[PATCH v2 25/41] media: atomisp: Avoid overflow in compute_blending

[Mauro Carvalho Chehab <mchehab+huawei@kernel.org>]

From: Nathan Chancellor <natechancellor@gmail.com>

Clang warns:

drivers/staging/media/atomisp/pci/isp/kernels/xnr/xnr_3.0/ia_css_xnr3.host.c:129:35:
warning: implicit conversion from 'unsigned long' to 'int32_t' (aka
'int') changes value from 18446744073709543424 to -8192
[-Wconstant-conversion]
        return MAX(MIN(isp_strength, 0), -XNR_BLENDING_SCALE_FACTOR);
        ~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~

XNR_BLENDING_SCALE_FACTOR is BIT(13), or 8192, which will easily fit
into a signed 32-bit integer. However, it is an unsigned long, which
means that negating it is the same as subtracting that value from
ULONG_MAX + 1, which causes it to be larger than a signed 32-bit
integer so it gets implicitly converted.

We can avoid this by using the variable isp_scale, which holds the value
of XNR_BLENDING_SCALE_FACTOR already, where the implicit conversion from
unsigned long to s32 already happened. If that were to ever overflow,
clang would warn: https://godbolt.org/z/EeSxLG

Link: https://github.com/ClangBuiltLinux/linux/issues/1036

Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
---
 .../atomisp/pci/isp/kernels/xnr/xnr_3.0/ia_css_xnr3.host.c      | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/staging/media/atomisp/pci/isp/kernels/xnr/xnr_3.0/ia_css_xnr3.host.c b/drivers/staging/media/atomisp/pci/isp/kernels/xnr/xnr_3.0/ia_css_xnr3.host.c
index a9db6366d20b..629f07faf20a 100644
--- a/drivers/staging/media/atomisp/pci/isp/kernels/xnr/xnr_3.0/ia_css_xnr3.host.c
+++ b/drivers/staging/media/atomisp/pci/isp/kernels/xnr/xnr_3.0/ia_css_xnr3.host.c
@@ -126,7 +126,7 @@ compute_blending(int strength)
 	 * exactly as s0.11 fixed point, but -1.0 can.
 	 */
 	isp_strength = -(((strength * isp_scale) + offset) / host_scale);
-	return MAX(MIN(isp_strength, 0), -XNR_BLENDING_SCALE_FACTOR);
+	return MAX(MIN(isp_strength, 0), -isp_scale);
 }
 
 void
-- 
2.26.2