Re: [PATCH bpf-next v5 5/8] bpf: Update the struct_ops of a bpf_link.

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On 3/8/23 14:21, Martin KaFai Lau wrote:
On 3/7/23 4:50 PM, Kui-Feng Lee wrote:
By improving the BPF_LINK_UPDATE command of bpf(), it should allow you
to conveniently switch between different struct_ops on a single
bpf_link. This would enable smoother transitions from one struct_ops
to another.

The struct_ops maps passing along with BPF_LINK_UPDATE should have the
BPF_F_LINK flag.

Signed-off-by: Kui-Feng Lee <kuifeng@xxxxxxxx>
---
  include/linux/bpf.h            |  1 +
  include/uapi/linux/bpf.h       |  8 ++++--
  kernel/bpf/bpf_struct_ops.c    | 46 ++++++++++++++++++++++++++++++++++
  kernel/bpf/syscall.c           | 43 ++++++++++++++++++++++++++++---
  tools/include/uapi/linux/bpf.h |  7 +++++-
  5 files changed, 98 insertions(+), 7 deletions(-)

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index afca6c526fe4..29d555a82bad 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -1470,6 +1470,7 @@ struct bpf_link_ops {
      void (*show_fdinfo)(const struct bpf_link *link, struct seq_file *seq);
      int (*fill_link_info)(const struct bpf_link *link,
                    struct bpf_link_info *info);
+    int (*update_map)(struct bpf_link *link, struct bpf_map *new_map);
  };
  struct bpf_tramp_link {
diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index f9fc7b8af3c4..edef9cf7d596 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h
@@ -1555,8 +1555,12 @@ union bpf_attr {
      struct { /* struct used by BPF_LINK_UPDATE command */
          __u32        link_fd;    /* link fd */
-        /* new program fd to update link with */
-        __u32        new_prog_fd;
+        union {
+            /* new program fd to update link with */
+            __u32        new_prog_fd;
+            /* new struct_ops map fd to update link with */
+            __u32           new_map_fd;
+        };
          __u32        flags;        /* extra flags */
          /* expected link's program fd; is specified only if
           * BPF_F_REPLACE flag is set in flags */
diff --git a/kernel/bpf/bpf_struct_ops.c b/kernel/bpf/bpf_struct_ops.c
index 5a7e86cf67b5..79e663869e51 100644
--- a/kernel/bpf/bpf_struct_ops.c
+++ b/kernel/bpf/bpf_struct_ops.c
@@ -775,10 +775,56 @@ static int bpf_struct_ops_map_link_fill_link_info(const struct bpf_link *link,
      return 0;
  }
+static int bpf_struct_ops_map_link_update(struct bpf_link *link, struct bpf_map *new_map)
+{
+    struct bpf_struct_ops_value *kvalue;
+    struct bpf_struct_ops_map *st_map, *old_st_map;
+    struct bpf_struct_ops_link *st_link;
+    struct bpf_map *old_map;
+    int err = 0;
+
+    if (new_map->map_type != BPF_MAP_TYPE_STRUCT_OPS ||
+        !(new_map->map_flags & BPF_F_LINK))
+        return -EINVAL;
+
+    mutex_lock(&update_mutex);
+
+    st_link = container_of(link, struct bpf_struct_ops_link, link);
+
+    /* The new and old struct_ops must be the same type. */
+    st_map = container_of(new_map, struct bpf_struct_ops_map, map);

nit. move the st_link and st_map init out of the lock.


Ok

+
+    old_map = st_link->map;

rcu_dereference_protected(...)

+    old_st_map = container_of(old_map, struct bpf_struct_ops_map, map);
+    if (st_map->st_ops != old_st_map->st_ops ||
+        /* Pair with smp_store_release() during map_update */
+        smp_load_acquire(&st_map->kvalue.state) != BPF_STRUCT_OPS_STATE_READY) {

nit. test the smp_load_acquire(&st_map...) outside of the lock.
Do it together with the new_map checking at the beginning of the func.

Ok


+        err = -EINVAL;
+        goto err_out;
+    }
+
+    kvalue = &st_map->kvalue;
+
+    err = st_map->st_ops->update(kvalue->data, old_st_map->kvalue.data);
+    if (err)
+        goto err_out;
+
+    bpf_map_inc(new_map);
+    rcu_assign_pointer(st_link->map, new_map);
+
+    bpf_map_put(old_map);
+
+err_out:
+    mutex_unlock(&update_mutex);
+
+    return err;
+}
+
  static const struct bpf_link_ops bpf_struct_ops_map_lops = {
      .dealloc = bpf_struct_ops_map_link_dealloc,
      .show_fdinfo = bpf_struct_ops_map_link_show_fdinfo,
      .fill_link_info = bpf_struct_ops_map_link_fill_link_info,
+    .update_map = bpf_struct_ops_map_link_update,
  };
  int bpf_struct_ops_link_create(union bpf_attr *attr)
diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index 3a4503987a48..c087dd2e2c08 100644
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c
@@ -4658,6 +4658,30 @@ static int link_create(union bpf_attr *attr, bpfptr_t uattr)
      return ret;
  }
+static int link_update_map(struct bpf_link *link, union bpf_attr *attr)
+{
+    struct bpf_map *new_map;
+    int ret = 0;
+
+    new_map = bpf_map_get(attr->link_update.new_map_fd);
+    if (IS_ERR(new_map))
+        return -EINVAL;
+
+    if (new_map->map_type != BPF_MAP_TYPE_STRUCT_OPS) {

This is unnecessary test. The individual '.update_map()' should test for its own map_type and the new bpf_struct_ops_map_link_update() does test it.


Will remove it.

+        ret = -EINVAL;
+        goto out_put_map;
+    }
+
+    if (link->ops->update_map)

This has just been tested in link_update() before calling link_update_map().

Sure


+        ret = link->ops->update_map(link, new_map);
+    else
+        ret = -EINVAL;
+
+out_put_map:
+    bpf_map_put(new_map);
+    return ret;
+}
+
  #define BPF_LINK_UPDATE_LAST_FIELD link_update.old_prog_fd
  static int link_update(union bpf_attr *attr)
@@ -4670,14 +4694,25 @@ static int link_update(union bpf_attr *attr)
      if (CHECK_ATTR(BPF_LINK_UPDATE))
          return -EINVAL;
-    flags = attr->link_update.flags;
-    if (flags & ~BPF_F_REPLACE)
-        return -EINVAL;
-
      link = bpf_link_get_from_fd(attr->link_update.link_fd);
      if (IS_ERR(link))
          return PTR_ERR(link);
+    flags = attr->link_update.flags;
+
+    if (link->ops->update_map) {
+        if (flags)    /* always replace the existing one */
+            ret = -EINVAL;
+        else
+            ret = link_update_map(link, attr);
+        goto out_put_link;
+    }
+
+    if (flags & ~BPF_F_REPLACE) {
+        ret = -EINVAL;
+        goto out_put_link;
+    }
+
      new_prog = bpf_prog_get(attr->link_update.new_prog_fd);
      if (IS_ERR(new_prog)) {
          ret = PTR_ERR(new_prog);




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