afs: Fix characters allowed into cell names

The afs filesystem needs to prohibit certain characters from cell names,
such as '/', as these are used to form filenames in procfs, leading to
the following warning being generated:

WARNING: CPU: 0 PID: 3489 at fs/proc/generic.c:178

Fix afs_alloc_cell() to disallow nonprintable characters, '/', '@' and
names that begin with a dot.

Remove the check for "@cell" as that is then redundant.

This can be tested by running:

echo add foo/.bar 1.2.3.4 >/proc/fs/afs/cells

Note that we will also need to deal with:

- Names ending in ".invalid" shouldn't be passed to the DNS.

- Names that contain non-valid domainname chars shouldn't be passed to
the DNS.

- DNS replies that say "your-dns-needs-immediate-attention.<gTLD>" and
replies containing A records that say 127.0.53.53 should be
considered invalid.
[https://www.icann.org/en/system/files/files/name-collision-mitigation-01aug14-en.pdf]

but these need to be dealt with by the kafs-client DNS program rather
than the kernel.

Reported-by: [e-mail]
Cc: [e-mail]
Signed-off-by: David Howells <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

fs/afs/cell.c

···
 		_leave(" = -ENAMETOOLONG");
 		return ERR_PTR(-ENAMETOOLONG);
 	}
-	if (namelen == 5 && memcmp(name, "@cell", 5) == 0)
+
+	/* Prohibit cell names that contain unprintable chars, '/' and '@' or
+	 * that begin with a dot.  This also precludes "@cell".
+	 */
+	if (name[0] == '.')
 		return ERR_PTR(-EINVAL);
+	for (i = 0; i < namelen; i++) {
+		char ch = name[i];
+		if (!isprint(ch) || ch == '/' || ch == '@')
+			return ERR_PTR(-EINVAL);
+	}
 
 	_enter("%*.*s,%s", namelen, namelen, name, addresses);