namei: allow nd_jump_link() to produce errors

In preparation for LOOKUP_NO_MAGICLINKS, it's necessary to add the
ability for nd_jump_link() to return an error which the corresponding
get_link() caller must propogate back up to the VFS.
Suggested-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/namei.c

@@ -859,7 +859,7 @@ static int nd_jump_root(struct nameidata *nd)
  * Helper to directly jump to a known parsed path from ->get_link,
  * caller must have taken a reference to path beforehand.
  */
-void nd_jump_link(struct path *path)
+int nd_jump_link(struct path *path)
 {
 	struct nameidata *nd = current->nameidata;
 	path_put(&nd->path);
@@ -867,6 +867,7 @@ void nd_jump_link(struct path *path)
 	nd->path = *path;
 	nd->inode = nd->path.dentry->d_inode;
 	nd->flags |= LOOKUP_JUMPED;
+	return 0;
 }
 
 static inline void put_link(struct nameidata *nd)

fs/proc/base.c

@@ -1626,8 +1626,7 @@ static const char *proc_pid_get_link(struct dentry *dentry,
 	if (error)
 		goto out;
 
-	nd_jump_link(&path);
-	return NULL;
+	error = nd_jump_link(&path);
 out:
 	return ERR_PTR(error);
 }

fs/proc/namespaces.c

@@ -51,11 +51,15 @@ static const char *proc_ns_get_link(struct dentry *dentry,
 	if (!task)
 		return ERR_PTR(-EACCES);
 
-	if (ptrace_may_access(task, PTRACE_MODE_READ_FSCREDS)) {
+	if (!ptrace_may_access(task, PTRACE_MODE_READ_FSCREDS))
+		goto out;
+
 	error = ns_get_path(&ns_path, task, ns_ops);
-		if (!error)
-			nd_jump_link(&ns_path);
-	}
+	if (error)
+		goto out;
+
+	error = nd_jump_link(&ns_path);
+out:
 	put_task_struct(task);
 	return ERR_PTR(error);
 }

include/linux/namei.h

@@ -69,7 +69,7 @@ extern int follow_up(struct path *);
 extern struct dentry *lock_rename(struct dentry *, struct dentry *);
 extern void unlock_rename(struct dentry *, struct dentry *);
 
-extern void nd_jump_link(struct path *path);
+extern int __must_check nd_jump_link(struct path *path);
 
 static inline void nd_terminate_link(void *name, size_t len, size_t maxlen)
 {

security/apparmor/apparmorfs.c

@@ -2573,16 +2573,18 @@ static const char *policy_get_link(struct dentry *dentry,
 {
 	struct aa_ns *ns;
 	struct path path;
+	int error;
 
 	if (!dentry)
 		return ERR_PTR(-ECHILD);
+
 	ns = aa_get_current_ns();
 	path.mnt = mntget(aafs_mnt);
 	path.dentry = dget(ns_dir(ns));
-	nd_jump_link(&path);
+	error = nd_jump_link(&path);
 	aa_put_ns(ns);
 
-	return NULL;
+	return ERR_PTR(error);
 }
 
 static int policy_readlink(struct dentry *dentry, char __user *buffer,