Re: [PATCH v3 bpf-next 02/11] bpf: add generic support for lookup and lookup_and_delete batch ops

[Yonghong Song <yhs@fb.com>]

On 12/11/19 2:33 PM, Brian Vazquez wrote:
> This commit introduces generic support for the bpf_map_lookup_batch and
> bpf_map_lookup_and_delete_batch ops. This implementation can be used by
> almost all the bpf maps since its core implementation is relying on the
> existing map_get_next_key, map_lookup_elem and map_delete_elem
> functions. The bpf syscall subcommands introduced are:
> 
>    BPF_MAP_LOOKUP_BATCH
>    BPF_MAP_LOOKUP_AND_DELETE_BATCH
> 
> The UAPI attribute is:
> 
>    struct { /* struct used by BPF_MAP_*_BATCH commands */
>           __aligned_u64   in_batch;       /* start batch,
>                                            * NULL to start from beginning
>                                            */
>           __aligned_u64   out_batch;      /* output: next start batch */
>           __aligned_u64   keys;
>           __aligned_u64   values;
>           __u32           count;          /* input/output:
>                                            * input: # of key/value
>                                            * elements
>                                            * output: # of filled elements
>                                            */
>           __u32           map_fd;
>           __u64           elem_flags;
>           __u64           flags;
>    } batch;
> 
> in_batch/out_batch are opaque values use to communicate between
> user/kernel space, in_batch/out_batch must be of key_size length.
> 
> To start iterating from the beginning in_batch must be null,
> count is the # of key/value elements to retrieve. Note that the 'keys'
> buffer must be a buffer of key_size * count size and the 'values' buffer
> must be value_size * count, where value_size must be aligned to 8 bytes
> by userspace if it's dealing with percpu maps. 'count' will contain the
> number of keys/values successfully retrieved. Note that 'count' is an
> input/output variable and it can contain a lower value after a call.
> 
> If there's no more entries to retrieve, ENOENT will be returned. If error
> is ENOENT, count might be > 0 in case it copied some values but there were
> no more entries to retrieve.
> 
> Note that if the return code is an error and not -EFAULT,
> count indicates the number of elements successfully processed.
> 
> Suggested-by: Stanislav Fomichev <sdf@google.com>
> Signed-off-by: Brian Vazquez <brianvv@google.com>
> Signed-off-by: Yonghong Song <yhs@fb.com>
> ---
>   include/linux/bpf.h      |  11 +++
>   include/uapi/linux/bpf.h |  19 +++++
>   kernel/bpf/syscall.c     | 172 +++++++++++++++++++++++++++++++++++++++
>   3 files changed, 202 insertions(+)
[...]
> diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
> index 2530266fa6477..708aa89fe2308 100644
> --- a/kernel/bpf/syscall.c
> +++ b/kernel/bpf/syscall.c
> @@ -1206,6 +1206,120 @@ static int map_get_next_key(union bpf_attr *attr)
>   	return err;
>   }
>   
> +#define MAP_LOOKUP_RETRIES 3
> +
> +static int __generic_map_lookup_batch(struct bpf_map *map,
> +				      const union bpf_attr *attr,
> +				      union bpf_attr __user *uattr,
> +				      bool do_delete)
> +{
> +	void __user *ubatch = u64_to_user_ptr(attr->batch.in_batch);
> +	void __user *uobatch = u64_to_user_ptr(attr->batch.out_batch);
> +	void __user *values = u64_to_user_ptr(attr->batch.values);
> +	void __user *keys = u64_to_user_ptr(attr->batch.keys);
> +	void *buf, *prev_key, *key, *value;
> +	u32 value_size, cp, max_count;
> +	bool first_key = false;
> +	int err, retry = MAP_LOOKUP_RETRIES;

Could you try to use reverse Christmas tree style declaration here?

> +
> +	if (attr->batch.elem_flags & ~BPF_F_LOCK)
> +		return -EINVAL;
> +
> +	if ((attr->batch.elem_flags & BPF_F_LOCK) &&
> +	    !map_value_has_spin_lock(map))
> +		return -EINVAL;
> +
> +	value_size = bpf_map_value_size(map);
> +
> +	max_count = attr->batch.count;
> +	if (!max_count)
> +		return 0;
> +
> +	buf = kmalloc(map->key_size + value_size, GFP_USER | __GFP_NOWARN);
> +	if (!buf)
> +		return -ENOMEM;
> +
> +	err = -EFAULT;
> +	first_key = false;
> +	if (ubatch && copy_from_user(buf, ubatch, map->key_size))
> +		goto free_buf;
> +	key = buf;
> +	value = key + map->key_size;
> +	if (!ubatch) {
> +		prev_key = NULL;
> +		first_key = true;
> +	}
> +
> +	for (cp = 0; cp < max_count;) {
> +		if (cp || first_key) {
> +			rcu_read_lock();
> +			err = map->ops->map_get_next_key(map, prev_key, key);
> +			rcu_read_unlock();
> +			if (err)
> +				break;
> +		}
> +		err = bpf_map_copy_value(map, key, value,
> +					 attr->batch.elem_flags, do_delete);
> +
> +		if (err == -ENOENT) {
> +			if (retry) {
> +				retry--;
> +				continue;
> +			}
> +			err = -EINTR;
> +			break;
> +		}
> +
> +		if (err)
> +			goto free_buf;
> +
> +		if (copy_to_user(keys + cp * map->key_size, key,
> +				 map->key_size)) {
> +			err = -EFAULT;
> +			goto free_buf;
> +		}
> +		if (copy_to_user(values + cp * value_size, value, value_size)) {
> +			err = -EFAULT;
> +			goto free_buf;
> +		}
> +
> +		prev_key = key;
> +		retry = MAP_LOOKUP_RETRIES;
> +		cp++;
> +	}
> +
> +	if (!err) {
> +		rcu_read_lock();
> +		err = map->ops->map_get_next_key(map, prev_key, key);
> +		rcu_read_unlock();
> +	}
> +
> +	if (err)
> +		memset(key, 0, map->key_size);

So if any error happens due to above map_get_next_key() or earlier 
error, the next "batch" returned to user could be "0". What should
user space handle this? Ultimately, the user space needs to start
from the beginning again?

What I mean is here how we could design an interface so user
space, if no -EFAULT error, can successfully get all elements
without duplication.

One way to do here is just return -EFAULT if we cannot get
proper next key. But maybe we could have better mechanism
when we try to implement what user space codes will look like.

> +
> +	if ((copy_to_user(&uattr->batch.count, &cp, sizeof(cp)) ||
> +		    (copy_to_user(uobatch, key, map->key_size))))
> +		err = -EFAULT;
> +
> +free_buf:
> +	kfree(buf);
> +	return err;
> +}
> +
[...]