svn commit: r352231 - head/lib/libc/sys

[Alan Somers]

On Wed, Sep 11, 2019 at 3:50 PM Ian Lepore <ian at freebsd.org> wrote:

> On Wed, 2019-09-11 at 19:48 +0000, Alan Somers wrote:
> > Author: asomers
> > Date: Wed Sep 11 19:48:32 2019
> > New Revision: 352231
> > URL: https://svnweb.freebsd.org/changeset/base/352231
> >
> > Log:
> >   getsockopt.2: clarify that SO_TIMESTAMP is not 100% reliable
> >
> >   When SO_TIMESTAMP is set, the kernel will attempt to attach a
> timestamp as
> >   ancillary data to each IP datagram that is received on the socket.
> However,
> >   it may fail, for example due to insufficient memory. In that case the
> >   packet will still be received but not timestamp will be attached.
> >
> >   Reviewed by:        kib
> >   MFC after:  3 days
> >   Differential Revision:      https://reviews.freebsd.org/D21607
> >
> > Modified:
> >   head/lib/libc/sys/getsockopt.2
> >
> > Modified: head/lib/libc/sys/getsockopt.2
> >
> ==============================================================================
> > --- head/lib/libc/sys/getsockopt.2    Wed Sep 11 19:29:40 2019
> (r352230)
> > +++ head/lib/libc/sys/getsockopt.2    Wed Sep 11 19:48:32 2019
> (r352231)
> > @@ -28,7 +28,7 @@
> >  .\"     @(#)getsockopt.2     8.4 (Berkeley) 5/2/95
> >  .\" $FreeBSD$
> >  .\"
> > -.Dd February 10, 2019
> > +.Dd September 11, 2019
> >  .Dt GETSOCKOPT 2
> >  .Os
> >  .Sh NAME
> > @@ -431,7 +431,8 @@ option is enabled on a
> >  .Dv SOCK_DGRAM
> >  socket, the
> >  .Xr recvmsg 2
> > -call will return a timestamp corresponding to when the datagram was
> received.
> > +call may return a timestamp corresponding to when the datagram was
> received.
> > +However, it may not, for example due to a resource shortage.
> >  The
> >  .Va msg_control
> >  field in the
> >
>
> So I guess this actually happened to someone... is it a common thing
> for the timestamp to fail?  I ask because ntpd relies on SO_TIMESTAMP
> and if this situation really happens and can persist for a long time,
> ntpd would effectively stop working.
>
> -- Ian
>

pho discovered how to trigger it.  If you start 50 ping processes
simultaneously, sometimes a few will fail.  Will ntpd be ok with a single
failure, as long as the timestamp is received correctly in a subsequent
packet?
-Alan