Re: [PATCH bpf-next v2 1/7] bpf: Implement bpf_probe_read_kernel_dynptr helper

[Alexei Starovoitov]

On Sat, Jan 25, 2025 at 5:05 PM Levi Zim <rsworktech@xxxxxxxxxxx> wrote:
>
> On 2025/1/26 00:58, Alexei Starovoitov wrote:
>  > On Sat, Jan 25, 2025 at 12:30 AM Levi Zim via B4 Relay
>  > <devnull+rsworktech.outlook.com@xxxxxxxxxx> wrote:
>  >> From: Levi Zim <rsworktech@xxxxxxxxxxx>
>  >>
>  >> This patch add a helper function bpf_probe_read_kernel_dynptr:
>  >>
>  >> long bpf_probe_read_kernel_dynptr(const struct bpf_dynptr *dst,
>  >>          u32 offset, u32 size, const void *unsafe_ptr, u64 flags);
>  > We stopped adding helpers years ago.
>  > Only new kfuncs are allowed.
>
> Sorry, I didn't know that. Just asking, is there any
> documentation/discussion
> about stopping adding helpers?
>
> I will switch the implementation to kfuncs in v3.
>
>  > This particular one doesn't look useful as-is.
>  > The same logic can be expressed with
>  > - create dynptr
>  > - dynptr_slice
>  > - copy_from_kernel
>
> By copy_from_kernel I assume you mean bpf_probe_read_kernel. The problem
> with dynptr_slice_rdwr and probe_read_kernel is that they only support a
> compile-time constant size [1].
>
> But in order to best utilize the space on a BPF ringbuf, it is possible
> to reserve a
> variable length of space as dynptr on a ringbuf with
> bpf_ringbuf_reserve_dynptr.

That makes sense. The commit log didn't call it out.
Please spell out the motivation clearly.
Also why bpf_probe_read_kernel_common ?
Do we need to memset() it on failure?