linux-kernel - Re: [PATCH v2 1/4] perf-security: document perf

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Message-ID: <969b6ea5-a5bf-70b7-7031-38b07538f744@linux.intel.com>
Date:   Mon, 11 Feb 2019 15:46:02 +0300
From:   Alexey Budankov <alexey.budankov@...ux.intel.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Jonatan Corbet <corbet@....net>, Kees Cook <keescook@...omium.org>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Jann Horn <jannh@...gle.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Mark Rutland <mark.rutland@....com>,
        Tvrtko Ursulin <tvrtko.ursulin@...ux.intel.com>,
        "kernel-hardening@...ts.openwall.com" 
        <kernel-hardening@...ts.openwall.com>,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 1/4] perf-security: document perf_events/Perf resource
 control


On 11.02.2019 1:34, Thomas Gleixner wrote:
> On Thu, 7 Feb 2019, Alexey Budankov wrote:
> 
> General note: Please stay in the 80 char limit for all of the text.

Yes, sure. [PATCH v2 4/4] implements wrapping at 72 columns.

> 
>> +The perf_events system call API [2]_ allocates file descriptors for every configured
>> +PMU event. Open file descriptors are a per-process accountable resource governed
>> +by the RLIMIT_NOFILE [11]_ limit (ulimit -n), which is usually derived from the login
>> +shell process. When configuring Perf collection for a long list of events on a
>> +large server system, this limit can be easily hit preventing required monitoring
>> +configuration.
> 
> I'd move this sentence into a different paragraph and keep those related to
> RLIMIT_NOFILE together.

Makes sense. Let's have these two paragraphs:

Open file descriptors
+++++++++++++++++++++

Memory allocation
+++++++++++++++++

> 
>> ... RLIMIT_NOFILE limit can be increased on per-user basis modifying
>> +content of the limits.conf file [12]_ on some systems.
> 
> On some systems?

Well, let's avoid this subtlety and have it like:

'RLIMIT_NOFILE limit can be increased on per-user basis 
 modifying content of the limits.conf file [12]_ .'

> 
>> Ordinarily, a Perf sampling session
>> +(perf record) requires an amount of open perf_event file descriptors that is not
>> +less than a number of monitored events multiplied by a number of monitored CPUs.
> 
>   s/a number of/the number of/

Accepted.

> 
> The ordinary use case is:
> 
>     perf CMD pile-of-events PROCESS
> 
> which does not specify the monitored CPUs at all. Then the number of file
> descriptors is NR_EVENTS * NR_ONLINE_CPUS.
> 
>> +An amount of memory available to user processes for capturing performance monitoring
> 
> The amount ...

Accepted.

> 
>> +data is governed by the perf_event_mlock_kb [2]_ setting. This perf_event specific
>> +resource setting defines overall per-cpu limits of memory allowed for mapping
>> +by the user processes to execute performance monitoring. The setting essentially
>> +extends the RLIMIT_MEMLOCK [11]_ limit, but only for memory regions mapped specially
> 
> s/specially/specifically/

Accepted.

> 
>> +for capturing monitored performance events and related data.
>> +
>> +For example, if a machine has eight cores and perf_event_mlock_kb limit is set
>> +to 516 KiB, then a user process is provided with 516 KiB * 8 = 4128 KiB of memory
>> +above the RLIMIT_MEMLOCK limit (ulimit -l) for perf_event mmap buffers. In particular,
>> +this means that, if the user wants to start two or more performance monitoring
>> +processes, the user is required to manually distribute available 4128 KiB between the
> 
> distribute the available

Accepted.

> 
>> +monitoring processes, for example, using the --mmap-pages Perf record mode option.
>> +Otherwise, the first started performance monitoring process allocates all available
>> +4128 KiB and the other processes will fail to proceed due to the lack of memory.
>> +
>> +RLIMIT_MEMLOCK and perf_event_mlock_kb resource costraints are ignored for
> 
> constraints.

Accepted.

> 
>> +processes with the CAP_IPC_LOCK capability. Thus, perf_events/Perf privileged users
> 
> what means perf_events/Perf ?

'perf_events/Perf privileged users' refers to the paragraph about privileged users.
'perf_events/Perf' means exact combination of the kernel subsystem (perf_events) and 
the privileged Perf tool (Perf) executable that enables certain group of users with
performance monitoring capabilities without scope limit.

> 
>> +can be provided with memory above the constraints for perf_events/Perf performance
>> +monitoring purpose by providing the Perf executable with CAP_IPC_LOCK capability.
> 
> Thanks,
> 
> 	tglx
> 

Thanks,
Alexey