Re: [PATCH v4 07/17] scs: add support for stack usage debugging

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On Fri, Nov 01, 2019 at 03:11:40PM -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>
> ---
>  kernel/scs.c | 39 +++++++++++++++++++++++++++++++++++++++
>  1 file changed, 39 insertions(+)
> 
> diff --git a/kernel/scs.c b/kernel/scs.c
> index 7780fc4e29ac..67c43af627d1 100644
> --- a/kernel/scs.c
> +++ b/kernel/scs.c
> @@ -167,6 +167,44 @@ int scs_prepare(struct task_struct *tsk, int node)
>  	return 0;
>  }
>  
> +#ifdef CONFIG_DEBUG_STACK_USAGE
> +static inline unsigned long scs_used(struct task_struct *tsk)
> +{
> +	unsigned long *p = __scs_base(tsk);
> +	unsigned long *end = scs_magic(tsk);
> +	uintptr_t s = (uintptr_t)p;

As previously, please use unsigned long for consistency.

> +
> +	while (p < end && *p)
> +		p++;

I think this is the only place where we legtimately access the shadow
call stack directly. When using SCS and KASAN, are the
compiler-generated accesses to the SCS instrumented?

If not, it might make sense to make this:

	while (p < end && READ_ONCE_NOCKECK(*p))

... and poison the allocation from KASAN's PoV, so that we can find
unintentional accesses more easily. 

Mark.

> +
> +	return (uintptr_t)p - s;
> +}
> +
> +static void scs_check_usage(struct task_struct *tsk)
> +{
> +	static DEFINE_SPINLOCK(lock);
> +	static unsigned long highest;
> +	unsigned long used = scs_used(tsk);
> +
> +	if (used <= highest)
> +		return;
> +
> +	spin_lock(&lock);
> +
> +	if (used > highest) {
> +		pr_info("%s: highest shadow stack usage %lu bytes\n",
> +			__func__, used);
> +		highest = used;
> +	}
> +
> +	spin_unlock(&lock);
> +}
> +#else
> +static inline void scs_check_usage(struct task_struct *tsk)
> +{
> +}
> +#endif
> +
>  bool scs_corrupted(struct task_struct *tsk)
>  {
>  	return *scs_magic(tsk) != SCS_END_MAGIC;
> @@ -181,6 +219,7 @@ void scs_release(struct task_struct *tsk)
>  		return;
>  
>  	WARN_ON(scs_corrupted(tsk));
> +	scs_check_usage(tsk);
>  
>  	scs_account(tsk, -1);
>  	task_set_scs(tsk, NULL);
> -- 
> 2.24.0.rc1.363.gb1bccd3e3d-goog
> 



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