Re: [PATCH v1 bpf-next 1/5] bpf: Add bpf_dynptr_trim and bpf_dynptr_advance

[Joanne Koong <joannelkoong@gmail.com>]

On Wed, Apr 12, 2023 at 2:46 PM Andrii Nakryiko
<andrii.nakryiko@gmail.com> wrote:
>
> On Sat, Apr 8, 2023 at 8:34 PM Joanne Koong <joannelkoong@gmail.com> wrote:
> >
> > bpf_dynptr_trim decreases the size of a dynptr by the specified
> > number of bytes (offset remains the same). bpf_dynptr_advance advances
> > the offset of the dynptr by the specified number of bytes (size
> > decreases correspondingly).
> >
> > Trimming or advancing the dynptr may be useful in certain situations.
> > For example, when hashing which takes in generic dynptrs, if the dynptr
> > points to a struct but only a certain memory region inside the struct
> > should be hashed, advance/trim can be used to narrow in on the
> > specific region to hash.
> >
> > Signed-off-by: Joanne Koong <joannelkoong@gmail.com>
> > ---
> >  kernel/bpf/helpers.c | 49 ++++++++++++++++++++++++++++++++++++++++++++
> >  1 file changed, 49 insertions(+)
> >
> > diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
> > index b6a5cda5bb59..51b4c4b5dbed 100644
> > --- a/kernel/bpf/helpers.c
> > +++ b/kernel/bpf/helpers.c
> > @@ -1448,6 +1448,13 @@ u32 bpf_dynptr_get_size(const struct bpf_dynptr_kern *ptr)
> >         return ptr->size & DYNPTR_SIZE_MASK;
> >  }
> >
> > +static void bpf_dynptr_set_size(struct bpf_dynptr_kern *ptr, u32 new_size)
> > +{
> > +       u32 metadata = ptr->size & ~DYNPTR_SIZE_MASK;
> > +
> > +       ptr->size = new_size | metadata;
> > +}
> > +
> >  int bpf_dynptr_check_size(u32 size)
> >  {
> >         return size > DYNPTR_MAX_SIZE ? -E2BIG : 0;
> > @@ -2275,6 +2282,46 @@ __bpf_kfunc void *bpf_dynptr_slice_rdwr(const struct bpf_dynptr_kern *ptr, u32 o
> >         return bpf_dynptr_slice(ptr, offset, buffer, buffer__szk);
> >  }
> >
> > +/* For dynptrs, the offset may only be advanced and the size may only be decremented */
> > +static int bpf_dynptr_adjust(struct bpf_dynptr_kern *ptr, u32 off_inc, u32 sz_dec)
>
> it feels like this helper just makes it a bit harder to follow what's
> going on. Half of this function isn't actually executed for
> bpf_dynptr_trim, so I don't think we are saving all that much code,
> maybe let's code each of advance and trim explicitly?
>

Sounds good, I will change this in v2 to handle advance and trim separately

> > +{
> > +       u32 size;
> > +
> > +       if (!ptr->data)
> > +               return -EINVAL;
> > +
> > +       size = bpf_dynptr_get_size(ptr);
> > +
> > +       if (sz_dec > size)
> > +               return -ERANGE;
> > +
> > +       if (off_inc) {
> > +               u32 new_off;
> > +
> > +               if (off_inc > size)
>
> like here it becomes confusing if off_inc includes sz_dec, or they
> should be added to each other. I think it's convoluted as is.
>
>
> > +                       return -ERANGE;
> > +
> > +               if (check_add_overflow(ptr->offset, off_inc, &new_off))
>
> why do we need to worry about overflow, we checked all the error
> conditions above?..

Ahh you're right, this cant overflow u32. The dynptr max supported
size is 2^24 - 1 as well

>
> > +                       return -ERANGE;
> > +
> > +               ptr->offset = new_off;
> > +       }
> > +
> > +       bpf_dynptr_set_size(ptr, size - sz_dec);
> > +
> > +       return 0;
> > +}
> > +
> > +__bpf_kfunc int bpf_dynptr_advance(struct bpf_dynptr_kern *ptr, u32 len)
> > +{
> > +       return bpf_dynptr_adjust(ptr, len, len);
> > +}
> > +
> > +__bpf_kfunc int bpf_dynptr_trim(struct bpf_dynptr_kern *ptr, u32 len)
>
> I'm also wondering if trim operation is a bit unusual for dealing
> ranges? Instead of a relative size decrement, maybe it's more
> straightforward to have bpf_dynptr_resize() to set new desired size?
> So if someone has original dynptr with 100 bytes but wants to have
> dynptr for bytes [10, 30), they'd do a pretty natural:
>
> bpf_dynptr_advance(&dynptr, 10);
> bpf_dynptr_resize(&dynptr, 20);
>
> ?
>

Yeah! I like this idea a lot, that way they dont' need to know the
current size of the dynptr before they trim. This seems a lot more
ergonomic

> > +{
> > +       return bpf_dynptr_adjust(ptr, 0, len);
> > +}
> > +
> >  __bpf_kfunc void *bpf_cast_to_kern_ctx(void *obj)
> >  {
> >         return obj;
> > @@ -2347,6 +2394,8 @@ BTF_ID_FLAGS(func, bpf_dynptr_slice_rdwr, KF_RET_NULL)
> >  BTF_ID_FLAGS(func, bpf_iter_num_new, KF_ITER_NEW)
> >  BTF_ID_FLAGS(func, bpf_iter_num_next, KF_ITER_NEXT | KF_RET_NULL)
> >  BTF_ID_FLAGS(func, bpf_iter_num_destroy, KF_ITER_DESTROY)
> > +BTF_ID_FLAGS(func, bpf_dynptr_trim)
> > +BTF_ID_FLAGS(func, bpf_dynptr_advance)
> >  BTF_SET8_END(common_btf_ids)
> >
> >  static const struct btf_kfunc_id_set common_kfunc_set = {
> > --
> > 2.34.1
> >