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Date:   Tue, 18 Dec 2018 11:46:22 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Suren Baghdasaryan <surenb@google.com>
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Tejun Heo <tj@kernel.org>, lizefan@huawei.com,
        Johannes Weiner <hannes@cmpxchg.org>, axboe@kernel.dk,
        dennis@kernel.org, Dennis Zhou <dennisszhou@gmail.com>,
        Ingo Molnar <mingo@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Jonathan Corbet <corbet@lwn.net>, cgroups@vger.kernel.org,
        linux-mm <linux-mm@kvack.org>, linux-doc@vger.kernel.org,
        LKML <linux-kernel@vger.kernel.org>, kernel-team@android.com
Subject: Re: [PATCH 6/6] psi: introduce psi monitor
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On Mon, Dec 17, 2018 at 05:21:05PM -0800, Suren Baghdasaryan wrote:
> On Mon, Dec 17, 2018 at 8:22 AM Peter Zijlstra <peterz@infradead.org> wrote:

> > How well has this thing been fuzzed? Custom string parser, yay!
> 
> Honestly, not much. Normal cases and some obvious corner cases. Will
> check if I can use some fuzzer to get more coverage or will write a
> script.
> I'm not thrilled about writing a custom parser, so if there is a
> better way to handle this please advise.

The grammar seems fairly simple, something like:

  some-full = "some" | "full" ;
  threshold-abs = integer ;
  threshold-pct = integer, { "%" } ;
  threshold = threshold-abs | threshold-pct ;
  window = integer ;
  trigger = some-full, space, threshold, space, window ;

And that could even be expressed as two scanf formats:

 "%4s %u%% %u" , "%4s %u %u"

which then gets your something like:

  char type[5];

  if (sscanf(input, "%4s %u%% %u", &type, &pct, &window) == 3) {
  	// do pct thing
  } else if (sscanf(intput, "%4s %u %u", &type, &thres, &window) == 3) {
  	// do abs thing
  } else return -EFAIL;

  if (!strcmp(type, "some")) {
  	// some
  } else if (!strcmp(type, "full")) {
  	// full
  } else return -EFAIL;

  // do more

which seems like a lot less error prone. Alternatively you can use 4
formats:

  "some %u%% %u" "some %u %u"
  "full %u%% %u" "full %u %u"

and avoid the whole 'type' thing.