drm/tinydrm: fix a loop in debugfs code

This code will likely crash if we try to do a zero byte write. The code looks like this:

        /* strip trailing whitespace */
        for (i = count - 1; i > 0; i--)
                if (isspace(buf[i]))
			...

We're writing zero bytes so count = 0. You would think that "count - 1" would be negative one, but because "i" is unsigned it is a large positive numer instead. The "i > 0" condition is true and the "buf[i]" access will be out of bounds.

The fix is to make "i" signed and now everything works as expected. The upper bound of "count" is capped in __kernel_write() at MAX_RW_COUNT so we don't have to worry about it being higher than INT_MAX.

---
 drivers/gpu/drm/drm_mipi_dbi.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

index 1961f713aaab..c4ee2709a6f3 100644
@@ -1187,8 +1187,7 @@ static ssize_t mipi_dbi_debugfs_command_write(struct file *file,
 	struct mipi_dbi_dev *dbidev = m->private;
 	u8 val, cmd = 0, parameters[64];
 	char *buf, *pos, *token;
-	unsigned int i;
-	int ret, idx;
+	int i, ret, idx;
 
 	if (!drm_dev_enter(&dbidev->drm, &idx))
 		return -ENODEV;

2.20.1

Den 21.08.2019 09.24, skrev Dan Carpenter:

Thanks Dan, applied to drm-misc-next with adjusted title:
-drm/tinydrm: fix a loop in debugfs code +drm/mipi-dbi: fix a loop in debugfs code

Noralf.