Re: [PATCH v2 2/4] kcov: add interrupt handling self test

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Jun 11, 2024 at 09:50:31AM +0200, Dmitry Vyukov wrote:
> Add a boot self test that can catch sprious coverage from interrupts.
> The coverage callback filters out interrupt code, but only after the
> handler updates preempt count. Some code periodically leaks out
> of that section and leads to spurious coverage.
> Add a best-effort (but simple) test that is likely to catch such bugs.
> If the test is enabled on CI systems that use KCOV, they should catch
> any issues fast.
> 
> Signed-off-by: Dmitry Vyukov <dvyukov@google.com>
> Reviewed-by: Alexander Potapenko <glider@google.com>
> Cc: x86@kernel.org
> Cc: linux-kernel@vger.kernel.org
> Cc: syzkaller@googlegroups.com
> 
> ---
> 
> Changed since v1:
>  - renamed KCOV_TEST to KCOV_SELFTEST
>  - improved the config description
>  - loop for exactly 300ms in the test
> 
> In my local testing w/o the previous fix,
> it immidiatly produced the following splat:
> 
> kcov: running selftest
> BUG: TASK stack guard page was hit at ffffc90000147ff8
> Oops: stack guard page: 0000 [#1] PREEMPT SMP KASAN PTI
> ...
>  kvm_set_cpu_l1tf_flush_l1d+0x5/0x20
>  sysvec_call_function+0x15/0xb0
>  asm_sysvec_call_function+0x1a/0x20
>  kcov_init+0xe4/0x130
>  do_one_initcall+0xbc/0x470
>  kernel_init_freeable+0x4fc/0x930
>  kernel_init+0x1c/0x2b0

So I'm not entirely sure how the above BUG comes about, nor how this
selftest tickles it. Could you elaborate?
	
I've found check_kcov_mode() which has this !in_task() clause, but I'm
not entirely sure how failing that leads to the above mentioned failure.

> ---
>  kernel/kcov.c     | 31 +++++++++++++++++++++++++++++++
>  lib/Kconfig.debug |  8 ++++++++
>  2 files changed, 39 insertions(+)
> 
> diff --git a/kernel/kcov.c b/kernel/kcov.c
> index c3124f6d5536..72a5bf55107f 100644
> --- a/kernel/kcov.c
> +++ b/kernel/kcov.c
> @@ -11,6 +11,7 @@
>  #include <linux/fs.h>
>  #include <linux/hashtable.h>
>  #include <linux/init.h>
> +#include <linux/jiffies.h>
>  #include <linux/kmsan-checks.h>
>  #include <linux/mm.h>
>  #include <linux/preempt.h>
> @@ -1057,6 +1058,32 @@ u64 kcov_common_handle(void)
>  }
>  EXPORT_SYMBOL(kcov_common_handle);
>  
> +#ifdef CONFIG_KCOV_SELFTEST
> +static void __init selftest(void)
> +{
> +	unsigned long start;
> +
> +	pr_err("running self test\n");
> +	/*
> +	 * Test that interrupts don't produce spurious coverage.
> +	 * The coverage callback filters out interrupt code, but only
> +	 * after the handler updates preempt count. Some code periodically
> +	 * leaks out of that section and leads to spurious coverage.
> +	 * It's hard to call the actual interrupt handler directly,
> +	 * so we just loop here for a bit waiting for a timer interrupt.
> +	 * We set kcov_mode to enable tracing, but don't setup the area,
> +	 * so any attempt to trace will crash. Note: we must not call any
> +	 * potentially traced functions in this region.
> +	 */
> +	start = jiffies;
> +	current->kcov_mode = KCOV_MODE_TRACE_PC;

	barrier();

> +	while ((jiffies - start) * MSEC_PER_SEC / HZ < 300)
> +		;

	barrier();

> +	current->kcov_mode = 0;
> +	pr_err("done running self test\n");
> +}
> +#endif