Re: [PATCH bpf-next v5 3/8] bpf: Create links for BPF struct_ops maps.

[Martin KaFai Lau <martin.lau@linux.dev>]

On 3/7/23 4:50 PM, Kui-Feng Lee wrote:
> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> index 00b6e1a2edaf..afca6c526fe4 100644
> --- a/include/linux/bpf.h
> +++ b/include/linux/bpf.h
> @@ -1548,6 +1548,7 @@ static inline void bpf_module_put(const void *data, struct module *owner)
>   	else
>   		module_put(owner);
>   }
> +int bpf_struct_ops_link_create(union bpf_attr *attr);
>   
>   #ifdef CONFIG_NET
>   /* Define it here to avoid the use of forward declaration */
> @@ -1588,6 +1589,11 @@ static inline int bpf_struct_ops_map_sys_lookup_elem(struct bpf_map *map,
>   {
>   	return -EINVAL;
>   }
> +static inline int bpf_struct_ops_link_create(union bpf_attr *attr)
> +{
> +	return -EOPNOTSUPP;
> +}
> +
>   #endif
>   
>   #if defined(CONFIG_CGROUP_BPF) && defined(CONFIG_BPF_LSM)
> @@ -2379,6 +2385,11 @@ static inline void bpf_link_put(struct bpf_link *link)
>   {
>   }
>   
> +static inline int bpf_struct_ops_link_create(union bpf_attr *attr)
> +{
> +	return -EOPNOTSUPP;
> +}

The inline version is double defined. It does not look right. Please double check.

> +
>   static inline int bpf_obj_get_user(const char __user *pathname, int flags)
>   {
>   	return -EOPNOTSUPP;
> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> index 976b194eb775..f9fc7b8af3c4 100644
> --- a/include/uapi/linux/bpf.h
> +++ b/include/uapi/linux/bpf.h
> @@ -1033,6 +1033,7 @@ enum bpf_attach_type {
>   	BPF_PERF_EVENT,
>   	BPF_TRACE_KPROBE_MULTI,
>   	BPF_LSM_CGROUP,
> +	BPF_STRUCT_OPS,
>   	__MAX_BPF_ATTACH_TYPE
>   };
>   
> @@ -1266,6 +1267,9 @@ enum {
>   
>   /* Create a map that is suitable to be an inner map with dynamic max entries */
>   	BPF_F_INNER_MAP		= (1U << 12),
> +
> +/* Create a map that will be registered/unregesitered by the backed bpf_link */
> +	BPF_F_LINK		= (1U << 13),
>   };
>   
>   /* Flags for BPF_PROG_QUERY. */
> @@ -1507,7 +1511,10 @@ union bpf_attr {
>   	} task_fd_query;
>   
>   	struct { /* struct used by BPF_LINK_CREATE command */
> -		__u32		prog_fd;	/* eBPF program to attach */
> +		union {
> +			__u32		prog_fd;	/* eBPF program to attach */
> +			__u32		map_fd;		/* struct_ops to attach */
> +		};
>   		union {
>   			__u32		target_fd;	/* object to attach to */
>   			__u32		target_ifindex; /* target ifindex */
> @@ -6379,6 +6386,9 @@ struct bpf_link_info {
>   		struct {
>   			__u32 ifindex;
>   		} xdp;
> +		struct {
> +			__u32 map_id;
> +		} struct_ops;
>   	};
>   } __attribute__((aligned(8)));
>   
> diff --git a/kernel/bpf/bpf_struct_ops.c b/kernel/bpf/bpf_struct_ops.c
> index 9e097fcc9cf4..5a7e86cf67b5 100644
> --- a/kernel/bpf/bpf_struct_ops.c
> +++ b/kernel/bpf/bpf_struct_ops.c
> @@ -16,6 +16,7 @@ enum bpf_struct_ops_state {
>   	BPF_STRUCT_OPS_STATE_INIT,
>   	BPF_STRUCT_OPS_STATE_INUSE,
>   	BPF_STRUCT_OPS_STATE_TOBEFREE,
> +	BPF_STRUCT_OPS_STATE_READY,
>   };
>   
>   #define BPF_STRUCT_OPS_COMMON_VALUE			\
> @@ -58,6 +59,11 @@ struct bpf_struct_ops_map {
>   	struct bpf_struct_ops_value kvalue;
>   };
>   
> +struct bpf_struct_ops_link {
> +	struct bpf_link link;
> +	struct bpf_map __rcu *map;
> +};
> +
>   static DEFINE_MUTEX(update_mutex);
>   
>   #define VALUE_PREFIX "bpf_struct_ops_"
> @@ -496,11 +502,24 @@ static int bpf_struct_ops_map_update_elem(struct bpf_map *map, void *key,
>   		*(unsigned long *)(udata + moff) = prog->aux->id;
>   	}
>   
> -	bpf_map_inc(map);
> -
>   	set_memory_rox((long)st_map->image, 1);
> +	if (st_map->map.map_flags & BPF_F_LINK) {
> +		if (st_ops->validate) {
> +			err = st_ops->validate(kdata);
> +			if (err)
> +				goto unlock;

