USB: storage: avoid use of uninitialized values in error path

When usb_stor_bulk_transfer_sglist() returns with USB_STOR_XFER_ERROR, it
returns without writing to its parameter *act_len.

Further, the two callers of usb_stor_bulk_transfer_sglist():

    usb_stor_bulk_srb() and
    usb_stor_bulk_transfer_sg(),

use the passed variable partial without checking the return value. Hence,
the uninitialized value of partial is then used in the further execution
of those two functions.

Clang-analyzer detects this potential control and data flow and warns:

  drivers/usb/storage/transport.c:469:40:
    warning: The right operand of '-' is a garbage value
    [clang-analyzer-core.UndefinedBinaryOperatorResult]
          scsi_set_resid(srb, scsi_bufflen(srb) - partial);
                                                ^

  drivers/usb/storage/transport.c:495:15:
    warning: Assigned value is garbage or undefined
    [clang-analyzer-core.uninitialized.Assign]
                  length_left -= partial;
                              ^

When a transfer error occurs, the *act_len value is probably ignored by the
higher layers. But it won't hurt to set it to a valid number, just in case.

For the two early-return paths in usb_stor_bulk_transfer_sglist(), the
amount of data transferred is 0.  So if act_len is not NULL, set *act_len
to 0 in those paths. That makes clang-analyzer happy.

Proposal was discussed in this mail thread:
	https://lore.kernel.org/linux-usb/alpine.DEB.2.21.2011112146110.13119@felia/

Reviewed-by: Nathan Chancellor <natechancellor@gmail.com>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Link: https://lore.kernel.org/r/20201112191255.13372-1-lukas.bulwahn@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/storage/transport.c

@@ -416,7 +416,7 @@ static int usb_stor_bulk_transfer_sglist(struct us_data *us, unsigned int pipe,
 
 	/* don't submit s-g requests during abort processing */
 	if (test_bit(US_FLIDX_ABORTING, &us->dflags))
-		return USB_STOR_XFER_ERROR;
+		goto usb_stor_xfer_error;
 
 	/* initialize the scatter-gather request block */
 	usb_stor_dbg(us, "xfer %u bytes, %d entries\n", length, num_sg);
@@ -424,7 +424,7 @@ static int usb_stor_bulk_transfer_sglist(struct us_data *us, unsigned int pipe,
 			sg, num_sg, length, GFP_NOIO);
 	if (result) {
 		usb_stor_dbg(us, "usb_sg_init returned %d\n", result);
-		return USB_STOR_XFER_ERROR;
+		goto usb_stor_xfer_error;
 	}
 
 	/*
@@ -452,6 +452,11 @@ static int usb_stor_bulk_transfer_sglist(struct us_data *us, unsigned int pipe,
 		*act_len = us->current_sg.bytes;
 	return interpret_urb_result(us, pipe, length, result,
 			us->current_sg.bytes);
+
+usb_stor_xfer_error:
+	if (act_len)
+		*act_len = 0;
+	return USB_STOR_XFER_ERROR;
 }
 
 /*