libfs: make simple_read_from_buffer conventional

Impact: have simple_read_from_buffer conform to standards

It was brought to my attention by Andrew Morton, Theodore Tso, and H.
Peter Anvin that a read from userspace should only return -EFAULT if
nothing was actually read.

Looking at the simple_read_from_buffer I noticed that this function does
not conform to that rule.  This patch fixes that function.

[[email protected]: simplification suggested by hpa]
[[email protected]: fix count==0 handling]
Signed-off-by: Steven Rostedt <[email protected]>
Cc: Al Viro <[email protected]>
Cc: Theodore Ts'o <[email protected]>
Cc: Ingo Molnar <[email protected]>
Signed-off-by: H. Peter Anvin <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Al Viro <[email protected]>

fs/libfs.c

@@ -527,14 +527,18 @@ ssize_t simple_read_from_buffer(void __user *to, size_t count, loff_t *ppos,
 				const void *from, size_t available)
 {
 	loff_t pos = *ppos;
+	size_t ret;
+
 	if (pos < 0)
 		return -EINVAL;
-	if (pos >= available)
+	if (pos >= available || !count)
 		return 0;
 	if (count > available - pos)
 		count = available - pos;
-	if (copy_to_user(to, from + pos, count))
+	ret = copy_to_user(to, from + pos, count);
+	if (ret == count)
 		return -EFAULT;
+	count -= ret;
 	*ppos = pos + count;
 	return count;
 }