linux-kernel - Re: [PATCH 4/6] hw_breakpoint: Factor out __modify_user_hw

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Message-ID: <20171127170911.GA22026@krava>
Date:   Mon, 27 Nov 2017 18:09:11 +0100
From:   Jiri Olsa <jolsa@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Jiri Olsa <jolsa@...nel.org>, Ingo Molnar <mingo@...nel.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        lkml <linux-kernel@...r.kernel.org>,
        Namhyung Kim <namhyung@...nel.org>,
        David Ahern <dsahern@...il.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Milind Chabbi <chabbi.milind@...il.com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Hari Bathini <hbathini@...ux.vnet.ibm.com>,
        Jin Yao <yao.jin@...ux.intel.com>,
        Kan Liang <kan.liang@...el.com>,
        Sukadev Bhattiprolu <sukadev@...ux.vnet.ibm.com>,
        Oleg Nesterov <onestero@...hat.com>,
        Will Deacon <will.deacon@....com>
Subject: Re: [PATCH 4/6] hw_breakpoint: Factor out
 __modify_user_hw_breakpoint function

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

jirka