Re: [PATCH 12/14] libfdt: fdt_get_string(): Fix sequential write comparison warnings

[David Gibson <david-xT8FGy+AXnRB3Ne2BGzF6laj5H9X9Tb+@public.gmane.org>]

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On Mon, Sep 21, 2020 at 05:53:01PM +0100, Andre Przywara wrote:
> With -Wsign-compare, compilers warn about a mismatching signedness in
> comparisons in fdt_get_string().
> 
> The two occassions dealing with the sequential write case are a bit
> tricky, since we deliberately abuse the negative values here.
> 
> As we have just established that stroffset is negative, we can use
> casts to appease the compiler.
> 
> Signed-off-by: Andre Przywara <andre.przywara-5wv7dgnIgG8@public.gmane.org>
> ---
>  libfdt/fdt_ro.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/libfdt/fdt_ro.c b/libfdt/fdt_ro.c
> index ddb5a2d..059d302 100644
> --- a/libfdt/fdt_ro.c
> +++ b/libfdt/fdt_ro.c
> @@ -68,9 +68,9 @@ const char *fdt_get_string(const void *fdt, int stroffset, int *lenp)
>  		}
>  	} else if (fdt_magic(fdt) == FDT_SW_MAGIC) {
>  		if ((stroffset >= 0)
> -		    || (stroffset < -fdt_size_dt_strings(fdt)))
> +		    || (stroffset < -(int)fdt_size_dt_strings(fdt)))

So, I'm pretty sure at this point we've tested that
fdt_size_dt_strings < INT_MAX.  But to make it more obvious that it's
safe, I think it would be preferable to case (-stroffset) to unsigned
rather than casting size_dt_strings to signed.

>  			goto fail;
> -		if ((-stroffset) < len)
> +		if ((unsigned)(-stroffset) < len)
>  			len = -stroffset;
>  	} else {
>  		err = -FDT_ERR_INTERNAL;

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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