Re: [PATCH v4 43/45] namei: initialize parameters passed to step_into()

[Alexander Potapenko]

On Mon, Jul 4, 2022 at 6:00 PM Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote:
>
> On Mon, Jul 04, 2022 at 02:44:00PM +0100, Al Viro wrote:
> > On Mon, Jul 04, 2022 at 10:20:53AM +0200, Alexander Potapenko wrote:
> >
> > > What makes you think they are false positives? Is the scenario I
> > > described above:
> > >
> > > """
> > > In particular, if the call to lookup_fast() in walk_component()
> > > returns NULL, and lookup_slow() returns a valid dentry, then the
> > > `seq` and `inode` will remain uninitialized until the call to
> > > step_into()
> > > """
> > >
> > > impossible?
> >
> > Suppose step_into() has been called in non-RCU mode.  The first
> > thing it does is
> >       int err = handle_mounts(nd, dentry, &path, &seq);
> >       if (err < 0)
> >               return ERR_PTR(err);
> >
> > And handle_mounts() in non-RCU mode is
> >       path->mnt = nd->path.mnt;
> >       path->dentry = dentry;
> >       if (nd->flags & LOOKUP_RCU) {
> >               [unreachable code]
> >       }
> >       [code not touching seqp]
> >       if (unlikely(ret)) {
> >               [code not touching seqp]
> >       } else {
> >               *seqp = 0; /* out of RCU mode, so the value doesn't matter */
> >       }
> >       return ret;
> >
> > In other words, the value seq argument of step_into() used to have ends up
> > being never fetched and, in case step_into() gets past that if (err < 0)
> > that value is replaced with zero before any further accesses.
> >
> > So it's a false positive; yes, strictly speaking compiler is allowd
> > to do anything whatsoever if it manages to prove that the value is
> > uninitialized.  Realistically, though, especially since unsigned int
> > is not allowed any trapping representations...
>
> FWIW, update (and yet untested) branch is in #work.namei.  Compared to the
> previous, we store sampled ->d_seq of the next dentry in nd->next_seq,
> rather than bothering with local variables.  AFAICS, it ends up with
> better code that way.  And both ->seq and ->next_seq are zeroed at the
> moments when we switch to non-RCU mode (as well as non-RCU path_init()).
>
> IMO it looks saner that way.  NOTE: it still needs to be tested and probably
> reordered and massaged; it's not for merge at the moment.  Current cumulative
> diff follows:

I confirm all KMSAN reports are gone as a result of applying this patch.


-- 
Alexander Potapenko
Software Engineer

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