Re: [PATCH 00/14] Present useful limits to user (v2)

Topi Miettinen
2016-07-19T16:53:50Z

On 07/18/16 22:05, Doug Ledford wrote:
> On 7/15/2016 12:35 PM, Topi Miettinen wrote:
>> On 07/15/16 13:04, Balbir Singh wrote:
>>> On Fri, Jul 15, 2016 at 01:35:47PM +0300, Topi Miettinen wrote:
>>>> Hello,
>>>>
>>>> There are many basic ways to control processes, including capabilities,
>>>> cgroups and resource limits. However, there are far fewer ways to find out
>>>> useful values for the limits, except blind trial and error.
>>>>
>>>> This patch series attempts to fix that by giving at least a nice starting
>>>> point from the highwater mark values of the resources in question.
>>>> I looked where each limit is checked and added a call to update the mark
>>>> nearby.
>>>>
>>>> Example run of program from Documentation/accounting/getdelauys.c:
>>>>
>>>> ./getdelays -R -p `pidof smartd`
>>>> printing resource accounting
>>>> RLIMIT_CPU=0
>>>> RLIMIT_FSIZE=0
>>>> RLIMIT_DATA=18198528
>>>> RLIMIT_STACK=135168
>>>> RLIMIT_CORE=0
>>>> RLIMIT_RSS=0
>>>> RLIMIT_NPROC=1
>>>> RLIMIT_NOFILE=55
>>>> RLIMIT_MEMLOCK=0
>>>> RLIMIT_AS=130879488
>>>> RLIMIT_LOCKS=0
>>>> RLIMIT_SIGPENDING=0
>>>> RLIMIT_MSGQUEUE=0
>>>> RLIMIT_NICE=0
>>>> RLIMIT_RTPRIO=0
>>>> RLIMIT_RTTIME=0
>>>>
>>>> ./getdelays -R -C /sys/fs/cgroup/systemd/system.slice/smartd.service/
>>>> printing resource accounting
>>>> sleeping 1, blocked 0, running 0, stopped 0, uninterruptible 0
>>>> RLIMIT_CPU=0
>>>> RLIMIT_FSIZE=0
>>>> RLIMIT_DATA=18198528
>>>> RLIMIT_STACK=135168
>>>> RLIMIT_CORE=0
>>>> RLIMIT_RSS=0
>>>> RLIMIT_NPROC=1
>>>> RLIMIT_NOFILE=55
>>>> RLIMIT_MEMLOCK=0
>>>> RLIMIT_AS=130879488
>>>> RLIMIT_LOCKS=0
>>>> RLIMIT_SIGPENDING=0
>>>> RLIMIT_MSGQUEUE=0
>>>> RLIMIT_NICE=0
>>>> RLIMIT_RTPRIO=0
>>>> RLIMIT_RTTIME=0
>>>
>>> Does this mean that rlimit_data and rlimit_stack should be set to the
>>> values as specified by the data above?
>>
>> My plan is that either system administrator, distro maintainer or even
>> upstream developer can get reasonable values for the limits. They may
>> still be wrong, but things would be better than without any help to
>> configure the system.
>
> This is not necessarily true. It seems like there is a disconnect
> between what these various values are for and what you are positioning
> them as. Most of these limits are meant to protect the system from
> resource starvation crashes. They aren't meant to be any sort of double
> check on a specific application. The vast majority of applications can
> have bugs, leak resources, and do all sorts of other bad things and
> still not hit these limits. A program that leaks a file handle an hour
> but only normally has 50 handles in use would take 950 hours of constant
> leaking before these limits would kick in to bring the program under
> control. That's over a month. What's more though, the kernel couldn't
> really care less that a single application leaked files until it got to
> 1000 open. The real point of the limit on file handles (since they are
> cheap) is just not to let the system get brought down. Someone could
> maliciously fire up 1000 processes, and they could all attempt to open
> up as many files as possible in order to drown the system in open
> inodes. The combination of the limit on maximum user processes and
> maximum files per process are intended to prevent this. They are not
> intended to prevent a single, properly running application from
> operating. In fact, there are very few applications that are likely to
> break the 1000 file per process limit. It is outrageously high for most
> applications. They will leak files and do all sorts of bad things
> without this ever stopping them. But it does stop malicious programs.
> And the process limit stops malicious users too. The max locked memory
> is used by almost no processes, and for the very few that use it, the
> default is more than enough. The major exception is the RDMA stack,
> which uses it so much that we just disable it on large systems because
> it's impossible to predict how much we'll need and we don't want a job
> to get killed because it couldn't get the memory it needs for buffers.
> The limit on POSIX message queues is another one where it's more than
> enough for most applications which don't use this feature at all, and
> the few systems that use this feature adjust the limit to something sane
> on their system (we can't make the default sane for these special
> systems or else it becomes an avenue for Denial of Service attack, so
> the default must stay low and servers that make extensive use of this
> feature must up their limit on a case by case basis).
>
>>>
>>> Do we expect a smart user space daemon to then tweak the RLIMIT values?
>>
>> Someone could write an autotuning daemon that checks if the system has
>> changed (for example due to upgrade) and then run some tests to
>> reconfigure the system. But the limits are a bit too fragile, or rather,
>> applications can't handle failure, so I don't know if that would really
>> work.
>
> This misses the point of most of these limits. They aren't there to
> keep normal processes and normal users in check. They are there to stop
> runaway use. This runaway situation might be accidental, or it might be
> a nefarious users. They are generally set exceedingly high for those
> things every application uses, and fairly low for those things that
> almost no application uses but which could be abused by the nefarious
> user crowd.
>
> Moreover, for a large percentage of applications, the highwatermark is a
> source of great trickery. For instance, if you have a web server that
> is hosting web pages written in python, and therefore are using
> mod_python in the httpd server (assuming apache here), then your
> highwatermark will never be a reliable, stable thing. If you get 1000
> web requests in a minute, all utilizing the mod_python resource in the
> web server, and you don't have your httpd configured to restart after
> every few hundred requests handled, then mod_python in your httpd
> process will grow seemingly without limit. It will consume tons of
> memory. And the only limit on how much memory it will consume is
> determined by how many web requests it handles in between its garbage
> collection intervals * how much memory it allocates per request. If you
> don't happen to catch the absolute highest amount while you are
> gathering your watermarks, then when you actually switch the system to
> enforcing the limits you learned from all your highwatermarks (you are
> planning on doing that aren't you?....I didn't see a copy of the patch
> 1/14, so I don't know if this infrastructure ever goes back to enforcing
> the limits or not, but I would assume so, what point is there in
> learning what the limits should be if you then never turn around and
> enforce them?), load spikes will cause random program failures.
>
> Really, this looks like a solution in search of a problem. Right now,
> the limits are set where they are because they do two things:
>
> 1) Stay out of the way of the vast majority of applications. Those
> applications that get tripped up by the defaults (like RDMA applications
> getting stopped by memlock settings) have setup guides that spell out
> which limits need changed and hints on what to change them too.
>
> 2) Stop nefarious users or errant applications from a total runaway
> situation on a machine.
>
> If your applications run without fail unless they have already failed,
> and the whole machine doesn't go down with your failed application, then
> the limits are working as designed. If your typical machine
> configuration includes 256GB of RAM, then you could probably stand to
> increase some of the limits safely if you wanted to. But unless you
> have applications getting killed because of these limits, why would you?

