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

Aleksa Sarai
2019-11-13T03:56:14Z

[-- Attachment #1: Type: text/plain, Size: 2141 bytes --]

On 2019-11-13, Al Viro <viro@zeniv.linux.org.uk> wrote:
> On Wed, Nov 13, 2019 at 01:44:14PM +1100, Aleksa Sarai wrote:
> > On 2019-11-13, Al Viro <viro@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".

Ah okay, I'd misunderstood the point you were making in that thread.

> 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.

Yup, agreed.

--
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>

[-- Attachment #2: signature.asc --]
[-- Type: application/pgp-signature, Size: 228 bytes --]