4.4-stable review patch. If anyone has any objections, please let me know.
------------------
From: Konstantin Khlebnikov <khlebnikov@xxxxxxxxxxxxxx>
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.
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
Acked-by: Tejun Heo <tj@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
fs/sysfs/file.c | 8 +++++++-
1 file changed, 7 insertions(+), 1 deletion(-)
--- a/fs/sysfs/file.c
+++ b/fs/sysfs/file.c
@@ -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);
}
--
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