ISDN: fix drivers, by handling errors thrown by ->readstat()

This is a particularly ugly on-failure bug, possibly security, since the
lack of error handling here is covering up another class of bug: failure to
handle copy_to_user() return values.

The I4L API function ->readstat() returns an integer, and by looking at
several existing driver implementations, it is clear that a negative return
value was meant to indicate an error.

Given that several drivers already return a negative value indicating an
errno-style error, the current code would blindly accept that [negative]
value as a valid amount of bytes read.  Obvious damage ensues.

Correcting ->readstat() handling to properly notice errors fixes the
existing code to work correctly on error, and enables future patches to
more easily indicate errors during operation.

Signed-off-by: Jeff Garzik <jeff@garzik.org>
Signed-off-by: Adrian Bunk <bunk@stusta.de>

diff --git a/drivers/isdn/i4l/isdn_common.c b/drivers/isdn/i4l/isdn_common.c
index 22759c01746af5d71a1c016d79716d4ec3406c91..4cd1a5a0b7103a7fe70b727080882ede187daf19 100644 (file)
--- a/drivers/isdn/i4l/isdn_common.c
+++ b/drivers/isdn/i4l/isdn_common.c
@@ -1125,9 +1125,12 @@ isdn_read(struct file *file, char __user *buf, size_t count, loff_t * off)
                if (dev->drv[drvidx]->interface->readstat) {
                        if (count > dev->drv[drvidx]->stavail)
                                count = dev->drv[drvidx]->stavail;
-                       len = dev->drv[drvidx]->interface->
-                               readstat(buf, count, drvidx,
-                                        isdn_minor2chan(minor));
+                       len = dev->drv[drvidx]->interface->readstat(buf, count,
+                                               drvidx, isdn_minor2chan(minor));
+                       if (len < 0) {
+                               retval = len;
+                               goto out;
+                       }
                } else {
                        len = 0;
                }