Re: [PATCH v3 bpf-next 1/4] bpf: Add bpf_loop helper

["Toke Høiland-Jørgensen" <toke@redhat.com>]

Joanne Koong <joannekoong@fb.com> writes:

> This patch adds the kernel-side and API changes for a new helper
> function, bpf_loop:
>
> long bpf_loop(u32 nr_loops, void *callback_fn, void *callback_ctx,
> u64 flags);
>
> where long (*callback_fn)(u32 index, void *ctx);
>
> bpf_loop invokes the "callback_fn" **nr_loops** times or until the
> callback_fn returns 1. The callback_fn can only return 0 or 1, and
> this is enforced by the verifier. The callback_fn index is zero-indexed.
>
> A few things to please note:
> ~ The "u64 flags" parameter is currently unused but is included in
> case a future use case for it arises.
> ~ In the kernel-side implementation of bpf_loop (kernel/bpf/bpf_iter.c),
> bpf_callback_t is used as the callback function cast.
> ~ A program can have nested bpf_loop calls but the program must
> still adhere to the verifier constraint of its stack depth (the stack depth
> cannot exceed MAX_BPF_STACK))
> ~ Recursive callback_fns do not pass the verifier, due to the call stack
> for these being too deep.
> ~ The next patch will include the tests and benchmark
>
> Signed-off-by: Joanne Koong <joannekoong@fb.com>

Acked-by: Toke Høiland-Jørgensen <toke@redhat.com>