[PATCH v15 5/9] namei: LOOKUP_IN_ROOT: chroot-like scoped resolution

[Al Viro]

On Wed, Nov 13, 2019 at 01:44:14PM +1100, Aleksa Sarai wrote:
> On 2019-11-13, Al Viro <viro at zeniv.linux.org.uk> wrote:
> > On Tue, Nov 05, 2019 at 08:05:49PM +1100, Aleksa Sarai wrote:
> > 
> > > @@ -2277,12 +2277,20 @@ static const char *path_init(struct nameidata *nd, unsigned flags)
> > >  
> > >  	nd->m_seq = read_seqbegin(&mount_lock);
> > >  
> > > -	/* Figure out the starting path and root (if needed). */
> > > -	if (*s == '/') {
> > > +	/* Absolute pathname -- fetch the root. */
> > > +	if (flags & LOOKUP_IN_ROOT) {
> > > +		/* With LOOKUP_IN_ROOT, act as a relative path. */
> > > +		while (*s == '/')
> > > +			s++;
> > 
> > Er...  Why bother skipping slashes?  I mean, not only link_path_walk()
> > will skip them just fine, you are actually risking breakage in this:
> >                 if (*s && unlikely(!d_can_lookup(dentry))) {
> >                         fdput(f);
> >                         return ERR_PTR(-ENOTDIR);
> >                 }
> > which is downstream from there with you patch, AFAICS.
> 
> I switched to stripping the slashes at your suggestion a few revisions
> ago[1], and had (wrongly) assumed we needed to handle "/" somehow in
> path_init(). But you're quite right about link_path_walk() -- and I'd be
> more than happy to drop it.

That, IIRC, was about untangling the weirdness around multiple calls of
dirfd_path_init() and basically went "we might want just strip the slashes
in case of that flag very early in the entire thing, so that later the
normal logics for absolute/relative would DTRT".  Since your check is
right next to checking for absolute pathnames (and not in the very
beginning of path_init()), we might as well turn the check for
absolute pathname into *s == '/' && !(flags & LOOKUP_IN_ROOT) and be
done with that.