Re: [PATCH 1/2] clone3: add CLONE3_CLEAR_SIGHAND

[Christian Brauner <christian.brauner@ubuntu.com>]

On Sat, Oct 12, 2019 at 01:46:54PM +0200, Michael Kerrisk (man-pages) wrote:
> On 10/12/19 9:48 AM, Christian Brauner wrote:
> > On Sat, Oct 12, 2019 at 08:53:34AM +0200, Michael Kerrisk (man-pages) wrote:
> >> Hello Aleksa,
> >>
> >> On Sat, 12 Oct 2019 at 00:12, Aleksa Sarai <cyphar@cyphar.com> wrote:
> >>>
> >>> On 2019-10-11, Michael Kerrisk <mtk.manpages@gmail.com> wrote:
> >>>> Why CLONE3_CLEAR_SIGHAND rather than just CLONE_CLEAR_SIGHAND?
> > 
> > I don't care much how we name this apart from the "_CLEAR_SIGHAND"
> > suffix. But see for a little rationale below.
> > 
> >>>
> >>> There are no more flag bits left for the classic clone()/clone2() (the
> >>> last one was used up by CLONE_PIDFD) -- thus this flag is clone3()-only.
> >>
> >> Yes, I understand that. But, I'm not sure that the "3" in the prefix
> >> is necessary. "CLONE_" still seems better to me.
> >>
> >> Consider this: sometime in the near future we will probably have time
> >> namespaces. The new flag for those namespaces will only be usable with
> >> clone3(). It should NOT be called CLONE3_NEWTIME, but rather
> >> CLONE_NEWTIME (or similar), because that same flag will presumably
> >> also be used in other APIs such as unshare() and setns(). (Hmm -- I
> > 
> > There are some noteable differences though. CLONE_NEWTIME takes the
> > CSIGNAL bit which is in the range of a 32bit integer and thus useable by
> > unshare() too. The same does not hold for CLONE{3}_CLEAR_SIGHAND. You
> > can't pass it to unshare(). unshare() also just deals with
> > namespace-relevant stuff so CLONE{3}_CLEAR_SIGHAND doesn't make much
> > sense there.
> 
> Sure, but going forward there's very likely to be more CLONE flags
> for whatever reason, and some will be usable just in clone3()
> while others will be more widely used (in other APIs such as
> unshare() and setns()). Using two different prefixes for these
> flags (CLONE_/CLONE3_) would be just confusing. AFAICS, the CLONE3_

I do agree with that part. And as I said in my previous mail, I don't
care about the prefix.

> prefix really provides no advantage, but does have the potential to
> cause confusion down the track for the aforementioned reasons.
> (Furthermore... Shudder! What if there's a clone4() one day. I
> know you might say: "won't happen, we got things right this time",
> but API history suggests that "right" now not infrequently becomes
> "oops" later.) I do recommend CLONE_ for all the flags...

I do love your trust in our ability to design syscalls (//Cc Aleksa ;)). :)

> 
> >> wonder if we are going to need a new unshare2() or some such...)
> > 
> > We still have one 32bit bit left (CLONE_DETACHED) which we can't reuse
> > with clone()/clone2() but we can reuse with clone3(). We can simply
> > earmark it for namespace-related stuff and thus still have one bit left
> > for unshare() before we have to go for unshare2() (If we have to go
> > there at all since I'm not sure how much more namespaces we can come up
> > with.).
> 
> I'm sure there'll be more namespaces...

Let's hope not. :) Anyway, no real reason to do unshare2() any time
soon. :)

Christian