update_opts {
        size_t sz; /* size of this struct for forward/backward compatibility */
        __u32 flags;       /* extra flags */
        __u32 old_prog_fd; /* expected old program FD */
+       __u32 old_map_fd;  /* expected old map FD */
 };
-#define bpf_link_update_opts__last_field old_prog_fd
+#define bpf_link_update_opts__last_field old_map_fd
 
 LIBBPF_API int bpf_link_update(int link_fd, int new_prog_fd,
                               const struct bpf_link_update_opts *opts);

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index 4ff82ba..2b5ea25 100644 (file)
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -11677,6 +11677,41 @@ struct bpf_link *bpf_map__attach_struct_ops(const struct bpf_map *map)
        return &link->link;
 }
 
+/*
+ * Swap the back struct_ops of a link with a new struct_ops map.
+ */
+int bpf_link__update_map(struct bpf_link *link, const struct bpf_map *map)
+{
+       struct bpf_link_struct_ops *st_ops_link;
+       __u32 zero = 0;
+       int err;
+
+       if (!bpf_map__is_struct_ops(map) || map->fd < 0)
+               return -EINVAL;
+
+       st_ops_link = container_of(link, struct bpf_link_struct_ops, link);
+       /* Ensure the type of a link is correct */
+       if (st_ops_link->map_fd < 0)
+               return -EINVAL;
+
+       err = bpf_map_update_elem(map->fd, &zero, map->st_ops->kern_vdata, 0);
+       /* It can be EBUSY if the map has been used to create or
+        * update a link before.  We don't allow updating the value of
+        * a struct_ops once it is set.  That ensures that the value
+        * never changed.  So, it is safe to skip EBUSY.
+        */
+       if (err && err != -EBUSY)
+               return err;
+
+       err = bpf_link_update(link->fd, map->fd, NULL);
+       if (err < 0)
+               return err;
+
+       st_ops_link->map_fd = map->fd;
+
+       return 0;
+}
+
 typedef enum bpf_perf_event_ret (*bpf_perf_event_print_t)(struct perf_event_header *hdr,
                                                          void *private_data);
 

diff --git a/tools/lib/bpf/libbpf.h b/tools/lib/bpf/libbpf.h
index db4992a..1615e55 100644 (file)
--- a/tools/lib/bpf/libbpf.h
+++ b/tools/lib/bpf/libbpf.h
@@ -719,6 +719,7 @@ bpf_program__attach_freplace(const struct bpf_program *prog,
 struct bpf_map;
 
 LIBBPF_API struct bpf_link *bpf_map__attach_struct_ops(const struct bpf_map *map);
+LIBBPF_API int bpf_link__update_map(struct bpf_link *link, const struct bpf_map *map);
 
 struct bpf_iter_attach_opts {
        size_t sz; /* size of this struct for forward/backward compatibility */

diff --git a/tools/lib/bpf/libbpf.map b/tools/lib/bpf/libbpf.map
index 50dde1f..a5aa3a3 100644 (file)
--- a/tools/lib/bpf/libbpf.map
+++ b/tools/lib/bpf/libbpf.map
@@ -386,6 +386,7 @@ LIBBPF_1.1.0 {
 LIBBPF_1.2.0 {
        global:
                bpf_btf_get_info_by_fd;
+               bpf_link__update_map;
                bpf_link_get_info_by_fd;
                bpf_map_get_info_by_fd;
                bpf_prog_get_info_by_fd;