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

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Date:   Wed, 5 Aug 2020 18:46:15 +0530
From:   B K Karthik <bkkarthik@...u.pes.edu>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     "Vabhav Sharma (OSS)" <vabhav.sharma@....nxp.com>,
        Jiri Slaby <jirislaby@...nel.org>,
        "linux-serial@...r.kernel.org" <linux-serial@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "bhuvanchandra.dv@...adex.com" <bhuvanchandra.dv@...adex.com>
Subject: Re: [PATCH v2] tty: serial: fsl_lpuart.c: prevent a bad shift operation

On Wed, Aug 5, 2020 at 6:26 PM Greg Kroah-Hartman
<gregkh@...uxfoundation.org> wrote:
>
> On Wed, Aug 05, 2020 at 12:09:34PM +0000, Vabhav Sharma (OSS) wrote:
> >
> >
> > > -----Original Message-----
> > > From: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> > > Sent: Wednesday, July 29, 2020 9:34 PM
> > > To: B K Karthik <bkkarthik@...u.pes.edu>
> > > Cc: Jiri Slaby <jirislaby@...nel.org>; linux-serial@...r.kernel.org; linux-
> > > kernel@...r.kernel.org; Vabhav Sharma (OSS)
> > > <vabhav.sharma@....nxp.com>; bhuvanchandra.dv@...adex.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@....nxp.com>
> > > > Signed-off-by: B K Karthik <bkkarthik@...u.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'm assuming he's talking about the value after : in the ? operator.

I am checking for the wrong thing in the if statement. We will have to
check for the return value of fls() before performing the shift.
I can change it and send a v3 if you agree.

Please let me know.

thanks,

karthik