[added to the 4.1 stable tree] sysfs: correctly handle read offset on PREALLOC attrs

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From: Konstantin Khlebnikov <khlebnikov@xxxxxxxxxxxxxx>

This patch has been added to the 4.1 stable tree. If you have any
objections, please let us know.

===============

[ Upstream commit 17d0774f80681020eccc9638d925a23f1fc4f671 ]

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.

Signed-off-by: Konstantin Khlebnikov <khlebnikov@xxxxxxxxxxxxxx>
Fixes: 4ef67a8c95f3 ("sysfs/kernfs: make read requests on pre-alloc files use the buffer.")
Link: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=787950
Cc: Stable <stable@xxxxxxxxxxxxxxx> # v3.19+
Acked-by: Tejun Heo <tj@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
Signed-off-by: Sasha Levin <alexander.levin@xxxxxxxxxxx>
---
 fs/sysfs/file.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/fs/sysfs/file.c b/fs/sysfs/file.c
index 9905595..167f80e 100644
--- a/fs/sysfs/file.c
+++ b/fs/sysfs/file.c
@@ -114,9 +114,15 @@ static ssize_t sysfs_kf_read(struct kernfs_open_file *of, char *buf,
 	 * 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);
 }
 
-- 
2.7.4
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