[PATCH] uefi: uefivariable: fix build error on older compilers

[Matt Fleming]

On Fri, 2013-03-15 at 11:26 +0000, Colin King wrote:
> From: Colin Ian King <colin.king at canonical.com>
> 
> Curiously, we are getting the following error on older versions
> of GCC:
> 
> cc1: warnings being treated as errors
> uefi/uefirtvariable/uefirtvariable.c: In function 'getnextvariable_test2':
> uefi/uefirtvariable/uefirtvariable.c:352:5: error: array subscript is above array bounds
> 
> ..and it only happens on 32 bit builds.
> 
> Looking at the compiled object code, it seems that:
> 
> c = variablename[(len / sizeof(c)) - 1];
> 
> produces an add of 0xffffffff todo the subtraction of -1
> which triggers the array subscript array bounds warning, where as:
> 
> uint64_t i = (len / sizeof(c)) - 1;
> 
> c = variablename[i];
> 
> produces a subtraction of 1 and everthing is fine.
> 
> This patch is a workaround.  I've hand checked the result, and it
> produces the code we require, so I think the original code was OK
> but GCC was being overly zealous.
> 
> Signed-off-by: Colin Ian King <colin.king at canonical.com>
> ---
>  src/uefi/uefirtvariable/uefirtvariable.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/src/uefi/uefirtvariable/uefirtvariable.c b/src/uefi/uefirtvariable/uefirtvariable.c
> index 7242afd..c5caa33 100644
> --- a/src/uefi/uefirtvariable/uefirtvariable.c
> +++ b/src/uefi/uefirtvariable/uefirtvariable.c
> @@ -349,7 +349,9 @@ static bool strlen_valid(uint16_t *variablename, uint64_t variablenamesize)
>  	uint16_t c;
>  
>  	for (len = 2; len <= variablenamesize; len += sizeof(c)) {
> -		c = variablename[(len / sizeof(c)) - 1];
> +		uint64_t i = (len / sizeof(c)) - 1;
> +
> +		c = variablename[i];
>  		if (!c)
>  			break;
>  	}

Looks good to me. Thanks for fixing this up.

-- 
Matt Fleming, Intel Open Source Technology Center