Re: [PATCH v2] usb: storage: initialize variable

[Alan Stern]

On Mon, Aug 24, 2020 at 02:18:39PM -0700, Vito Caputo wrote:
> On Mon, Aug 24, 2020 at 02:10:27PM -0700, trix@xxxxxxxxxx wrote:
> > From: Tom Rix <trix@xxxxxxxxxx>
> > 
> > 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 set partial on entry to 0.
> > 
> > Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> > Signed-off-by: Tom Rix <trix@xxxxxxxxxx>
> > ---
> >  drivers/usb/storage/transport.c | 3 +++
> >  1 file changed, 3 insertions(+)
> > 
> > diff --git a/drivers/usb/storage/transport.c b/drivers/usb/storage/transport.c
> > index 238a8088e17f..044429717dcc 100644
> > --- a/drivers/usb/storage/transport.c
> > +++ b/drivers/usb/storage/transport.c
> > @@ -414,6 +414,9 @@ static int usb_stor_bulk_transfer_sglist(struct us_data *us, unsigned int pipe,
> >  {
> >  	int result;
> >  
> > +	if (act_len)
> > +		*act_len = 0;
> > +
> >  	/* don't submit s-g requests during abort processing */
> >  	if (test_bit(US_FLIDX_ABORTING, &us->dflags))
> >  		return USB_STOR_XFER_ERROR;
> 
> At a glance this seems odd to me.  If the caller insists on ignoring
> the return value, shouldn't it just initialize partial to zero?

In this case, the callers are not the final consumers of the return 
value or of partial.  They merely copy those values back up to _their_ 
callers, and those copy operations are what the static analyzer objects 
to.

> In my experience it's generally frowned upon for functions to store
> results in error paths.

I don't see any reason for such an attitude, at least not here.  It 
makes perfectly good sense, if an error prevents transmission of an 
entire data buffer, to store the amount of data that did get 
transmitted.

Alan Stern