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

On 03/19/2018 09:30 AM, Alexei Starovoitov wrote:

On Sun, Mar 18, 2018 at 12:57:20PM -0700, John Fastabend wrote:

quoted

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?

Correct, setting cork_bytes or apply_bytes to zero is just a nop.

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