usb: usbtmc: Fix read_stb function and get_stb ioctl

commit acb3dac upstream.

The usbtmc488_ioctl_read_stb function relied on a positive return from
usbtmc_get_stb to reset the srq condition in the driver. The
USBTMC_IOCTL_GET_STB case tested for a positive return to return the stb
to the user.

Commit: <cac01bd178d6> ("usb: usbtmc: Fix erroneous get_stb ioctl
error returns") changed the return value of usbtmc_get_stb to 0 on
success instead of returning the value of usb_control_msg which is
positive in the normal case. This change caused the function
usbtmc488_ioctl_read_stb and the USBTMC_IOCTL_GET_STB ioctl to no
longer function correctly.

Change the test in usbtmc488_ioctl_read_stb to test for failure
first and return the failure code immediately.
Change the test for the USBTMC_IOCTL_GET_STB ioctl to test for 0
instead of a positive value.

Fixes: cac01bd ("usb: usbtmc: Fix erroneous get_stb ioctl error returns")
Cc: stable@vger.kernel.org
Signed-off-by: Dave Penkler <dpenkler@gmail.com>
Link: https://lore.kernel.org/r/20250521121656.18174-3-dpenkler@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: dpenkler

drivers/usb/class/usbtmc.c

@@ -565,14 +565,15 @@ static int usbtmc488_ioctl_read_stb(struct usbtmc_file_data *file_data,
 
 	rv = usbtmc_get_stb(file_data, &stb);
 
-	if (rv > 0) {
-		srq_asserted = atomic_xchg(&file_data->srq_asserted,
-					srq_asserted);
-		if (srq_asserted)
-			stb |= 0x40; /* Set RQS bit */
+	if (rv < 0)
+		return rv;
+
+	srq_asserted = atomic_xchg(&file_data->srq_asserted, srq_asserted);
+	if (srq_asserted)
+		stb |= 0x40; /* Set RQS bit */
+
+	rv = put_user(stb, (__u8 __user *)arg);
 
-		rv = put_user(stb, (__u8 __user *)arg);
-	}
 	return rv;
 
 }
@@ -2201,7 +2202,7 @@ static long usbtmc_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
 
 	case USBTMC_IOCTL_GET_STB:
 		retval = usbtmc_get_stb(file_data, &tmp_byte);
-		if (retval > 0)
+		if (!retval)
 			retval = put_user(tmp_byte, (__u8 __user *)arg);
 		break;