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

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

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?

> +{
> +       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?..

> +                       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);

?

> +{
> +       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
>