Subject: Re: [PATCH 2/5] bpf: convert queue and stack map to map_copy_value

From: Jakub Sitnicki <[email-masked]>
Date: Wed, 11 Mar 2020 15:00:57 +0100

On Tue, Mar 10, 2020 at 06:47 PM CET, Lorenz Bauer wrote:
> Migrate BPF_MAP_TYPE_QUEUE and BPF_MAP_TYPE_STACK to map_copy_value,
> by introducing small wrappers that discard the (unused) key argument.
>
> Signed-off-by: Lorenz Bauer <[email-masked]>
> ---
>  kernel/bpf/queue_stack_maps.c | 18 ++++++++++++++++++
>  kernel/bpf/syscall.c          |  5 +----
>  2 files changed, 19 insertions(+), 4 deletions(-)
>
> diff --git a/kernel/bpf/queue_stack_maps.c b/kernel/bpf/queue_stack_maps.c
> index f697647ceb54..5c89b7583cd2 100644
> --- a/kernel/bpf/queue_stack_maps.c
> +++ b/kernel/bpf/queue_stack_maps.c
> @@ -262,11 +262,28 @@ static int queue_stack_map_get_next_key(struct bpf_map *map, void *key,
>  	return -EINVAL;
>  }
>
> +/* Called from syscall */
> +static int queue_map_copy_value(struct bpf_map *map, void *key, void *value)
> +{
> +	(void)key;

Alternatively, there's is the __always_unused compiler attribute from
include/linux/compiler_attributes.h that seems to be in wide use.

> +
> +	return queue_map_peek_elem(map, value);
> +}
> +
> +/* Called from syscall */
> +static int stack_map_copy_value(struct bpf_map *map, void *key, void *value)
> +{
> +	(void)key;
> +
> +	return stack_map_peek_elem(map, value);
> +}
> +
>  const struct bpf_map_ops queue_map_ops = {
>  	.map_alloc_check = queue_stack_map_alloc_check,
>  	.map_alloc = queue_stack_map_alloc,
>  	.map_free = queue_stack_map_free,
>  	.map_lookup_elem = queue_stack_map_lookup_elem,
> +	.map_copy_value = queue_map_copy_value,
>  	.map_update_elem = queue_stack_map_update_elem,
>  	.map_delete_elem = queue_stack_map_delete_elem,
>  	.map_push_elem = queue_stack_map_push_elem,
> @@ -280,6 +297,7 @@ const struct bpf_map_ops stack_map_ops = {
>  	.map_alloc = queue_stack_map_alloc,
>  	.map_free = queue_stack_map_free,
>  	.map_lookup_elem = queue_stack_map_lookup_elem,
> +	.map_copy_value = stack_map_copy_value,
>  	.map_update_elem = queue_stack_map_update_elem,
>  	.map_delete_elem = queue_stack_map_delete_elem,
>  	.map_push_elem = queue_stack_map_push_elem,
> diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
> index 6503824e81e9..20c6cdace275 100644
> --- a/kernel/bpf/syscall.c
> +++ b/kernel/bpf/syscall.c
> @@ -218,10 +218,7 @@ static int bpf_map_copy_value(struct bpf_map *map, void *key, void *value,
>  		return bpf_map_offload_lookup_elem(map, key, value);
>
>  	bpf_disable_instrumentation();
> -	if (map->map_type == BPF_MAP_TYPE_QUEUE ||
> -	    map->map_type == BPF_MAP_TYPE_STACK) {
> -		err = map->ops->map_peek_elem(map, value);
> -	} else if (map->ops->map_copy_value) {
> +	if (map->ops->map_copy_value) {
>  		err = map->ops->map_copy_value(map, key, value);
>  	} else {
>  		rcu_read_lock();