Re: [PATCH v6 8/9] kfence: add test suite

[Jann Horn]

On Thu, Oct 29, 2020 at 2:17 PM Marco Elver <elver@xxxxxxxxxx> 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.
[...]
> diff --git a/mm/kfence/kfence_test.c b/mm/kfence/kfence_test.c
[...]
> +static void *test_alloc(struct kunit *test, size_t size, gfp_t gfp, enum allocation_policy policy)
> +{
> +       void *alloc;
> +       unsigned long timeout, resched_after;
[...]
> +       /*
> +        * 100x the sample interval should be more than enough to ensure we get
> +        * a KFENCE allocation eventually.
> +        */
> +       timeout = jiffies + msecs_to_jiffies(100 * CONFIG_KFENCE_SAMPLE_INTERVAL);
> +       /*
> +        * Especially for non-preemption kernels, ensure the allocation-gate
> +        * timer has time to catch up.
> +        */
> +       resched_after = jiffies + msecs_to_jiffies(CONFIG_KFENCE_SAMPLE_INTERVAL);
> +       do {
[...]
> +               if (time_after(jiffies, resched_after))
> +                       cond_resched();

You probably meant to recalculate resched_after after the call to
cond_resched()?

> +       } while (time_before(jiffies, timeout));
> +
> +       KUNIT_ASSERT_TRUE_MSG(test, false, "failed to allocate from KFENCE");
> +       return NULL; /* Unreachable. */
> +}
[...]
> +/*
> + * KFENCE is unable to detect an OOB if the allocation's alignment requirements
> + * leave a gap between the object and the guard page. Specifically, an
> + * allocation of e.g. 73 bytes is aligned on 8 and 128 bytes for SLUB or SLAB
> + * respectively. Therefore it is impossible for the allocated object to adhere
> + * to either of the page boundaries.

Should this be "to the left page boundary" instead of "to either of
the page boundaries"?

> + * However, we test that an access to memory beyond the gap result in KFENCE

*results



> + * detecting an OOB access.
> + */
> +static void test_kmalloc_aligned_oob_read(struct kunit *test)