Re: [bpf-next PATCH v3 07/18] bpf: sockmap, add msg_cork_bytes() helper

Alexei Starovoitov
2018-03-19T16:30:16Z

On Sun, Mar 18, 2018 at 12:57:20PM -0700, John Fastabend wrote:
> In the case where we need a specific number of bytes before a
> verdict can be assigned, even if the data spans multiple sendmsg
> or sendfile calls. The BPF program may use msg_cork_bytes().
>
> The extreme case is a user can call sendmsg repeatedly with
> 1-byte msg segments. Obviously, this is bad for performance but
> is still valid. If the BPF program needs N bytes to validate
> a header it can use msg_cork_bytes to specify N bytes and the
> BPF program will not be called again until N bytes have been
> accumulated. The infrastructure will attempt to coalesce data
> if possible so in many cases (most my use cases at least) the
> data will be in a single scatterlist element with data pointers
> pointing to start/end of the element. However, this is dependent
> on available memory so is not guaranteed. So BPF programs must
> validate data pointer ranges, but this is the case anyways to
> convince the verifier the accesses are valid.
>
> Signed-off-by: John Fastabend <john.fastabend@gmail.com>
> ---
> include/uapi/linux/bpf.h | 3 ++-
> net/core/filter.c | 16 ++++++++++++++++
> 2 files changed, 18 insertions(+), 1 deletion(-)
>
> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> index a557a2a..1765cfb 100644
> --- a/include/uapi/linux/bpf.h
> +++ b/include/uapi/linux/bpf.h
> @@ -792,7 +792,8 @@ struct bpf_stack_build_id {
> FN(override_return), \
> FN(sock_ops_cb_flags_set), \
> FN(msg_redirect_map), \
> - FN(msg_apply_bytes),
> + FN(msg_apply_bytes), \
> + FN(msg_cork_bytes),
>
> /* integer value in 'imm' field of BPF_CALL instruction selects which helper
> * function eBPF program intends to call
> diff --git a/net/core/filter.c b/net/core/filter.c
> index 17d6775..0c9daf6 100644
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -1942,6 +1942,20 @@ struct sock *do_msg_redirect_map(struct sk_msg_buff *msg)
> .arg2_type = ARG_ANYTHING,
> };
>
> +BPF_CALL_2(bpf_msg_cork_bytes, struct sk_msg_buff *, msg, u32, bytes)
> +{
> + msg->cork_bytes = bytes;
> + return 0;
> +}

my understanding that setting it here and in the other helper *_bytes to zero
will be effectively a nop. Right?

Acked-by: Alexei Starovoitov <ast@kernel.org>