Re: [PATCH bpf-next] bpf: ringbuf: Support consuming BPF_MAP_TYPE_RINGBUF from prog

[Daniel Xu]

On Tue, Sep 10, 2024, at 2:07 PM, Daniel Xu wrote:
> On Tue, Sep 10, 2024 at 01:41:41PM GMT, Andrii Nakryiko wrote:
>> On Tue, Sep 10, 2024 at 11:36 AM Alexei Starovoitov
[...]
>
>> 
>> Also, Daniel, can you please make sure that dynptr we return for each
>> sample is read-only? We shouldn't let consumer BPF program ability to
>> corrupt ringbuf record headers (accidentally or otherwise).
>
> Sure.

So the sample is not read-only. But I think prog is prevented from messing
with header regardless.

__bpf_user_ringbuf_peek() returns sample past the header:

        *sample = (void *)((uintptr_t)rb->data +
                           (uintptr_t)((cons_pos + BPF_RINGBUF_HDR_SZ) & rb->mask));

dynptr is initialized with the above ptr:

        bpf_dynptr_init(&dynptr, sample, BPF_DYNPTR_TYPE_LOCAL, 0, size);

So I don't think there's a way for the prog to access the header thru the dynptr.

Thanks,
Daniel