Re: [PATCH v2] tty: serial: fsl_lpuart.c: prevent a bad shift operation

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Wed, Aug 05, 2020 at 12:09:34PM +0000, Vabhav Sharma (OSS) wrote:
> 
> 
> > -----Original Message-----
> > From: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> > Sent: Wednesday, July 29, 2020 9:34 PM
> > To: B K Karthik <bkkarthik@pesu.pes.edu>
> > Cc: Jiri Slaby <jirislaby@kernel.org>; linux-serial@vger.kernel.org; linux-
> > kernel@vger.kernel.org; Vabhav Sharma (OSS)
> > <vabhav.sharma@oss.nxp.com>; bhuvanchandra.dv@toradex.com
> > Subject: Re: [PATCH v2] tty: serial: fsl_lpuart.c: prevent a bad shift operation
> > 
> > On Tue, Jul 21, 2020 at 11:12:29PM +0530, B K Karthik wrote:
> > > prevent a bad shift operation by verifying that the argument to fls is
> > > non zero.
> > >
> > > Reported-by: "Vabhav Sharma (OSS)" <vabhav.sharma@oss.nxp.com>
> > > Signed-off-by: B K Karthik <bkkarthik@pesu.pes.edu>
> > > ---
> > > v1 -> v2:
> > > 	added Reported-by tag
> > >
> > >  drivers/tty/serial/fsl_lpuart.c | 3 ++-
> > >  1 file changed, 2 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/drivers/tty/serial/fsl_lpuart.c
> > > b/drivers/tty/serial/fsl_lpuart.c index 7ca642249224..0cc64279cd2d
> > > 100644
> > > --- a/drivers/tty/serial/fsl_lpuart.c
> > > +++ b/drivers/tty/serial/fsl_lpuart.c
> > > @@ -1168,7 +1168,8 @@ static inline int lpuart_start_rx_dma(struct
> > lpuart_port *sport)
> > >  	 * 10ms at any baud rate.
> > >  	 */
> > >  	sport->rx_dma_rng_buf_len = (DMA_RX_TIMEOUT * baud /  bits /
> > 1000) * 2;
> > > -	sport->rx_dma_rng_buf_len = (1 << (fls(sport->rx_dma_rng_buf_len)
> > - 1));
> > > +	if (sport->rx_dma_rng_buf_len != 0)
> > 
> > How can this variable become 0?
> Condition x, taking false branch
> Explicitly returning zero 
> 
> static __always_inline int fls(unsigned int x)
> {
> 	return x ? sizeof(x) * 8 - __builtin_clz(x) : 0;
> }

What false branch?

I don't see how this can ever be an issue in "the real world", can you
explain how it could ever be a problem?

thanks,

greg k-h