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

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.