On 03/15/2018 02:45 PM, Alexei Starovoitov wrote: > 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? >
Yep just a hold-over from an earlier patch description. I'll remove that entire note2 and fixup a couple small things Daniel noticed with a v3. > The semantics of the helper looks great. > Great!
