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

[Brian Vazquez]

On Fri, Dec 13, 2019 at 11:26 AM Yonghong Song <yhs@xxxxxx> wrote:
>
>
>
> 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@xxxxxxxxxx>
> > Signed-off-by: Brian Vazquez <brianvv@xxxxxxxxxx>
> > Signed-off-by: Yonghong Song <yhs@xxxxxx>
> > ---
> >   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?

ACK
>
> > +
> > +     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.

I was thinking that instead of using the "next key" as a token we
could use the last value successfully copied as the token, that way
user space code would always be able to start/retry from the last
processed entry. Do you think this would work?
>
> > +
> > +     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;
> > +}
> > +
> [...]