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Date:   Mon, 21 Sep 2020 10:48:27 -0700
From:   "Paul E. McKenney" <paulmck@kernel.org>
To:     Marco Elver <elver@google.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Alexander Potapenko <glider@google.com>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Andrey Konovalov <andreyknvl@google.com>,
        Andrey Ryabinin <aryabinin@virtuozzo.com>,
        Andy Lutomirski <luto@kernel.org>,
        Borislav Petkov <bp@alien8.de>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Christoph Lameter <cl@linux.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        David Rientjes <rientjes@google.com>,
        Dmitry Vyukov <dvyukov@google.com>,
        Eric Dumazet <edumazet@google.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Hillf Danton <hdanton@sina.com>,
        Ingo Molnar <mingo@redhat.com>, Jann Horn <jannh@google.com>,
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        Linux Memory Management List <linux-mm@kvack.org>
Subject: Re: [PATCH v3 10/10] kfence: add test suite
Message-ID: <20200921174827.GG29330@paulmck-ThinkPad-P72>
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On Mon, Sep 21, 2020 at 07:37:13PM +0200, Marco Elver wrote:
> On Mon, 21 Sep 2020 at 19:13, Paul E. McKenney <paulmck@kernel.org> wrote:
> >
> > On Mon, Sep 21, 2020 at 03:26:11PM +0200, Marco Elver wrote:
> > > Add KFENCE test suite, testing various error detection scenarios. Makes
> > > use of KUnit for test organization. Since KFENCE's interface to obtain
> > > error reports is via the console, the test verifies that KFENCE outputs
> > > expected reports to the console.
> > >
> > > Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
> > > Co-developed-by: Alexander Potapenko <glider@google.com>
> > > Signed-off-by: Alexander Potapenko <glider@google.com>
> > > Signed-off-by: Marco Elver <elver@google.com>
> >
> > [ . . . ]
> >
> > > +/* Test SLAB_TYPESAFE_BY_RCU works. */
> > > +static void test_memcache_typesafe_by_rcu(struct kunit *test)
> > > +{
> > > +     const size_t size = 32;
> > > +     struct expect_report expect = {
> > > +             .type = KFENCE_ERROR_UAF,
> > > +             .fn = test_memcache_typesafe_by_rcu,
> > > +     };
> > > +
> > > +     setup_test_cache(test, size, SLAB_TYPESAFE_BY_RCU, NULL);
> > > +     KUNIT_EXPECT_TRUE(test, test_cache); /* Want memcache. */
> > > +
> > > +     expect.addr = test_alloc(test, size, GFP_KERNEL, ALLOCATE_ANY);
> > > +     *expect.addr = 42;
> > > +
> > > +     rcu_read_lock();
> > > +     test_free(expect.addr);
> > > +     KUNIT_EXPECT_EQ(test, *expect.addr, (char)42);
> > > +     rcu_read_unlock();
> >
> > It won't happen very often, but memory really could be freed at this point,
> > especially in CONFIG_RCU_STRICT_GRACE_PERIOD=y kernels ...
> 
> Ah, thanks for pointing it out.
> 
> > > +     /* No reports yet, memory should not have been freed on access. */
> > > +     KUNIT_EXPECT_FALSE(test, report_available());
> >
> > ... so the above statement needs to go before the rcu_read_unlock().
> 
> You mean the comment (and not the KUNIT_EXPECT_FALSE that no reports
> were generated), correct?
> 
> Admittedly, the whole comment is a bit imprecise, so I'll reword.

I freely confess that I did not research exactly what might generate
a report.  But if this KUNIT_EXPECT_FALSE() was just verifying that the
previous KUNIT_EXPECT_TRUE() did not trigger, then yes, the code is just
fine as it is.

							Thanx, Paul

> > > +     rcu_barrier(); /* Wait for free to happen. */
> >
> > But you are quite right that the memory is not -guaranteed- to be freed
> > until we get here.
> 
> Right, I'll update the comment.
> 
> Thanks,
> -- Marco