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

[Peter Zijlstra]

On Mon, Nov 27, 2017 at 06:09:11PM +0100, Jiri Olsa wrote:
> On Mon, Nov 27, 2017 at 05:46:39PM +0100, Peter Zijlstra wrote:
> > On Mon, Nov 27, 2017 at 05:21:31PM +0100, Jiri Olsa wrote:
> > > +static int __modify_user_hw_breakpoint(struct perf_event *bp, struct perf_event_attr *attr)
> > > +{
> > > +	u64 old_addr = bp->attr.bp_addr;
> > > +	u64 old_len  = bp->attr.bp_len;
> > > +	int old_type = bp->attr.bp_type;
> > > +	bool modify  = attr->bp_type != old_type;
> > > +	int err = 0;
> > > +
> > > +	bp->attr.bp_addr = attr->bp_addr;
> > > +	bp->attr.bp_type = attr->bp_type;
> > > +	bp->attr.bp_len  = attr->bp_len;
> > > +
> > > +	err = validate_hw_breakpoint(bp);
> > > +	if (!err && modify)
> > > +		err = modify_bp_slot(bp, old_type);
> > > +
> > > +	if (err) {
> > > +		bp->attr.bp_addr = old_addr;
> > > +		bp->attr.bp_type = old_type;
> > > +		bp->attr.bp_len  = old_len;
> > > +		return err;
> > > +	}
> > > +
> > > +	bp->attr.disabled = attr->disabled;
> > > +	return 0;
> > > +}
> > 
> > I think this function is failing to check if anything else in the attr
> > changes.
> > 
> > For example, someone could have added PERF_SAMPLE_BRANCH_STACK. That's
> > something you'll fail to create breakpoints with, but this modification
> > would 'accept'.
> > 
> 
> hum, I dont think so.. the only things you're allowed to change
> are bp_addr, bp_type and bp_len.. we put new values in those
> fields and keep the rest untouched.. apart from 'disabled' bit

But what validates the input attr is the same as the event attr, aside
from those fields?