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

On Sun, Jul 07, 2019 at 12:57:28AM +1000, Aleksa Sarai wrote:

quoted

@@ -514,7 +516,14 @@ static void set_nameidata(struct nameidata *p, int dfd, struct filename *name)
 	p->stack = p->internal;
 	p->dfd = dfd;
 	p->name = name;
-	p->total_link_count = old ? old->total_link_count : 0;
+	p->total_link_count = 0;
+	p->acc_mode = 0;
+	p->opath_mask = FMODE_PATH_READ | FMODE_PATH_WRITE;
+	if (old) {
+		p->total_link_count = old->total_link_count;
+		p->acc_mode = old->acc_mode;
+		p->opath_mask = old->opath_mask;
+	}

Huh?  Could somebody explain why traversals of NFS4 referrals should inherit
->acc_mode and ->opath_mask?

I'll be honest -- I don't understand what set_nameidata() did so I just
did what I thought would be an obvious change (to just copy the
contents). I thought it was related to some aspect of the symlink stack
handling.

In that case, should they both be set to 0 on set_nameidata()? This will
mean that fd re-opening (or magic-link opening) through a
set_nameidata() would always fail.

quoted

 static __always_inline
-const char *get_link(struct nameidata *nd)
+const char *get_link(struct nameidata *nd, bool trailing)
 {
 	struct saved *last = nd->stack + nd->depth - 1;
 	struct dentry *dentry = last->link.dentry;

@@ -1081,6 +1134,44 @@ const char *get_link(struct nameidata *nd)
 		} else {
 			res = get(dentry, inode, &last->done);
 		}
+		/* If we just jumped it was because of a magic-link. */
+		if (unlikely(nd->flags & LOOKUP_JUMPED)) {

[...]
In any case, this "bool trailing" is completely wrong; whether that
check belongs in trailing_symlink() or (some of) its callers, putting
it into get_link() is a mistake, forced by kludgy check for procfs-style
symlinks.

The error path for LOOKUP_JUMPED comes from the old O_BENEATH patchset,
but all of the "bool trailing" logic is definitely my gaff (I was
quietly hoping you'd have a much better solution than the whole
get_link() thing -- it definitely felt very kludgey to write).

I will work on the suggestion in your follow-up email. Thanks!

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