git: select: Don't allow unwanted/leftover fds being returned.

[Sepherosa Ziehau]

commit ce4975442fa0524017fb3c1aef93bbe6880ae770
Author: Sepherosa Ziehau <sephe at dragonflybsd.org>
Date:   Mon Aug 28 21:49:00 2017 +0800

    select: Don't allow unwanted/leftover fds being returned.
    
    The root cause is that the lwp_kqueue_serial will wrap pretty quickly,
    6 seconds on my laptop, if the select(2) is polling, either due to heavy
    workload or 0 timeout.  The POC test:
    https://leaf.dragonflybsd.org/~sephe/select_wrap.c
    
    Fixing this issue by saving the original fd_sets and do additional
    kevent filtering before return the fd to userland.
    
    poll(2) suffers the similar issue and will be fixed in later commit.
    
    Reported-by: many

Summary of changes:
 sys/kern/sys_generic.c | 106 ++++++++++++++++++++++++++++++++-----------------
 1 file changed, 69 insertions(+), 37 deletions(-)

http://gitweb.dragonflybsd.org/dragonfly.git/commitdiff/ce4975442fa0524017fb3c1aef93bbe6880ae770


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