This should at least be 'goto reset_unlock' to release the progs.

set_memory_rox(..., 1) should also be done after validate?

> +		}
> +		/* Let bpf_link handle registration & unregistration.
> +		 *
> +		 * Pair with smp_load_acquire() during lookup_elem().
> +		 */
> +		smp_store_release(&kvalue->state, BPF_STRUCT_OPS_STATE_READY);
> +		goto unlock;
> +	}
> +
>   	err = st_ops->reg(kdata);
>   	if (likely(!err)) {
> +		bpf_map_inc(map);
>   		/* Pair with smp_load_acquire() during lookup_elem().
>   		 * It ensures the above udata updates (e.g. prog->aux->id)
>   		 * can be seen once BPF_STRUCT_OPS_STATE_INUSE is set.
> @@ -516,7 +535,6 @@ static int bpf_struct_ops_map_update_elem(struct bpf_map *map, void *key,
>   	 */
>   	set_memory_nx((long)st_map->image, 1);
>   	set_memory_rw((long)st_map->image, 1);
> -	bpf_map_put(map);
>   
>   reset_unlock:
>   	bpf_struct_ops_map_put_progs(st_map);
> @@ -534,6 +552,9 @@ static int bpf_struct_ops_map_delete_elem(struct bpf_map *map, void *key)
>   	struct bpf_struct_ops_map *st_map;
>   
>   	st_map = (struct bpf_struct_ops_map *)map;
> +	if (st_map->map.map_flags & BPF_F_LINK)
> +		return -EOPNOTSUPP;
> +
>   	prev_state = cmpxchg(&st_map->kvalue.state,
>   			     BPF_STRUCT_OPS_STATE_INUSE,
>   			     BPF_STRUCT_OPS_STATE_TOBEFREE);
> @@ -601,7 +622,7 @@ static void bpf_struct_ops_map_free(struct bpf_map *map)
>   static int bpf_struct_ops_map_alloc_check(union bpf_attr *attr)
>   {
>   	if (attr->key_size != sizeof(unsigned int) || attr->max_entries != 1 ||
> -	    attr->map_flags || !attr->btf_vmlinux_value_type_id)
> +	    (attr->map_flags & ~BPF_F_LINK) || !attr->btf_vmlinux_value_type_id)
>   		return -EINVAL;
>   	return 0;
>   }
> @@ -712,3 +733,98 @@ void bpf_struct_ops_put(const void *kdata)
>   
>   	bpf_map_put(&st_map->map);
>   }
> +
> +static void bpf_struct_ops_map_link_dealloc(struct bpf_link *link)
> +{
> +	struct bpf_struct_ops_link *st_link;
> +	struct bpf_struct_ops_map *st_map;
> +
> +	st_link = container_of(link, struct bpf_struct_ops_link, link);
> +	st_map = (struct bpf_struct_ops_map *)st_link->map;

