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

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Message-ID: <20171127164639.3ymnc6io3eae7n4c@hirez.programming.kicks-ass.net>
Date:   Mon, 27 Nov 2017 17:46:39 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Jiri Olsa <jolsa@...nel.org>
Cc:     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 05:21:31PM +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'.