Re: [PATCH v11 03/12] scs: add support for stack usage debugging

[Sami Tolvanen]

On Mon, Apr 20, 2020 at 06:17:42PM +0100, Will Deacon wrote:
> > +#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(p);
> > +	unsigned long s = (unsigned long)p;
> > +
> > +	while (p < end && READ_ONCE_NOCHECK(*p))
> > +		p++;
> 
> I think the expectation is that the caller has already checked that the
> stack is not corrupted, so I'd probably throw a couple of underscores
> in front of the function name, along with a comment.

Correct. I'll do that.

> Also, is tsk ever != current?

This is only called from scs_release(), so tsk is never current.

> > +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 (%d): highest shadow stack usage: %lu bytes\n",
> > +			tsk->comm, task_pid_nr(tsk), used);
> > +		highest = used;
> > +	}
> > +
> > +	spin_unlock(&lock);
> 
> Do you really need this lock? I'd have thought you could cmpxchg()
> highest instead.

This is similar to check_stack_usage in kernel/exit.c, but yes, I can
change this to a cmpxchg() loop instead.

Sami