Re: recvfrom/recvmsg performance and CONFIG_HARDENED_USERCOPY

[Andy Lutomirski]

> On Dec 9, 2019, at 3:01 AM, David Laight <David.Laight@xxxxxxxxxx> wrote:
> 
> ïFrom: Eric Dumazet
>> Sent: 06 December 2019 14:22
> ...
>> Real question is : Do you actually need to use recvmsg() instead of recvfrom() ?
>> If recvmsg() provides additional cmsg, this is not surprising it is more expensive.
> 
> Except I'm not passing in a buffer for it.
> The reason I'm looking at recvmsg is that I'd like to use recvmmsg it order to
> read out more than one message from a socket without doing an extra poll().
> Note that I don't expect there to be a second message most of the time and
> almost never a third one.
> 
> Although I think that will only ever 'win' if recvmmsg() called vfs_poll() to find
> if there was more data to read before doing any of the copy_from_user() etc

I would suggest a more general improvement: add a -EAGAIN fast path to recvmsg().  If the socket is nonblocking and has no data to read, then there shouldnât be a need to process the iovec at all.