Re: [PATCH v12 03/12] scs: add support for stack usage debugging

[Will Deacon]

On Mon, Apr 20, 2020 at 07:14:44PM -0700, Sami Tolvanen wrote:
> Implements CONFIG_DEBUG_STACK_USAGE for shadow stacks. When enabled,
> also prints out the highest shadow stack usage per process.
> 
> Signed-off-by: Sami Tolvanen <samitolvanen@xxxxxxxxxx>
> Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>
> ---
>  kernel/scs.c | 38 ++++++++++++++++++++++++++++++++++++++
>  1 file changed, 38 insertions(+)
> 
> diff --git a/kernel/scs.c b/kernel/scs.c
> index 7eea2d97bd2d..147917e31adf 100644
> --- a/kernel/scs.c
> +++ b/kernel/scs.c
> @@ -68,6 +68,43 @@ int scs_prepare(struct task_struct *tsk, int node)
>  	return 0;
>  }
>  
> +#ifdef CONFIG_DEBUG_STACK_USAGE
> +static unsigned long __scs_used(struct task_struct *tsk)
> +{
> +	unsigned long *p = __scs_base(tsk);
> +	unsigned long *end = __scs_magic(p);
> +	unsigned long s = (unsigned long)p;
> +
> +	while (p < end && READ_ONCE_NOCHECK(*p))
> +		p++;
> +
> +	return (unsigned long)p - s;
> +}
> +
> +static void scs_check_usage(struct task_struct *tsk)
> +{
> +	static unsigned long highest;
> +	unsigned long used = __scs_used(tsk);
> +	unsigned long prev;
> +	unsigned long curr = highest;
> +
> +	while (used > curr) {
> +		prev = cmpxchg(&highest, curr, used);

I think this can be cmpxchg_relaxed(), since we don't care about ordering
here afaict.

With that:

Acked-by: Will Deacon <will@xxxxxxxxxx>

Cheers,

Will