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

[Andrey Konovalov]

On Tue, Jun 11, 2024 at 9:50 AM 'Dmitry Vyukov' via syzkaller
<syzkaller@xxxxxxxxxxxxxxxx> 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@xxxxxxxxxx>
> Reviewed-by: Alexander Potapenko <glider@xxxxxxxxxx>
> Cc: x86@xxxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> Cc: syzkaller@xxxxxxxxxxxxxxxx
>
> ---
>
> 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
> ---
>  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;
> +       while ((jiffies - start) * MSEC_PER_SEC / HZ < 300)
> +               ;
> +       current->kcov_mode = 0;
> +       pr_err("done running self test\n");
> +}
> +#endif
> +
>  static int __init kcov_init(void)
>  {
>         int cpu;
> @@ -1076,6 +1103,10 @@ static int __init kcov_init(void)
>          */
>         debugfs_create_file_unsafe("kcov", 0600, NULL, NULL, &kcov_fops);
>
> +#ifdef CONFIG_KCOV_SELFTEST
> +       selftest();
> +#endif
> +
>         return 0;
>  }
>
> diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
> index 59b6765d86b8..695a437a52d9 100644
> --- a/lib/Kconfig.debug
> +++ b/lib/Kconfig.debug
> @@ -2171,6 +2171,14 @@ config KCOV_IRQ_AREA_SIZE
>           soft interrupts. This specifies the size of those areas in the
>           number of unsigned long words.
>
> +config KCOV_SELFTEST
> +       bool "Perform short selftests on boot"
> +       depends on KCOV
> +       help
> +         Run short KCOV coverage collection selftests on boot.
> +         On test failure, causes the kernel to panic. Recommended to be

Nit: "causes the kernel to panic" => "causes a kernel panic" or "panic
the kernel"

> +         enabled, ensuring critical functionality works as intended.
> +
>  menuconfig RUNTIME_TESTING_MENU
>         bool "Runtime Testing"
>         default y
> --
> 2.45.2.505.gda0bf45e8d-goog

Reviewed-by: Andrey Konovalov <andreyknvl@xxxxxxxxx>