bpf: Support default .validate() and .update() behavior for struct_ops links

Currently, if a struct_ops map is loaded with BPF_F_LINK, it must also
define the .validate() and .update() callbacks in its corresponding
struct bpf_struct_ops in the kernel. Enabling struct_ops link is useful
in its own right to ensure that the map is unloaded if an application
crashes. For example, with sched_ext, we want to automatically unload
the host-wide scheduler if the application crashes. We would likely
never support updating elements of a sched_ext struct_ops map, so we'd
have to implement these callbacks showing that they _can't_ support
element updates just to benefit from the basic lifetime management of
struct_ops links.

Let's enable struct_ops maps to work with BPF_F_LINK even if they
haven't defined these callbacks, by assuming that a struct_ops map
element cannot be updated by default.

Acked-by: Kui-Feng Lee <thinker.li@gmail.com>
Signed-off-by: David Vernet <void@manifault.com>
Link: https://lore.kernel.org/r/20230814185908.700553-2-void@manifault.com
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>

diff --git a/kernel/bpf/bpf_struct_ops.c b/kernel/bpf/bpf_struct_ops.c
index eaff04e..fdc3e87 100644 (file)
--- a/kernel/bpf/bpf_struct_ops.c
+++ b/kernel/bpf/bpf_struct_ops.c
@@ -509,9 +509,12 @@ static long bpf_struct_ops_map_update_elem(struct bpf_map *map, void *key,
        }
 
        if (st_map->map.map_flags & BPF_F_LINK) {
-               err = st_ops->validate(kdata);
-               if (err)
-                       goto reset_unlock;
+               err = 0;
+               if (st_ops->validate) {
+                       err = st_ops->validate(kdata);
+                       if (err)
+                               goto reset_unlock;
+               }
                set_memory_rox((long)st_map->image, 1);
                /* Let bpf_link handle registration & unregistration.
                 *
@@ -663,9 +666,6 @@ static struct bpf_map *bpf_struct_ops_map_alloc(union bpf_attr *attr)
        if (attr->value_size != vt->size)
                return ERR_PTR(-EINVAL);
 
-       if (attr->map_flags & BPF_F_LINK && (!st_ops->validate || !st_ops->update))
-               return ERR_PTR(-EOPNOTSUPP);
-
        t = st_ops->type;
 
        st_map_size = sizeof(*st_map) +
@@ -823,6 +823,9 @@ static int bpf_struct_ops_map_link_update(struct bpf_link *link, struct bpf_map
        if (!bpf_struct_ops_valid_to_reg(new_map))
                return -EINVAL;
 
+       if (!st_map->st_ops->update)
+               return -EOPNOTSUPP;
+
        mutex_lock(&update_mutex);
 
        old_map = rcu_dereference_protected(st_link->map, lockdep_is_held(&update_mutex));