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SubjectRe: [PATCH v9 05/10] namei: O_BENEATH-style path resolution flags
On 2019-07-12, Al Viro <viro@zeniv.linux.org.uk> wrote:
> On Sun, Jul 07, 2019 at 12:57:32AM +1000, Aleksa Sarai wrote:
> > @@ -1442,8 +1464,11 @@ static int follow_dotdot_rcu(struct nameidata *nd)
> > struct inode *inode = nd->inode;
> >
> > while (1) {
> > - if (path_equal(&nd->path, &nd->root))
> > + if (path_equal(&nd->path, &nd->root)) {
> > + if (unlikely(nd->flags & LOOKUP_BENEATH))
> > + return -EXDEV;
>
> > @@ -1468,6 +1493,8 @@ static int follow_dotdot_rcu(struct nameidata *nd)
> > return -ECHILD;
> > if (&mparent->mnt == nd->path.mnt)
> > break;
> > + if (unlikely(nd->flags & LOOKUP_XDEV))
> > + return -EXDEV;
> > /* we know that mountpoint was pinned */
> > nd->path.dentry = mountpoint;
> > nd->path.mnt = &mparent->mnt;
> > @@ -1482,6 +1509,8 @@ static int follow_dotdot_rcu(struct nameidata *nd)
> > return -ECHILD;
> > if (!mounted)
> > break;
> > + if (unlikely(nd->flags & LOOKUP_XDEV))
> > + return -EXDEV;
>
> Are you sure these failure exits in follow_dotdot_rcu() won't give
> suprious hard errors?

I could switch to -ECHILD for the *_rcu() checks if you'd prefer that.
Though, I'd have (probably naively) thought that you'd have already
gotten -ECHILD from the seqlock checks if there was a race during ".."
handling.

> > + if (unlikely(nd->flags & LOOKUP_BENEATH)) {
> > + error = dirfd_path_init(nd);
> > + if (unlikely(error))
> > + return ERR_PTR(error);
> > + nd->root = nd->path;
> > + if (!(nd->flags & LOOKUP_RCU))
> > + path_get(&nd->root);
> > + }
> > if (*s == '/') {
> > if (likely(!nd->root.mnt))
> > set_root(nd);
> > @@ -2350,9 +2400,11 @@ static const char *path_init(struct nameidata *nd, unsigned flags)
> > s = ERR_PTR(error);
> > return s;
> > }
> > - error = dirfd_path_init(nd);
> > - if (unlikely(error))
> > - return ERR_PTR(error);
> > + if (likely(!nd->path.mnt)) {
>
> Is that a weird way of saying "if we hadn't already called dirfd_path_init()"?

Yes. I did it to be more consistent with the other "have we got the
root" checks elsewhere. Is there another way you'd prefer I do it?

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