On Mon, May 04, 2020 at 12:09:46PM -0500, Eric W. Biederman wrote: > Christian Brauner <christian.brauner@xxxxxxxxxx> writes: > > > On Mon, May 04, 2020 at 11:25:07AM -0500, Eric W. Biederman wrote: > >> > >> I am not thrilled about treating nstype as a flags fields when it is not > >> currently. It was my hope when I designed the interface that not > >> treating nstype as a flags field would save us from the problem of bits > >> running out. > > > > Hm, I researched the setns() syscall history before that and I didn't > > see that reasoning anywhere. The "nstype" arg was originally advertised > > on the list as "having a flags field is useful in general". > > Take a look at the code. At the end of the day nstype is not treated at > all like a flags field. Oh, I wasn't trying to dispute that. I was just pointing at the history where using it as a flags field in the future wasn't in principle out of the question. > > It isn't a very important point. And it was certainly easier to use > the existing bits for essentially their existing meanings. But it was > certainly something I was thinking at the time. > > I think I left it as we can see either way, depending on how things > evolve. > > I can imagine a use for a nstype being a single namespace from a pidfd. > Do you have any actual usecases for setting some but not all of the > namespaces from a pidfd? If we don't have a compelling reason > I would like to kick that can down the road a ways farther. Yeah, I think so. We already have a few use-cases. The syscall interception stuff selectively attaches to subsets of namespaces depending on what namespaces are needed to emulate a given syscall. And the exec logic let's users select what namespaces to attach to. It's common to setns to a subset of namespaces to perform operations with privilege and then later attach others (often the userns). > > I am also remembering that that setns freed the low 8 bits. Which gave > some freedom beyond clone. > > >> That aside. It would be very good if the default version of setting > >> everything from a pidfd would set the root directory from the process it > >> is copying everything else from. > > > > I'm not sure I follow completely. If you specify CLONE_NEWNS then we do > > set the root directory with set_fs_root() in commit_nsset(). Or are you > > saying we should always do that independent of whether or not > > CLONE_NEWNS is specified? And if so could you explain why we'd want > > that? I'm sure I'm missing something! > > I am suggesting that when we do: > > "setns(pidfd, 0)" or "setns(pidfd, SETNS_PIDFD)" > > That the result is not just the namespaces changing but also the root > directory changing to the pids root directory. Something where the > whole is greater than the parts. Ok, I can see that being useful. But If we do this, then a new flag would be quite helpful. (I also think Michael had some reservations against re-using 0 for something like this.) But let me suggest moving your phrase from above down to here and say that we could kick that can down the road for a follow-up extension? Christian
