Commit 17d0774f authored Jun 22, 2016 by Konstantin Khlebnikov Committed by Greg Kroah-Hartman Aug 31, 2016

sysfs: correctly handle read offset on PREALLOC attrs

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@yandex-team.ru>
Fixes: 4ef67a8c ("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@vger.kernel.org> # v3.19+
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

parent 96b0af4b

fs/sysfs/file.c

+7 −1

Original line number	Diff line number	Diff line
		@@ -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);
		}

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