Patch "sysfs: correctly handle read offset on PREALLOC attrs" has been added to the 4.4-stable tree

This is a note to let you know that I've just added the patch titled

sysfs: correctly handle read offset on PREALLOC attrs

to the 4.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
sysfs-correctly-handle-read-offset-on-prealloc-attrs.patch
and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Konstantin Khlebnikov <[email protected]>
Date: Wed, 22 Jun 2016 21:42:16 +0300
Subject: sysfs: correctly handle read offset on PREALLOC attrs

From: Konstantin Khlebnikov <[email protected]>

commit 17d0774f80681020eccc9638d925a23f1fc4f671 upstream.

Attributes declared with __ATTR_PREALLOC use sysfs_kf_read() which returns zero bytes for non-zero offset. This breaks script checkarray in mdadm tool in debian where /bin/sh is 'dash' because its builtin 'read' reads only one byte at a time. Script gets 'i' instead of 'idle' when reads current action from /sys/block/$dev/md/sync_action and as a result does nothing.

This patch adds trivial implementation of partial read: generate whole string and move required part into buffer head.

---
 fs/sysfs/file.c |    8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

@@ -114,9 +114,15 @@ static ssize_t sysfs_kf_read(struct kern
 	 * If buf != of->prealloc_buf, we don't know how
 	 * large it is, so cannot safely pass it to ->show
 	 */
-	if (pos || WARN_ON_ONCE(buf != of->prealloc_buf))
+	if (WARN_ON_ONCE(buf != of->prealloc_buf))
 		return 0;
 	len = ops->show(kobj, of->kn->priv, buf);
+	if (pos) {
+		if (len <= pos)
+			return 0;
+		len -= pos;
+		memmove(buf, buf + pos, len);
+	}
 	return min(count, len);
 }
 

Patches currently in stable-queue which might be from [email protected] are

queue-4.4/sysfs-correctly-handle-read-offset-on-prealloc-attrs.patch