Re: Potentially undesirable interactions between vfork() and time namespaces

[Christian Brauner <brauner@kernel.org>]

On Wed, Sep 07, 2022 at 10:15:51AM -0700, Andrei Vagin wrote:
> On Wed, Sep 07, 2022 at 08:33:20AM +0300, Alexey Izbyshev wrote:
> > > 
> > > That is something to be double checked.
> > > 
> > > I can't see where it would make sense to unshare a time namespace and
> > > then call exec, instead of calling exit.  So I suspect we can just
> > > change this behavior and no one will notice.
> > > 
> > One can imagine a helper binary that calls unshare, forks some children in
> > new namespaces, and then calls exec to hand off actual work to another
> > binary (which might not expect being in the new time namespace). I'm purely
> > theorizing here, however. Keeping a special case for vfork() based only on
> > FUD is likely a net negative, so it'd be nice to hear actual time namespace
> > users speak up, and switch to the solution you suggested if they don't care.
> 
> I can speak for one tool that uses time namespaces for the right
> reasons. It is CRIU.  When a process is restored, the monotonic and
> boottime clocks have to be adjusted to match old values. It is for what
> the timens was designed for. These changes doesn't affect CRIU.
> 
> Honestly, I haven't heard about other users of timens yet. I don't take
> into account tools like unshare.

LXC/LXD does

unshare(CLONE_NEWTIME)
// write offsets to /proc/self/timens_offsets
timens_fd = open("/proc/self/ns/time_for_children", O_RDONLY | O_CLOEXEC)
setns(timens_fd, CLONE_NEWTIME)
exec(payload)

so I agree don't change the uapi, please.

But as you can see what we do is basically emulating changing time
namespace during exec via the setns() prior to the exec call.