linux-kernel - Re: [PATCH 2/5] perf/hw_breakpoint: Modify breakpoint even if the new attr has disabled set

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Date:   Fri, 17 Aug 2018 16:27:27 +0200
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Jiri Olsa <jolsa@...nel.org>
Cc:     Arnaldo Carvalho de Melo <acme@...nel.org>,
        lkml <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Namhyung Kim <namhyung@...nel.org>,
        David Ahern <dsahern@...il.com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Peter Zijlstra <a.p.zijlstra@...llo.nl>,
        Milind Chabbi <chabbi.milind@...il.com>,
        Oleg Nesterov <oleg@...hat.com>
Subject: Re: [PATCH 2/5] perf/hw_breakpoint: Modify breakpoint even if the
 new attr has disabled set

On Fri, Aug 10, 2018 at 12:47:27PM +0200, Jiri Olsa wrote:
> We need to change the breakpoint even if the attr with
> new fields has disabled set to true.
> 
> Current code prevents following user code to change
> the breakpoint address:
> 
>   ptrace(PTRACE_POKEUSER, child, offsetof(struct user, u_debugreg[0]), addr_1)
>   ptrace(PTRACE_POKEUSER, child, offsetof(struct user, u_debugreg[0]), addr_2)
>   ptrace(PTRACE_POKEUSER, child, offsetof(struct user, u_debugreg[7]), dr7)
> 
> The first PTRACE_POKEUSER creates the breakpoint with
> attr.disabled set to true:
> 
>   ptrace_set_breakpoint_addr(nr = 0)
>     struct perf_event *bp = t->ptrace_bps[nr];
> 
>     ptrace_register_breakpoint(..., disabled = true)
>       ptrace_fill_bp_fields(..., disabled)
>       register_user_hw_breakpoint
> 
> So the second PTRACE_POKEUSER will be omitted:
> 
>   ptrace_set_breakpoint_addr(nr = 0)
>     struct perf_event *bp = t->ptrace_bps[nr];
>     struct perf_event_attr attr = bp->attr;
> 
>     modify_user_hw_breakpoint(bp, &attr)
>       if (!attr->disabled)
>         modify_user_hw_breakpoint_check
> 
> Acked-by: Oleg Nesterov <oleg@...hat.com>
> Reported-by: Milind Chabbi <chabbi.milind@...il.com>
> Link: http://lkml.kernel.org/n/tip-yjhgplc28gk5gfzt7ceooe6z@git.kernel.org
> Signed-off-by: Jiri Olsa <jolsa@...nel.org>

Right, now that we fixed the breakpoint modification arch code, it should
be safe to do that.

Thanks!

Acked-by: Frederic Weisbecker <frederic@...nel.org>