On 2019-07-12, Al Viro [off-list ref] wrote:

On Sun, Jul 07, 2019 at 12:57:32AM +1000, Aleksa Sarai wrote:

quoted

@@ -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;

quoted

@@ -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.

quoted

+	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/>