/* protected by refcnt and no one is replacing it */
rcu_dereference_protected(st_link->map, true);

st_link->map is with __rcu. It should have warning when compile with 'make C=1 
...'. Patchwork also reports this: 
https://patchwork.kernel.org/project/netdevbpf/patch/20230308005050.255859-4-kuifeng@meta.com/. 
Please pay attention to patchwork for errors.

> +	st_map->st_ops->unreg(&st_map->kvalue.data);
> +	bpf_map_put(st_link->map);

Same here. Reading __rcu pointer without rcu_dereference_xxx.

or simply use &st_map->map here. Otherwise, it will also have type mismatch warning.

> +	kfree(st_link);
> +}
> +
> +static void bpf_struct_ops_map_link_show_fdinfo(const struct bpf_link *link,
> +					    struct seq_file *seq)
> +{
> +	struct bpf_struct_ops_link *st_link;
> +	struct bpf_map *map;
> +
> +	st_link = container_of(link, struct bpf_struct_ops_link, link);
> +	rcu_read_lock();
> +	map = rcu_dereference(st_link->map);
> +	if (map)

map cannot be NULL?

> +		seq_printf(seq, "map_id:\t%d\n", map->id);
> +	rcu_read_unlock();
> +}
> +
> +static int bpf_struct_ops_map_link_fill_link_info(const struct bpf_link *link,
> +					       struct bpf_link_info *info)
> +{
> +	struct bpf_struct_ops_link *st_link;
> +	struct bpf_map *map;
> +
> +	st_link = container_of(link, struct bpf_struct_ops_link, link);
> +	rcu_read_lock();
> +	map = rcu_dereference(st_link->map);
> +	if (map)

Same here.

> +		info->struct_ops.map_id = map->id;
> +	rcu_read_unlock();
> +	return 0;
> +}
> +
> +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,
> +};
> +
> +int bpf_struct_ops_link_create(union bpf_attr *attr)
> +{
> +	struct bpf_struct_ops_link *link = NULL;
> +	struct bpf_link_primer link_primer;
> +	struct bpf_struct_ops_map *st_map;
> +	struct bpf_map *map;
> +	int err;
> +
> +	map = bpf_map_get(attr->link_create.map_fd);
> +	if (!map)
> +		return -EINVAL;
> +
> +	st_map = (struct bpf_struct_ops_map *)map;
> +
> +	if (map->map_type != BPF_MAP_TYPE_STRUCT_OPS || !(map->map_flags & BPF_F_LINK) ||
> +	    /* Pair with smp_store_release() during map_update */
> +	    smp_load_acquire(&st_map->kvalue.state) != BPF_STRUCT_OPS_STATE_READY) {
> +		err = -EINVAL;
> +		goto err_out;
> +	}
> +
> +	link = kzalloc(sizeof(*link), GFP_USER);
> +	if (!link) {
> +		err = -ENOMEM;
> +		goto err_out;
> +	}
> +	bpf_link_init(&link->link, BPF_LINK_TYPE_STRUCT_OPS, &bpf_struct_ops_map_lops, NULL);
> +	RCU_INIT_POINTER(link->map, map);
> +
> +	err = bpf_link_prime(&link->link, &link_primer);
> +	if (err)
> +		goto err_out;
> +
> +	err = st_map->st_ops->reg(st_map->kvalue.data);
> +	if (err) {
> +		bpf_link_cleanup(&link_primer);
> +		goto err_out;
> +	}
> +
> +	return bpf_link_settle(&link_primer);
> +
> +err_out:
> +	bpf_map_put(map);
> +	kfree(link);
> +	return err;
> +}
> +