* Kees Cook: > On Sun, Jun 12, 2022 at 11:07:22PM -0700, Andrei Vagin wrote: >> Right now, a new process can't be forked in another time namespace >> if it shares mm with its parent. It is prohibited, because each time >> namespace has its own vvar page that is mapped into a process address >> space. >> >> When a process calls exec, it gets a new mm and so it could be "legal" >> to switch time namespace in that case. This was not implemented and >> now if we want to do this, we need to add another clone flag to not >> break backward compatibility. >> >> We don't have any user requests to switch times on exec except the >> vfork+exec combination, so there is no reason to add a new clone flag. >> As for vfork+exec, this should be safe to allow switching timens with >> the current clone flag. Right now, vfork (CLONE_VFORK | CLONE_VM) fails >> if a child is forked into another time namespace. With this change, >> vfork creates a new process in parent's timens, and the following exec >> does the actual switch to the target time namespace. > > This seems like a very special case. None of the other namespaces do > this, do they? I think this started with CLONE_NEWPID, which had a similar delayed effect with unshare: it happens only after fork, not for the current process image. I think it's just a limitation of the unshare interface. Some of the effects simply have to be delayed due to their nature. Thanks, Florian
