Re: Casting 0 to a __bitwise type

[Luc Van Oostenryck <luc.vanoostenryck@gmail.com>]

On Fri, Feb 17, 2017 at 11:01:53AM +0000, Edward Cree wrote:
> (The actual code that is hitting this is drivers/net/ethernet/sfc/ethtool.c
>  in recent Linux trees.)
> From this I was able to create a new synthetic example, which *does*
>  reproduce the warning:
> 
>     typedef unsigned int __attribute__((bitwise)) __be32;
> 
>     static inline void ip6_fill_mask(__be32 *mask)
>     {
>         mask[0] = mask[1] = mask[2] = mask[3] = ~(__be32)0;
>     }
> 
>     static void f(void)
>     {
>         __be32 mask[4];
> 
>         ip6_fill_mask(mask);
>     }
> 
> Feeding the above program to sparse yields the output
> "test.c:5:51: warning: cast to restricted __be32"
> 
> In fact the example can be reduced even further:
> 
>     typedef unsigned int __attribute__((bitwise)) __be32;
> 
>     static __be32 foo = (__be32)0;
> 
> yields "test.c:3:22: warning: cast to restricted __be32"
> 
> My apologies for previously providing an incorrect example; I should have
>  tested it rather than assuming, as I did, that a static inline that was
>  never called would still be type-checked.

No worries.

In fact this warning is correct, by design.
Your not supposed to cast a bitwise type, even zero.
Here is a few examples that doesn't give a warning:
	mask[1] = 0;
	mask[2] = ~msk;
	mask[3] = ~(__force __be32) 0;
	mask[4] = ~({ __be32 msk = 0; msk; });

The canonical way to do what you need is via the (__force ...) cast.
In fact, in the driver you're talking about, just a few lines above
this fill_mask() function, there is a few masks already defined that
do exactly this, like IP4_ADDR_FULL_MASK.

Luc

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