On Mon, Nov 29, 2021 at 2:39 PM Joanne Koong [off-list ref] wrote:

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 <redacted>
---

LGTM.

Acked-by: Andrii Nakryiko <andrii@kernel.org>

 include/linux/bpf.h            |  1 +
 include/uapi/linux/bpf.h       | 25 ++++++++++
 kernel/bpf/bpf_iter.c          | 35 ++++++++++++++
 kernel/bpf/helpers.c           |  2 +
 kernel/bpf/verifier.c          | 88 +++++++++++++++++++++-------------
 tools/include/uapi/linux/bpf.h | 25 ++++++++++
 6 files changed, 142 insertions(+), 34 deletions(-)

[...]