linux-kernel - Re: [PATCH] usb: storage: initialize variable

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Date:   Sat, 22 Aug 2020 20:49:03 -0400
From:   Alan Stern <stern@...land.harvard.edu>
To:     trix@...hat.com
Cc:     gregkh@...uxfoundation.org, linux-usb@...r.kernel.org,
        usb-storage@...ts.one-eyed-alien.net, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] usb: storage: initialize variable

On Sat, Aug 22, 2020 at 02:18:39PM -0700, trix@...hat.com wrote:
> From: Tom Rix <trix@...hat.com>
> 
> clang static analysis reports this representative problem
> 
> transport.c:495:15: warning: Assigned value is garbage or
>   undefined
>         length_left -= partial;
>                    ^  ~~~~~~~
> partial is set only when usb_stor_bulk_transfer_sglist()
> is successful.
> 
> So initialize to partial to 0.
> 
> Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> Signed-off-by: Tom Rix <trix@...hat.com>
> ---
>  drivers/usb/storage/transport.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/usb/storage/transport.c b/drivers/usb/storage/transport.c
> index 238a8088e17f..ce920851b1f2 100644
> --- a/drivers/usb/storage/transport.c
> +++ b/drivers/usb/storage/transport.c
> @@ -461,7 +461,7 @@ static int usb_stor_bulk_transfer_sglist(struct us_data *us, unsigned int pipe,
>  int usb_stor_bulk_srb(struct us_data* us, unsigned int pipe,
>  		      struct scsi_cmnd* srb)
>  {
> -	unsigned int partial;
> +	unsigned int partial = 0;
>  	int result = usb_stor_bulk_transfer_sglist(us, pipe, scsi_sglist(srb),
>  				      scsi_sg_count(srb), scsi_bufflen(srb),
>  				      &partial);
> @@ -484,7 +484,7 @@ int usb_stor_bulk_transfer_sg(struct us_data* us, unsigned int pipe,
>  		void *buf, unsigned int length_left, int use_sg, int *residual)
>  {
>  	int result;
> -	unsigned int partial;
> +	unsigned int partial = 0;
>  
>  	/* are we scatter-gathering? */
>  	if (use_sg) {

Yes, this is a bug.  But the right way to fix it is to change 
usb_stor_bulk_transfer_sglist(): Make it store 0 to *act_len at the 
start.

That way you change only one localized piece of code, instead of 
changing multiple callers and leaving a possibility of more errors being 
added in the future.

Alan Stern