linux-kernel - Re: [for-linus][PATCH 2/5] tracing/uprobe: Fix double perf

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Date:   Tue, 21 Jan 2020 15:50:09 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jiri Olsa <jolsa@...nel.org>,
        "Naveen N . Rao" <naveen.n.rao@...ux.ibm.com>,
        Anil S Keshavamurthy <anil.s.keshavamurthy@...el.com>,
        "David S . Miller" <davem@...emloft.net>,
        Namhyung Kim <namhyung@...nel.org>,
        Toke Høiland-Jørgensen <thoiland@...hat.com>,
        Jean-Tsung Hsiao <jhsiao@...hat.com>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        stable@...r.kernel.org, Arnaldo Carvalho de Melo <acme@...nel.org>,
        Arnaldo Carvalho de Melo <acme@...hat.com>,
        Masami Hiramatsu <mhiramat@...nel.org>
Subject: Re: [for-linus][PATCH 2/5] tracing/uprobe: Fix double perf_event
 linking on multiprobe uprobe

On Tue, Jan 21, 2020 at 09:38:49AM -0500, Steven Rostedt wrote:
> diff --git a/kernel/trace/trace_probe.h b/kernel/trace/trace_probe.h
> index 4ee703728aec..03e4e180058d 100644
> --- a/kernel/trace/trace_probe.h
> +++ b/kernel/trace/trace_probe.h
> @@ -230,6 +230,7 @@ struct trace_probe_event {
>  	struct trace_event_call		call;
>  	struct list_head 		files;
>  	struct list_head		probes;
> +	char				data[0];
>  };

Note that this relies on pure 'luck'. If you stick anything <4 bytes in
between the list_head and the data member it'll come unstuck real fast.

> +static struct trace_uprobe_filter *
> +trace_uprobe_get_filter(struct trace_uprobe *tu)
> +{
> +	struct trace_probe_event *event = tu->tp.event;
> +
> +	return (struct trace_uprobe_filter *)&event->data[0];
> +}

That is, the above does not consider the alignment requirements of
struct trace_uprobe_filter.