sysfs: Complain bitterly about attempts to remove files from nonexistent directories.

Recently an OOPS was observed from the usb serial io_ti driver when it tried to remove
sysfs directories.  Upon investigation it turns out this driver was always buggy
and that a recent sysfs change had stopped guarding itself against removing attributes
from sysfs directories that had already been removed. :(

Historically we have been silent about attempting to files from nonexistent sysfs
directories and have politely returned error codes.  That has resulted in people writing
broken code that ignores the error codes.

Issue a kernel WARNING and a stack backtrace to make it clear in no uncertain
terms that abusing sysfs is not ok, and the callers need to fix their code.

This change transforms the io_ti OOPS into a more comprehensible error message
and stack backtrace.
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
Reported-by: Wolfgang Frisch <wfpub@roembden.net>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

fs/sysfs/file.c

@@ -493,6 +493,12 @@ int sysfs_attr_ns(struct kobject *kobj, const struct attribute *attr,
 	const void *ns = NULL;
 	int err;
 
+	if (!dir_sd) {
+		WARN(1, KERN_ERR "sysfs: kobject %s without dirent\n",
+			kobject_name(kobj));
+		return -ENOENT;
+	}
+
 	err = 0;
 	if (!sysfs_ns_type(dir_sd))
 		goto out;

fs/sysfs/inode.c

@@ -318,8 +318,11 @@ int sysfs_hash_and_remove(struct sysfs_dirent *dir_sd, const void *ns, const cha
 	struct sysfs_addrm_cxt acxt;
 	struct sysfs_dirent *sd;
 
-	if (!dir_sd)
+	if (!dir_sd) {
+		WARN(1, KERN_WARNING "sysfs: can not remove '%s', no directory\n",
+			name);
 		return -ENOENT;
+	}
 
 	sysfs_addrm_start(&acxt, dir_sd);