Re: [PATCH bpf-next] bpf: Support default .validate() and .update() behavior for struct_ops links

[Yonghong Song <yonghong.song@linux.dev>]

On 8/10/23 3:04 PM, David Vernet wrote:
> 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.

Maybe you want to add one map_flag to indicate validate/update callbacks
are optional for a struct_ops link? In this case, some struct_ops maps
can still require validate() and update(), but others can skip them?

> 
> Signed-off-by: David Vernet <void@manifault.com>
> ---
>   kernel/bpf/bpf_struct_ops.c | 17 +++++++++++------
>   1 file changed, 11 insertions(+), 6 deletions(-)
> 
> diff --git a/kernel/bpf/bpf_struct_ops.c b/kernel/bpf/bpf_struct_ops.c
> index eaff04eefb31..3d2fb85186a9 100644
> --- 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) +
> @@ -838,6 +838,11 @@ static int bpf_struct_ops_map_link_update(struct bpf_link *link, struct bpf_map
>   		goto err_out;
>   	}
>   
> +	if (!st_map->st_ops->update) {
> +		err = -EOPNOTSUPP;
> +		goto err_out;
> +	}
> +
>   	err = st_map->st_ops->update(st_map->kvalue.data, old_st_map->kvalue.data);
>   	if (err)
>   		goto err_out;