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From: Andrew Morton <akpm@linux-foundation.org>
To: Marco Elver <elver@google.com>
Cc: Alexander Potapenko <glider@google.com>,
	Dmitry Vyukov <dvyukov@google.com>, Jann Horn <jannh@google.com>,
	Aleksandr Nogikh <nogikh@google.com>,
	Taras Madan <tarasmadan@google.com>,
	LKML <linux-kernel@vger.kernel.org>,
	Linux Memory Management List <linux-mm@kvack.org>,
	kasan-dev <kasan-dev@googlegroups.com>
Subject: Re: [PATCH v3 4/5] kfence: limit currently covered allocations when pool nearly full
Date: Thu, 23 Sep 2021 16:28:11 -0700	[thread overview]
Message-ID: <20210923162811.3cc8188d6a30d9eed2375468@linux-foundation.org> (raw)
In-Reply-To: <CANpmjNOh0ugPq90cVRPAbR-6qr=Q4CsQ_R1Qxk_Bi4TocgwUQA@mail.gmail.com>

On Thu, 23 Sep 2021 15:44:10 +0200 Marco Elver <elver@google.com> wrote:

> > > > + * time, the below parameters provide a probablity of 0.02-0.33 for false
> > > > + * positive hits respectively:
> > > > + *
> > > > + *     P(alloc_traces) = (1 - e^(-HNUM * (alloc_traces / SIZE)) ^ HNUM
> > > > + */
> > > > +#define ALLOC_COVERED_HNUM     2
> > > > +#define ALLOC_COVERED_SIZE     (1 << (const_ilog2(CONFIG_KFENCE_NUM_OBJECTS) + 2))
> > > > +#define ALLOC_COVERED_HNEXT(h) (1664525 * (h) + 1013904223)
> >
> > Unless we are planning to change these primes, can you use
> > next_pseudo_random32() instead?
> 
> I'm worried about next_pseudo_random32() changing their implementation
> to longer be deterministic or change in other ways that break our
> usecase. In this case we want pseudorandomness, but we're not
> implementing a PRNG.
> 
> Open-coding the constants (given they are from "Numerical Recipes") is
> more reliable and doesn't introduce unwanted reliance on
> next_pseudo_random32()'s behaviour.

Perhaps we could summarize this in an additional comment?

Also, this:

+static u32 get_alloc_stack_hash(unsigned long *stack_entries, size_t num_entries)
+{
+	/* Some randomness across reboots / different machines. */
+	u32 seed = (u32)((unsigned long)__kfence_pool >> (BITS_PER_LONG - 32));

seems a bit weak.  Would it be better to seed this at boot time with
a randomish number?

  parent reply	other threads:[~2021-09-23 23:28 UTC|newest]

Thread overview: 15+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-09-23 10:47 [PATCH v3 1/5] stacktrace: move filter_irq_stacks() to kernel/stacktrace.c Marco Elver
2021-09-23 10:48 ` [PATCH v3 2/5] kfence: count unexpectedly skipped allocations Marco Elver
2021-09-23 11:15   ` Alexander Potapenko
2021-09-23 10:48 ` [PATCH v3 3/5] kfence: move saving stack trace of allocations into __kfence_alloc() Marco Elver
2021-09-23 11:32   ` Alexander Potapenko
2021-09-23 10:48 ` [PATCH v3 4/5] kfence: limit currently covered allocations when pool nearly full Marco Elver
2021-09-23 11:18   ` Dmitry Vyukov
2021-09-23 13:23     ` Alexander Potapenko
2021-09-23 13:44       ` Marco Elver
2021-09-23 13:46         ` Alexander Potapenko
2021-09-23 23:28         ` Andrew Morton [this message]
2021-09-24 13:01           ` Marco Elver
2021-09-23 10:48 ` [PATCH v3 5/5] kfence: add note to documentation about skipping covered allocations Marco Elver
2021-09-23 15:46   ` Alexander Potapenko
2021-09-23 11:14 ` [PATCH v3 1/5] stacktrace: move filter_irq_stacks() to kernel/stacktrace.c Alexander Potapenko

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