ALSA: info: Return error for invalid read/write

Currently the ALSA proc handler allows read or write even if the proc
file were write-only or read-only.  It's mostly harmless, does thing
but allocating memory and ignores the input/output.  But it doesn't
tell user about the invalid use, and it's confusing and inconsistent
in comparison with other proc files.

This patch adds some sanity checks and let the proc handler returning
an -EIO error when the invalid read/write is performed.

Cc: <stable@vger.kernel.org> # v4.2+
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/core/info.c

@@ -325,6 +325,8 @@ static ssize_t snd_info_text_entry_write(struct file *file,
 	size_t next;
 	int err = 0;
 
+	if (!entry->c.text.write)
+		return -EIO;
 	pos = *offset;
 	if (!valid_pos(pos, count))
 		return -EIO;
@@ -369,7 +371,9 @@ static int snd_info_seq_show(struct seq_file *seq, void *p)
 	struct snd_info_private_data *data = seq->private;
 	struct snd_info_entry *entry = data->entry;
 
-	if (entry->c.text.read) {
+	if (!entry->c.text.read) {
+		return -EIO;
+	} else {
 		data->rbuffer->buffer = (char *)seq; /* XXX hack! */
 		entry->c.text.read(entry, data->rbuffer);
 	}