Re: [PATCH 00/38] Fixes related to incorrect usage of unsigned types

[Andrzej Hajda]

On 09/21/2015 03:42 PM, David Howells wrote:
> Andrzej Hajda  wrote:
> 
>> Semantic patch finds comparisons of types:
>> unsigned < 0
>> unsigned >= 0
>> The former is always false, the latter is always true.
>> Such comparisons are useless, so theoretically they could be
>> safely removed, but their presence quite often indicates bugs.
> 
> Or someone has left them in because they don't matter and there's the
> possibility that the type being tested might be or become signed under some
> circumstances.  If the comparison is useless, I'd expect the compiler to just
> discard it - for such cases your patch is pointless.
> 
> If I have, for example:
> 
>   unsigned x;
> 
>   if (x == 0 || x > 27)
>   give_a_range_error();
> 
> I will write this as:
> 
>   unsigned x;
> 
>   if (x <= 0 || x > 27)
>   give_a_range_error();
> 
> because it that gives a way to handle x being changed to signed at some point
> in the future for no cost.  In which case, your changing the <= to an ==
> "because the < part of the case is useless" is arguably wrong.

This is why I have not checked for such cases - I have skipped checks of type
unsigned <= 0
exactly for the reasons above.

However I have left two other checks as they seems to me more suspicious - they
are always true or false. But as Dmitry and Andrew pointed out Linus have quite
strong opinion against removing range checks in such cases as he finds it
clearer. I think it applies to patches 29-36. I am not sure about patches 
26-28,37.

Regards
Andrzej

> 
> David
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[PATCH 28/38] video/omap: remove invalid check

[Andrzej Hajda]

regno is unsigned so it cannot be negative.

The problem has been detected using proposed semantic patch
scripts/coccinelle/tests/unsigned_lesser_than_zero.cocci [1].

[1]: http://permalink.gmane.org/gmane.linux.kernel/2038576

Signed-off-by: Andrzej Hajda 
---
 drivers/video/fbdev/omap/omapfb_main.c | 5 -
 1 file changed, 5 deletions(-)

diff --git a/drivers/video/fbdev/omap/omapfb_main.c 
b/drivers/video/fbdev/omap/omapfb_main.c
index 1fb3ea3..393ae1b 100644
--- a/drivers/video/fbdev/omap/omapfb_main.c
+++ b/drivers/video/fbdev/omap/omapfb_main.c
@@ -276,11 +276,6 @@ static int _setcolreg(struct fb_info *info, u_int regno, 
u_int red, u_int green,
if (r != 0)
break;
 
-   if (regno < 0) {
-   r = -EINVAL;
-   break;
-   }
-
if (regno < 16) {
u16 pal;
pal = ((red >> (16 - var->red.length)) <<
-- 
1.9.1

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