On Tue, Aug 23, 2022 at 10:01 AM Shmulik Ladkani
<shmulik@xxxxxxxxxxxxxxxx> wrote:
>
> The task of calculating bpf_dynptr_kern's available size, and the
> current (offset) data pointer is common for BPF functions working with
> ARG_PTR_TO_DYNPTR parameters.
>
> Introduce 'bpf_dynptr_get_data' which returns the current data
> (with properer offset), and the number of usable bytes it has.
>
> This will void callers from directly calculating bpf_dynptr_kern's
> data, offset and size.
>
> Signed-off-by: Shmulik Ladkani <shmulik.ladkani@xxxxxxxxx>
> ---
> include/linux/bpf.h | 1 +
> kernel/bpf/helpers.c | 10 ++++++++++
> 2 files changed, 11 insertions(+)
>
> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> index 39bd36359c1e..6d288dfc302b 100644
> --- a/include/linux/bpf.h
> +++ b/include/linux/bpf.h
> @@ -2576,6 +2576,7 @@ void bpf_dynptr_init(struct bpf_dynptr_kern *ptr, void *data,
> enum bpf_dynptr_type type, u32 offset, u32 size);
> void bpf_dynptr_set_null(struct bpf_dynptr_kern *ptr);
> int bpf_dynptr_check_size(u32 size);
> +void *bpf_dynptr_get_data(struct bpf_dynptr_kern *ptr, u32 *avail_bytes);
>
> #ifdef CONFIG_BPF_LSM
> void bpf_cgroup_atype_get(u32 attach_btf_id, int cgroup_atype);
> diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
> index 3c1b9bbcf971..91f406a9c37f 100644
> --- a/kernel/bpf/helpers.c
> +++ b/kernel/bpf/helpers.c
> @@ -1461,6 +1461,16 @@ void bpf_dynptr_init(struct bpf_dynptr_kern *ptr, void *data,
> bpf_dynptr_set_type(ptr, type);
> }
>
> +void *bpf_dynptr_get_data(struct bpf_dynptr_kern *ptr, u32 *avail_bytes)
> +{
> + u32 size = bpf_dynptr_get_size(ptr);
> +
> + if (!ptr->data || ptr->offset > size)
The "ptr->offset > size" check isn't quite correct because size is the
number of usable bytes (more on this below :))
> + return NULL;
> + *avail_bytes = size - ptr->offset;
dynptr->size is already the number of usable bytes; this is noted in
include/linux/bpf.h
/* the implementation of the opaque uapi struct bpf_dynptr */
struct bpf_dynptr_kern {
void *data;
/* Size represents the number of usable bytes of dynptr data.
* If for example the offset is at 4 for a local dynptr whose data is
* of type u64, the number of usable bytes is 4.
*
* The upper 8 bits are reserved. It is as follows:
* Bits 0 - 23 = size
* Bits 24 - 30 = dynptr type
* Bit 31 = whether dynptr is read-only
*/
u32 size;
u32 offset;
} __aligned(8);
> + return ptr->data + ptr->offset;
> +}
> +
> void bpf_dynptr_set_null(struct bpf_dynptr_kern *ptr)
> {
> memset(ptr, 0, sizeof(*ptr));
> --
> 2.37.2
>
