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

Al Viro
2022-07-04T13:55:41Z

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;

Make that
[code assigning ret a non-negative value and never using seqp]
if (unlikely(ret)) {
[code never using seqp or ret]
} else {
*seqp = 0; /* out of RCU mode, so the value doesn't matter */
}
return ret;

so if (err < 0) in the caller is equivalent to if (err).