Joanne Koong wrote:
> Add a new kfunc
>
> int bpf_dynptr_adjust(struct bpf_dynptr_kern *ptr, u32 start, u32 end);
>
> which adjusts the dynptr to reflect the new [start, end) interval.
> In particular, it advances the offset of the dynptr by "start" bytes,
> and if end is less than the size of the dynptr, then this will trim the
> dynptr accordingly.
>
> Adjusting the dynptr interval 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, adjust can be used to narrow in on the
> specific region to hash.
Would you want to prohibit creating an empty dynptr with [start, start)?
>
> Signed-off-by: Joanne Koong <joannelkoong@xxxxxxxxx>
> ---
> kernel/bpf/helpers.c | 26 ++++++++++++++++++++++++++
> 1 file changed, 26 insertions(+)
>
> diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
> index 00e5fb0682ac..7ddf63ac93ce 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;
> @@ -2297,6 +2304,24 @@ __bpf_kfunc void *bpf_dynptr_slice_rdwr(const struct bpf_dynptr_kern *ptr, u32 o
> return bpf_dynptr_slice(ptr, offset, buffer, buffer__szk);
> }
>
> +__bpf_kfunc int bpf_dynptr_adjust(struct bpf_dynptr_kern *ptr, u32 start, u32 end)
> +{
> + u32 size;
> +
> + if (!ptr->data || start > end)
> + return -EINVAL;
> +
> + size = bpf_dynptr_get_size(ptr);
> +
> + if (start > size || end > size)
> + return -ERANGE;
maybe 'start >= size'? OTOH if the verifier doesn't mind I guess its OK
to create the thing even if it doesn't make much sense.
> +
> + ptr->offset += start;
> + bpf_dynptr_set_size(ptr, end - start);
> +
> + return 0;
> +}
> +
> __bpf_kfunc void *bpf_cast_to_kern_ctx(void *obj)
> {
> return obj;
> @@ -2369,6 +2394,7 @@ 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_adjust)
> BTF_SET8_END(common_btf_ids)
>
> static const struct btf_kfunc_id_set common_kfunc_set = {
> --
> 2.34.1
>
