• Linus Torvalds's avatar
    Merge branch 'fixes' of git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs · 0aecba61
    Linus Torvalds authored
    Pull vfs d_inode/d_flags memory ordering fixes from Al Viro:
     "Fallout from tree-wide audit for ->d_inode/->d_flags barriers use.
      Basically, the problem is that negative pinned dentries require
      careful treatment - unless ->d_lock is locked or parent is held at
      least shared, another thread can make them positive right under us.
    
      Most of the uses turned out to be safe - the main surprises as far as
      filesystems are concerned were
    
       - race in dget_parent() fastpath, that might end up with the caller
         observing the returned dentry _negative_, due to insufficient
         barriers. It is positive in memory, but we could end up seeing the
         wrong value of ->d_inode in CPU cache. Fixed.
    
       - manual checks that result of lookup_one_len_unlocked() is positive
         (and rejection of negatives). Again, insufficient barriers (we
         might end up with inconsistent observed values of ->d_inode and
         ->d_flags). Fixed by switching to a new primitive that does the
         checks itself and returns ERR_PTR(-ENOENT) instead of a negative
         dentry. That way we get rid of boilerplate converting negatives
         into ERR_PTR(-ENOENT) in the callers and have a single place to
         deal with the barrier-related mess - inside fs/namei.c rather than
         in every caller out there.
    
      The guts of pathname resolution *do* need to be careful - the race
      found by Ritesh is real, as well as several similar races.
      Fortunately, it turns out that we can take care of that with fairly
      local changes in there.
    
      The tree-wide audit had not been fun, and I hate the idea of repeating
      it. I think the right approach would be to annotate the places where
      we are _not_ guaranteed ->d_inode/->d_flags stability and have sparse
      catch regressions. But I'm still not sure what would be the least
      invasive way of doing that and it's clearly the next cycle fodder"
    
    * 'fixes' of git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs:
      fs/namei.c: fix missing barriers when checking positivity
      fix dget_parent() fastpath race
      new helper: lookup_positive_unlocked()
      fs/namei.c: pull positivity check into follow_managed()
    0aecba61
mount.c 9.56 KB