netdev - Re: [Y2038] Fourth draft of the Y2038 design document

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Date:   Wed, 7 Sep 2016 12:05:22 +0200
From:   Albert ARIBAUD <albert.aribaud@...ev.fr>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     y2038@...ts.linaro.org, Deepa Dinamani <deepa.kernel@...il.com>,
        libc-alpha@...rceware.org, netdev@...r.kernel.org,
        David Miller <davem@...emloft.net>
Subject: Re: [Y2038] Fourth draft of the Y2038 design document

Hi Arnd,

On Thu, 14 Jul 2016 13:39:13 +0200, Arnd Bergmann <arnd@...db.de>
wrote :

> On Wednesday, July 13, 2016 7:38:25 PM CEST Deepa Dinamani wrote:
> > On Wed, Jul 13, 2016 at 1:40 PM, Deepa Dinamani <deepa.kernel@...il.com> wrote:  
> 
>  [...]  
>  [...]  
> > >
> > > Another way of handling this would be to use the flags in sendmsg/recvmsg.
> > > Since cmsg is sent using these calls, at the time of call, sendmsg/recvmsg can
> > > indicate whether 64 bit time_t or 32 bit time_t is used.
> > > This will eliminate the need for new options and kernel need not depend on
> > > __USE_TIME_BITS64.  
> 
> Good point, I had forgotten how we discussed that a while ago.
> 
>  [...]  
>  [...]  
> > >
> > > But, if we are not using optlen here, then probably just using new numbers for
> > > timestamps also makes sense.  
> > 
> > Forgot to note one more thing here.
> > Since we are already using struct sizes for ioctls, shouldn't this be similar.
> > So optlen should also be okay?
> > Just as in the example Arnd pointed out above:
> > 
> > /* Set and get port timeout (struct timeval's) */
> > #define PPGETTIME       _IOR(PP_IOCTL, 0x95, struct timeval)
> > #define PPSETTIME       _IOW(PP_IOCTL, 0x96, struct timeval)  
> 
> I've added the netdev list and David Miller to Cc here, it seems
> either way would work, and I don't have a strong preference.
> 
> a) add a flag to recvmsg() that glibc always passes when called
>    using __USE_TIME_BITS64 to handle reading the timestamps, and
>    rely on the optlen value for SO_RCVTIMEO/SO_SNTTIMEO to
>    decide how to interpret the data
> 
> b) handle all five sockopts using conditional option numbers
>    and don't rely on the recvmsg() flag or optlen.
> 
> Using either a) or b) is probably better than a combination of
> them, so I've not listed that as another alternative.
> 
> To work around the header inclusion order problem we discussed
> earlier for approach b), I suppose we can do this using
> something like
> 
> 
> #define SO_RCVTIMEO_TIME32	21
> #define SO_RCVTIMEO_TIME64	53
> 
> #define SO_RCVTIMEO (__USE_TIME_BITS==64 ? SO_RCVTIMEO_TIME64 : SO_RCVTIMEO_TIME32)
> 
> where __USE_TIME_BITS is a macro defined by the libc. We cannot easily
> check for whether a macro is defined in a conditional expression,
> but I think the above should work, as long as we don't need use the
> value from assembler code.
> 
> 	Arnd

Seems like there was no reply on the netdev or libc-alpha lists or from
David. I personally am not fond of relying on length to determine which
variant of an argument we are dealing with, therefore I prefer option b
over option a and will update the design doc accordingly.

Cordialement,
Albert ARIBAUD
3ADEV