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

Will Deacon
2020-04-22T17:46:29Z

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@google.com>
> Reviewed-by: Kees Cook <keescook@chromium.org>
> ---
> 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@kernel.org>

Cheers,

Will