[Qemu-devel] [PATCH] hw/display/bcm2835_fb: Silence Coverity warning about multiply overflow

[Qemu-devel] [PATCH] hw/display/bcm2835_fb: Silence Coverity warning about multiply overflow

[Peter Maydell]

Coverity complains (CID 1395628) that the multiply in the calculation
of the framebuffer base is performed as 32x32 but then used in a
context that takes a 64-bit hwaddr. This can't actually ever
overflow the 32-bit result, because of the constraints placed on
the s->config values in bcm2835_fb_validate_config(). But we
can placate Coverity anyway, by explicitly casting one of the
inputs to a hwaddr, so the whole expression is calculated with
64-bit arithmetic.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
---
This is one of those cases where I was 50/50 about whether to just
mark the coverity issue as a false-positive.

 hw/display/bcm2835_fb.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/hw/display/bcm2835_fb.c b/hw/display/bcm2835_fb.c
index d534d00a65f..599863e4e17 100644
--- a/hw/display/bcm2835_fb.c
+++ b/hw/display/bcm2835_fb.c
@@ -190,7 +190,7 @@ static void fb_update_display(void *opaque)
     }
 
     if (s->invalidate) {
-        hwaddr base = s->config.base + xoff + yoff * src_width;
+        hwaddr base = s->config.base + xoff + (hwaddr)yoff * src_width;
         framebuffer_update_memory_section(&s->fbsection, s->dma_mr,
                                           base,
                                           s->config.yres, src_width);
-- 
2.19.0

Re: [Qemu-devel] [PATCH] hw/display/bcm2835_fb: Silence Coverity warning about multiply overflow

[Paolo Bonzini]

On 05/10/2018 15:30, Peter Maydell wrote:
> Coverity complains (CID 1395628) that the multiply in the calculation
> of the framebuffer base is performed as 32x32 but then used in a
> context that takes a 64-bit hwaddr. This can't actually ever
> overflow the 32-bit result, because of the constraints placed on
> the s->config values in bcm2835_fb_validate_config(). But we
> can placate Coverity anyway, by explicitly casting one of the
> inputs to a hwaddr, so the whole expression is calculated with
> 64-bit arithmetic.
> 
> Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
> ---
> This is one of those cases where I was 50/50 about whether to just
> mark the coverity issue as a false-positive.

Agreed, but in the end it's not too much noise, so

Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>

Thanks,

Paolo

>  hw/display/bcm2835_fb.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/hw/display/bcm2835_fb.c b/hw/display/bcm2835_fb.c
> index d534d00a65f..599863e4e17 100644
> --- a/hw/display/bcm2835_fb.c
> +++ b/hw/display/bcm2835_fb.c
> @@ -190,7 +190,7 @@ static void fb_update_display(void *opaque)
>      }
>  
>      if (s->invalidate) {
> -        hwaddr base = s->config.base + xoff + yoff * src_width;
> +        hwaddr base = s->config.base + xoff + (hwaddr)yoff * src_width;
>          framebuffer_update_memory_section(&s->fbsection, s->dma_mr,
>                                            base,
>                                            s->config.yres, src_width);
>