Thanks for the long explanation. I'd suppose loose limits are also used
because it's hard to know good tighter values. I was thinking of using
tighter settings to make things less easy for exploit writers. With
tight limits for RLIMIT_AS, RLIMIT_DATA and RLIMIT_STACK (also
RLIMIT_FSIZE in case a daemon is not supposed to create new files) it
would not so easy for the initial exploit to mmap() a large next stage
payload.

But there could be more direct ways to prevent that. For example, if
there was a way for seccomp filters to access a share state, they could
implement a state machine that could switch to a stricter mode after the
application has entered the event loop. Most of the limits or the denial
of service case are not interesting to me anyway.

> Right now, I'm inclined to NAK the patch set. I've only seen patch 9/14
> since you didn't Cc: everyone on the patch 1/14 that added the
> infrastructure. But, as I mentioned in another email, I think this can
> be accomplished via a systemtap script instead so we keep the clutter
> out of the kernel. And more importantly, these patches seem to be
> thinking about these limits as though they are supposed to be some sort
> of tight fitting container around applications that catch an errant
> application as soon as it steps out of bounds. Nothing could be further
> from the truth, and if we actually implemented something of that sort,
> programs susceptible to high resource usage during load spikes would
> suddenly start failing on a frequent basis. The proof that these limits
> are working is given by the fact that we rarely hear from users about
> their programs being killed for resource consumption, and yet we also
> don't hear from users about their systems going down due to runaway
> applications. From what I can tell from these patches, I would suspect
> complaints from one of those two issues to increase once these patches
> are in place and put in use, and that doesn't seem like a good thing.
>

Those complaints could increase either way if the users want to use
tight limits, with kernel assistance or with systemtap. Again, the cause
of lack of complaints could also be that users are unaware of how to get
tight limits, so the users have no option but to either ignore the
limits or to use loose settings.

-Topi