On Sun, Jan 19, 2020 at 10:03:13AM +1100, Aleksa Sarai wrote: > > possibly trigger? The only things that ever clean ->root.mnt are > > You're quite right -- the codepath I was worried about was pick_link() > failing (which *does* clear nd->path.mnt, and I must've misread it at > the time as nd->root.mnt). pick_link() (allocation failure of external stack in RCU case, followed by failure to legitimize the link) is, unfortunately, subtle and nasty. We *must* path_put() the link; if we'd managed to legitimize the mount and failed on dentry, the mount needs to be dropped. No way around it. And while everything else there can be left for soon-to-be-reached terminate_walk(), this cannot. We have no good way to pass what we need to drop to the place where that eventual terminate_walk() drops rcu_read_lock(). So we end up having to do what terminate_walk() would've done and do it right there, so we could do that path_put(link) before we bugger off. I'm not happy about that, but I don't see cleaner solutions, more's the pity. However, it doesn't mess with ->root - nor should it, since we don't have LOOKUP_ROOT_GRABBED (not in RCU mode), so it can and should be left alone. > We can drop this check, though now complete_walk()'s main defence > against a NULL nd->root.mnt is that path_is_under() will fail and > trigger -EXDEV (or set_root() will fail at some point in the future). > However, as you pointed out, a NULL nd->root.mnt won't happen with > things as they stand today -- I might be a little too paranoid. :P The only reason why complete_walk() zeroes nd->root in some cases is microoptimization - we *know* we won't be using it later, so we don't care whether it's stale or not and can spare unlazy_walk() a bit of work. All there is to that one. I don't see any reason for adding code that would clear nd->root in later work; if such thing does get added (again, I don't see what purpose could that possibly serve), we'll need to watch out for a lot of things. Starting with LOOKUP_ROOT case... It's not something likely to slip in unnoticed.
