linux-kernel - Re: [PATCH] firmware: arm_scpi: prevent ternary sign expansion bug

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Date:   Sat, 24 Apr 2021 09:43:07 +0300
From:   Dan Carpenter <dan.carpenter@...cle.com>
To:     Sudeep Holla <sudeep.holla@....com>
Cc:     Cristian Marussi <cristian.marussi@....com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        kernel-janitors@...r.kernel.org
Subject: Re: [PATCH] firmware: arm_scpi: prevent ternary sign expansion bug

On Thu, Apr 22, 2021 at 06:46:31PM +0100, Sudeep Holla wrote:
> (dropping Tixy as I am sure it will bounce, he left/retired from Linaro
> long back)
> 
> On Thu, Apr 22, 2021 at 12:02:29PM +0300, Dan Carpenter wrote:
> > How type promotion works in ternary expressions is a bit tricky.
> > The problem is that scpi_clk_get_val() returns longs, "ret" is a int
> > which holds a negative error code, and le32_to_cpu() is an unsigned int.
> 
> Agreed.
> 
> > We want the negative error code to be cast to a negative long.  But
> > because le32_to_cpu() is an u32 then "ret" is type promoted to u32 and
> > becomes a high positive and then it is promoted to long and it is still
> > a high positive value.
> >
> 
> Thanks a lot for finding and fixing the bug!
> 
> > Fix this by getting rid of the ternary.
> >
> > Fixes: 8cb7cf56c9fe ("firmware: add support for ARM System Control and Power Interface(SCPI) protocol")
> > Signed-off-by: Dan Carpenter <dan.carpenter@...cle.com>
> > ---
> >  drivers/firmware/arm_scpi.c | 4 +++-
> >  1 file changed, 3 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/firmware/arm_scpi.c b/drivers/firmware/arm_scpi.c
> > index d0dee37ad522..3bf61854121d 100644
> > --- a/drivers/firmware/arm_scpi.c
> > +++ b/drivers/firmware/arm_scpi.c
> > @@ -552,8 +552,10 @@ static unsigned long scpi_clk_get_val(u16 clk_id)
> >
> >  	ret = scpi_send_message(CMD_GET_CLOCK_VALUE, &le_clk_id,
> >  				sizeof(le_clk_id), &rate, sizeof(rate));
> > +	if (ret)
> > +		return ret;
> 
> This could be still an issue, ideally I would prefer to pass the return
> value via argument pointer and always return success/failure as return
> value. Can't remember any reason for this. Since this is old interface
> with limited platform to test, I think returning 0 as clock rate on error
> should be fine as Cristain suggested. If you agree with that, I can
> fix up when applying. 
> 

That sounds great.  Thanks.

regards,
dan carpenter