linux-kernel - Re: Fixing perf top --user shortcoming was: Re: [GIT PULL 0/9] perf/core improvements and fixes

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Date:	Thu, 26 Jan 2012 14:09:19 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Arnaldo Carvalho de Melo <acme@...radead.org>
Cc:	linux-kernel@...r.kernel.org, David Ahern <dsahern@...il.com>,
	David Daney <david.daney@...ium.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Jan Beulich <jbeulich@...e.com>,
	Joerg Roedel <joerg.roedel@....com>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Mike Galbraith <efault@....de>,
	Namhyung Kim <namhyung.kim@....com>,
	Paul Mackerras <paulus@...ba.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Stephane Eranian <eranian@...gle.com>
Subject: Re: Fixing perf top --user shortcoming was: Re: [GIT PULL 0/9]
 perf/core improvements and fixes


* Arnaldo Carvalho de Melo <acme@...radead.org> wrote:

> > So what does --uid do which perf record --pid 1234 wouldnt 
> > already do? By all means --uid ought to be a fancy way of 
> > doing a whole bunch of perf record --pid 1234 profiling 
> > sessions, at once.
> 
> I stopped at the kernel, i.e. used what can be done with what 
> is available from the kernel right now, the diagnosis was sent 
> in private, but boils down to:
> 
> +++ b/kernel/events/core.c
> @@ -2636,7 +2636,8 @@ find_lively_task_by_vpid(pid_t vpid)
>  
>  	/* Reuse ptrace permission checks for now. */
>  	err = -EACCES;
> -	if (!ptrace_may_access(task, PTRACE_MODE_READ))
> +	if (perf_paranoid_tracepoint_raw() &&
> +	    !ptrace_may_access(task, PTRACE_MODE_READ))
>  		goto errout;
>  
>  	return task;
> 
> ptrace_may_access(task, PTRACE_MODE_READ) fails for some tasks 
> owned by the user because, IIRC, in __ptrace_may_access:

Which tasks are these, are they privileged in any sense?

If yes and if most of the 'real' tasks a user have can be 
profiled just fine then i think we should just skip the 
privileged tasks and not abort the profiling session?

Thanks,

	Ingo
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