Re: [bpf-next PATCH v2 06/18] bpf: sockmap, add bpf_msg_apply_bytes() helper

On Mon, Mar 12, 2018 at 12:23:34PM -0700, John Fastabend wrote:

A single sendmsg or sendfile system call can contain multiple logical
messages that a BPF program may want to read and apply a verdict. But,
without an apply_bytes helper any verdict on the data applies to all
bytes in the sendmsg/sendfile. Alternatively, a BPF program may only
care to read the first N bytes of a msg. If the payload is large say
MB or even GB setting up and calling the BPF program repeatedly for
all bytes, even though the verdict is already known, creates
unnecessary overhead.

To allow BPF programs to control how many bytes a given verdict
applies to we implement a bpf_msg_apply_bytes() helper. When called
from within a BPF program this sets a counter, internal to the
BPF infrastructure, that applies the last verdict to the next N
bytes. If the N is smaller than the current data being processed
from a sendmsg/sendfile call, the first N bytes will be sent and
the BPF program will be re-run with start_data pointing to the N+1
byte. If N is larger than the current data being processed the
BPF verdict will be applied to multiple sendmsg/sendfile calls
until N bytes are consumed.

Note1 if a socket closes with apply_bytes counter non-zero this
is not a problem because data is not being buffered for N bytes
and is sent as its received.

Note2 if this is operating in the sendpage context the data
pointers may be zeroed after this call if the apply walks beyond
a msg_pull_data() call specified data range. (helper implemented
shortly in this series).

instead of 'shortly in this seris' you meant 'implemented earlier'?
patch 5 handles it, but it's set here, right?

The semantics of the helper looks great.