Re: [PATCH bpf-next 1/2] bpf: fix setting return values for htab batch ops

[Anton Protopopov <aspsk@isovalent.com>]

On Tue, Jul 18, 2023 at 05:52:38PM -0700, Alexei Starovoitov wrote:
> On Mon, Jul 17, 2023 at 4:42 AM Anton Protopopov <aspsk@isovalent.com> wrote:
> >
> > The map_lookup{,_and_delete}_batch operations are expected to set the
> > output parameter, counter, to the number of elements successfully copied
> > to the user space. This is also expected to be true if an error is
> > returned and the errno is set to a value other than EFAULT. The current
> > implementation can return -EINVAL without setting the counter to zero, so
> > some userspace programs may confuse this with a [partially] successful
> > operation. Move code which sets the counter to zero to the top of the
> > function so that we always return a correct value.
> >
> > Fixes: 057996380a42 ("bpf: Add batch ops to all htab bpf map")
> > Signed-off-by: Anton Protopopov <aspsk@isovalent.com>
> > ---
> >  kernel/bpf/hashtab.c | 14 +++++++-------
> >  1 file changed, 7 insertions(+), 7 deletions(-)
> >
> > diff --git a/kernel/bpf/hashtab.c b/kernel/bpf/hashtab.c
> > index a8c7e1c5abfa..fa8e3f1e1724 100644
> > --- a/kernel/bpf/hashtab.c
> > +++ b/kernel/bpf/hashtab.c
> > @@ -1692,6 +1692,13 @@ __htab_map_lookup_and_delete_batch(struct bpf_map *map,
> >         struct bucket *b;
> >         int ret = 0;
> >
> > +       max_count = attr->batch.count;
> > +       if (!max_count)
> > +               return 0;
> > +
> > +       if (put_user(0, &uattr->batch.count))
> > +               return -EFAULT;
> > +
> >         elem_map_flags = attr->batch.elem_flags;
> >         if ((elem_map_flags & ~BPF_F_LOCK) ||
> >             ((elem_map_flags & BPF_F_LOCK) && !btf_record_has_field(map->record, BPF_SPIN_LOCK)))
> > @@ -1701,13 +1708,6 @@ __htab_map_lookup_and_delete_batch(struct bpf_map *map,
> >         if (map_flags)
> >                 return -EINVAL;
> >
> > -       max_count = attr->batch.count;
> > -       if (!max_count)
> > -               return 0;
> > -
> > -       if (put_user(0, &uattr->batch.count))
> > -               return -EFAULT;
> > -
> 
> I hear your concern, but I don't think it's a good idea
> to return 0 when flags were incorrect.
> That will cause more suprises to user space.
> I think the code is fine as-is.

Yes, thanks, this makes sense. And actually we can do both:

   max_count = attr->batch.count;
   put_user(0, &uattr->batch.count);
   /* check flags */
   if (!max_count)
           return 0;

This way we always set the userspace counter to a correct value
and also check flags in the right place.