On Tue, Feb 18, 2020 at 06:11:26PM +0100, Adrian Reber wrote: > On Tue, Feb 18, 2020 at 12:03:31AM +0100, Christian Brauner wrote: > > On Mon, Feb 17, 2020 at 10:47:53PM +0100, Michael Kerrisk (man-pages) wrote: > > > Hello Christian, > > > > > > On Mon, 17 Feb 2020 at 16:15, Christian Brauner > > > <christian.brauner@xxxxxxxxxx> wrote: > > > > > > > > On Mon, Feb 17, 2020 at 03:20:55PM +0100, Michael Kerrisk wrote: > > > > > Hello Dmitry, Andrei, > > > > > > > > > > Is the CLONE_NEWTIME flag intended to be usable with clone3()? The > > > > > mail quoted below implies (in my reading) that this should be possible > > > > > once clone3() is available, which it is by now. (See also [1].) > > > > > > > > > > If the answer is yes, CLONE_NEWTIME should be usable with clone3(), > > > > > then I have a bug report and a question. > > > > > > > > > > I successfully used CLONE_NEWTIME with unshare(). But if I try to use > > > > > CLONE_NEWSIGNAL with clone3(), it errors out with EINVAL, because of > > > > > > > > s/CLONE_NEWSIGNAL/CLONE_NEWTIME/ > > > > > > > > > the following check in clone3_args_valid(): > > > > > > > > > > /* > > > > > * - make the CLONE_DETACHED bit reuseable for clone3 > > > > > * - make the CSIGNAL bits reuseable for clone3 > > > > > */ > > > > > if (kargs->flags & (CLONE_DETACHED | CSIGNAL)) > > > > > return false; > > > > > > > > > > The problem is that CLONE_NEWTIME matches one of the bits in the > > > > > CSIGNAL mask. If the intention is to allow CLONE_NEWTIME with > > > > > clone3(), then either the bit needs to be redefined, or the error > > > > > checking in clone3_args_valid() needs to be reworked. > > > > > > > > If this is intended to be useable with clone3() the check should be > > > > adapted to allow for CLONE_NEWTIME. (I asked about this a while ago I > > > > think.) > > > > But below rather sounds like it should simply be an unshare() flag. The > > > > code seems to set frozen_offsets to true right after copy_namespaces() > > > > in timens_on_fork(new_ns, tsk) and so the offsets can't be changed > > > > anymore unless I'm reading this wrong. > > > > Alternatives seem to either make timens_offsets writable once after fork > > > > and before exec, I guess - though that's probably not going to work > > > > with the vdso judging from timens_on_fork(). > > > > > > > > The other alternative is that Andrei and Dmitry send me a patch to > > > > enable CLONE_NEWTIME with clone3() by exposing struct timens_offsets (or > > > > a version of it) in the uapi and extend struct clone_args to include a > > > > pointer to a struct timens_offset that is _only_ set when CLONE_NEWTIME > > > > is set. > > > > Though the unshare() way sounds way less invasive simpler. > > > > > > Actually, I think the alternative you propose just here is better. I > > > imagine there are times when one will want to create multiple > > > namespaces with a single call to clone3(), including a time namespace. > > > I think this should be allowed by the API. And, otherwise, clone3() > > > becomes something of a second-class citizen for creating namespaces. > > > (I don't really get the "less invasive" argument. Implementing this is > > > just a piece of kernel to code to make user-space's life a bit simpler > > > and more consistent.) > > > > I don't particularly mind either way. If there's actual users that need > > to set it at clone3() time then we can extend it. So I'd like to hear > > what Adrian, Dmitry, and Thomas think since they are well-versed how > > this will be used in the wild. I'm weary of exposing a whole new uapi > > struct and extending clone3() without any real use-case but I'm happy to > > if there is! > > Re-creating a time namespace during restore via clone3() would be CRIU's > preferred way of doing this. If available CRIU is already using > clone3(), especially because of set_tid, and for the time namespace we > would also rely on clone3() to re-create it. > > I will provide a patch to extend clone3() to handle the creation of a > new time namespace. Great, thanks! Christian
