Re: [PATCH] kern/sys: Compat sysinfo syscall fix undefined behavior

[Andrew Morton <akpm@linux-foundation.org>]

On Thu, 04 Sep 2014 12:46:53 -0600 Scotty Bauer <sbauer@eng.utah.edu> wrote:

> Fix undefined behavior and compiler warning by replacing right
> shift 32 with upper_32_bits macro
> 
> Signed-off-by: Scotty Bauer <sbauer@eng.utah.edu>
> ---
>  kernel/sys.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/sys.c b/kernel/sys.c
> index ce81291..c78530b 100644
> --- a/kernel/sys.c
> +++ b/kernel/sys.c
> @@ -2135,7 +2135,7 @@ COMPAT_SYSCALL_DEFINE1(sysinfo, struct compat_sysinfo __user *, info)
>  	/* Check to see if any memory value is too large for 32-bit and scale
>  	 *  down if needed
>  	 */
> -	if ((s.totalram >> 32) || (s.totalswap >> 32)) {
> +	if (upper_32_bits(s.totalram) || upper_32_bits(s.totalswap)) {
>  		int bitcount = 0;
>  
>  		while (s.mem_unit < PAGE_SIZE) {

Fair enough.

If I'm reading it correctly, this is all dead code because si_meminfo()
unconditionally sets sysinfo.mem_unit to PAGE_SIZE.

It could all do with a bit of a cleanup, I suspect.