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

[Cristian Marussi <cristian.marussi@arm.com>]

On Thu, Apr 22, 2021 at 11:17:09AM +0100, Cristian Marussi wrote:
> Hi,
> 
> 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.
> > 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.
> > 
> > Fix this by getting rid of the ternary.
> 
> I wonder how/if the callers up in the stack check/expect ever effectively for a
> 2-complement negative value inside the returned unsigned long...given that this
> plugs finally into CLK framework struct clk_ops.recalc_rate via clk-scpi.c which
> also expects unsigned long....but that's another story.
> 
> FWIW regarding this patch:
> 
> Reviewed-by: Cristian Marussi <cristian.marussi@arm.com>
> 
> Thanks
> 
> Cristian

@Sudeep, as a second though, looking at .recalc_rate() definition inside

include/linux/clk-provider.h:struct clk_ops 

which is the direct caller of this SCPI clk function, I wonder if, instead,
on error we should not return here just ZERO as the returned clock rate value
as in:

	if (ret)
		return 0;

given that the error code returned inside the unsigned long won't be ever
considerd as such apparently, so not sure if it'd be worst to return a
very big fake value or zero...

Thanks
Cristian


> > 
> > Fixes: 8cb7cf56c9fe ("firmware: add support for ARM System Control and Power Interface(SCPI) protocol")
> > Signed-off-by: Dan Carpenter <dan.carpenter@oracle.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;
> >  
> > -	return ret ? ret : le32_to_cpu(rate);
> > +	return le32_to_cpu(rate);
> >  }
> >  
> >  static int scpi_clk_set_val(u16 clk_id, unsigned long rate)
> > -- 
> > 2.30.2
> >