Re: [PATCH v2 07/39] mount: attach mappings to mounts

On Tue, Nov 24, 2020 at 02:40:35PM +0100, Christian Brauner wrote: > On Tue, Nov 24, 2020 at 08:37:40AM -0500, Tycho Andersen wrote: > > On Tue, Nov 24, 2020 at 01:30:35PM +0100, Christian Brauner wrote: > > > On Mon, Nov 23, 2020 at 11:24:28AM -0500, Tycho Andersen wrote: > > > > On Mon, Nov 23, 2020 at 10:47:19AM -0500, Tycho Andersen wrote: > > > > > On Sun, Nov 15, 2020 at 11:36:46AM +0100, Christian Brauner wrote: > > > > > > +static inline struct user_namespace *mnt_user_ns(const struct vfsmount *mnt) > > > > > > +{ > > > > > > + return mnt->mnt_user_ns; > > > > > > +} > > > > > > > > > > I think you might want a READ_ONCE() here. Right now it seems ok, since the > > > > > mnt_user_ns can't change, but if we ever allow it to change (and I see you have > > > > > a idmapped_mounts_wip_v2_allow_to_change_idmapping branch on your public tree > > > > > :D), the pattern of, > > > > > > > > > > user_ns = mnt_user_ns(path->mnt); > > > > > if (mnt_idmapped(path->mnt)) { > > > > > uid = kuid_from_mnt(user_ns, uid); > > > > > gid = kgid_from_mnt(user_ns, gid); > > > > > } > > > > > > > > > > could race. > > > > > > > > Actually, isn't a race possible now? > > > > > > > > kuid_from_mnt(mnt_user_ns(path->mnt) /* &init_user_ns */); > > > > WRITE_ONCE(mnt->mnt.mnt_user_ns, user_ns); > > > > WRITE_ONCE(m->mnt.mnt_flags, flags); > > > > kgid_from_mnt(mnt_user_ns(path->mnt) /* the right user ns */); > > > > > > > > So maybe it should be: > > > > > > > > if (mnt_idmapped(path->mnt)) { > > > > barrier(); > > > > user_ns = mnt_user_ns(path->mnt); > > > > uid = kuid_from_mnt(user_ns, uid); > > > > gid = kgid_from_mnt(user_ns, gid); > > > > } > > > > > > > > since there's no data dependency between mnt_idmapped() and > > > > mnt_user_ns()? > > > > > > I think I had something to handle this case in another branch of mine. > > > The READ_ONCE() you mentioned in another patch I had originally dropped > > > because I wasn't sure whether it works on pointers but after talking to > > > Jann and David it seems that it handles pointers fine. > > > Let me take a look and fix it in the next version. I just finished > > > porting the test suite to xfstests as Christoph requested and I'm > > > looking at this now. > > > > Another way would be to just have mnt_idmapped() test > > mnt_user_ns() != &init_user_ns instead of the flags; then I think you > > get the data dependency and thus correct ordering for free. > > I indeed dropped mnt_idmapped() which is unnecessary. :) It still might be a nice helper to prevent people from checking the flags and forgetting that there's a memory ordering issue, though.