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.) Thanks, Michael -- Michael Kerrisk Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/ Linux/UNIX System Programming Training: http://man7.org/training/
