Re: [PATCH 4/6] hw_breakpoint: Factor out __modify_user_hw_breakpoint function

[Peter Zijlstra <peterz@infradead.org>]

On Mon, Nov 27, 2017 at 06:34:17PM +0100, Peter Zijlstra wrote:
> On Mon, Nov 27, 2017 at 06:25:32PM +0100, Jiri Olsa wrote:
> > On Mon, Nov 27, 2017 at 06:12:03PM +0100, Peter Zijlstra wrote:
> > > But what validates the input attr is the same as the event attr, aside
> > > from those fields?
> > 
> > we don't.. the attr serves as a holder to carry those fields
> > into the function
> 
> Then that's a straight up bug.
> 
> > the current kernel interface does not check anything else
> 
> Not enough, if the new attr would fail perf_event_open() it should also
> fail this modify thing.


On IRC you asked:

<jolsa> peterz, I dont follow.. why should we check fields that we dont use?

Suppose someone does:

	attr = malloc(sizeof(*attr)); // uninitialized memory
	attr->type = BP;
	attr->bp_addr = new_addr;
	attr->bp_type = bp_type;
	attr->bp_len = bp_len;
	ioctl(fd, PERF_IOC_MOD_ATTR, &attr);

And feeds absolute shite for the rest of the fields.

Then we later want to extend IOC_MOD_ATTR to allow changing
attr::sample_type but we can't, because that would break the above
application.

Therefore we must be very strict to check only the fields we can change
have changed.