regmap: debugfs: Avoid overflows for very small reads

commit db04328c upstream.

If count is less than the size of a register then we may hit integer
wraparound when trying to move backwards to check if we're still in
the buffer. Instead move the position forwards to check if it's still
in the buffer, we are unlikely to be able to allocate a buffer
sufficiently big to overflow here.
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
[bwh: Backported to 3.2: formatted length is tot_len]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

drivers/base/regmap/regmap-debugfs.c

@@ -67,7 +67,7 @@ static ssize_t regmap_map_read_file(struct file *file, char __user *user_buf,
 		/* If we're in the region the user is trying to read */
 		if (p >= *ppos) {
 			/* ...but not beyond it */
-			if (buf_pos >= count - 1 - tot_len)
+			if (buf_pos + 1 + tot_len >= count)
 				break;
 
 			/* Format the register */