Re: [PATCH] Reduce stack usage in time.c

[Jeff Garzik <jgarzik@pobox.com>]

Yum Rayan wrote:
> Attempt to reduce stack usage in time.c (linux-2.6.12-rc1-mm3). Stack
> usage was noted using checkstack.pl. Specifically:
> 
> Before patch
> ------------
> sys_adjtimex - 128
> 
> After patch
> -----------
> sys_adjtimex - none (register usage only)
> 
> Signed-off-by: Yum Rayan <yum.rayan@gmail.com>
> 
> --- a/kernel/time.c	2005-03-25 22:11:06.000000000 -0800
> +++ b/kernel/time.c	2005-03-30 16:59:51.000000000 -0800
> @@ -413,17 +413,27 @@
>  
>  asmlinkage long sys_adjtimex(struct timex __user *txc_p)
>  {
> -	struct timex txc;		/* Local copy of parameter */
> -	int ret;
> +	struct timex *txc;		/* Local copy of parameter */
> +	int retval;
> +
> +	txc = kmalloc(sizeof(struct timex), GFP_KERNEL);
> +	if (!txc)
> +		return -ENOMEM;
>  
>  	/* Copy the user data space into the kernel copy
>  	 * structure. But bear in mind that the structures
>  	 * may change
>  	 */
> -	if(copy_from_user(&txc, txc_p, sizeof(struct timex)))
> -		return -EFAULT;
> -	ret = do_adjtimex(&txc);
> -	return copy_to_user(txc_p, &txc, sizeof(struct timex)) ? -EFAULT : ret;
> +	if(copy_from_user(txc, txc_p, sizeof(struct timex))) {
> +		retval = -EFAULT;
> +		goto free_txc;
> +	}
> +	retval = do_adjtimex(txc);
> +	if (copy_to_user(txc_p, txc, sizeof(struct timex)))
> +		retval = -EFAULT;
> +free_txc:

It seems quite unhealthy to add a kmalloc(,GFP_KERNEL) allocation into a 
time-sensitive function.

	Jeff