linux-kernel - Re: [PATCH 2/3] tracing/user_events: Ensure bit is cleared on unregister

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Message-ID: <20230425170654.GA74@W11-BEAU-MD.localdomain>
Date:   Tue, 25 Apr 2023 10:06:54 -0700
From:   Beau Belgrave <beaub@...ux.microsoft.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     mhiramat@...nel.org, mathieu.desnoyers@...icios.com,
        dcook@...ux.microsoft.com, alanau@...ux.microsoft.com,
        linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org
Subject: Re: [PATCH 2/3] tracing/user_events: Ensure bit is cleared on
 unregister

On Mon, Apr 24, 2023 at 09:39:57PM -0400, Steven Rostedt wrote:
> On Tue, 11 Apr 2023 14:17:08 -0700
> Beau Belgrave <beaub@...ux.microsoft.com> wrote:
> 
> > +static int user_event_mm_clear_bit(struct user_event_mm *user_mm,
> > +				   unsigned long uaddr, unsigned char bit)
> > +{
> > +	struct user_event_enabler enabler;
> > +	int result;
> > +
> > +	memset(&enabler, 0, sizeof(enabler));
> > +	enabler.addr = uaddr;
> > +	enabler.values = bit;
> > +retry:
> > +	/* Prevents state changes from racing with new enablers */
> > +	mutex_lock(&event_mutex);
> > +
> > +	/* Force the bit to be cleared, since no event is attached */
> > +	mmap_read_lock(user_mm->mm);
> > +	result = user_event_enabler_write(user_mm, &enabler, false);
> > +	mmap_read_unlock(user_mm->mm);
> > +
> > +	mutex_unlock(&event_mutex);
> > +
> > +	if (result) {
> > +		/* Attempt to fault-in and retry if it worked */
> > +		if (!user_event_mm_fault_in(user_mm, uaddr))
> > +			goto retry;
> 
> Without looking into the functions of this call, I wonder if this can
> get into an infinite loop?
> 

That's a good point, user_event_mm_fault() is a wrapper around
fixup_user_fault(). We retry if it works, so I guess if the user could
somehow cause a fail on the write and succeed to page in repeatedly, it
could keep the loop going for that time period. I cannot think of a way
to achieve this forever, but that doesn't mean it couldn't happen.

I can certainly add an upper bound of retries (maybe 3 or so?) if you
think it would be possible for this to occur. I think we need retries of
some amount to handle spurious faults.

Thanks,
-Beau

> -- Steve
> 
> 
> > +	}
> > +
> > +	return result;
> > +}
> > +