[PATCH v2] kcov: properly check if we are in an interrupt

[PATCH v2] kcov: properly check if we are in an interrupt

[Andrey Konovalov]

in_interrupt() returns a nonzero value when we are either in an
interrupt or have bh disabled via local_bh_disable(). Since we are
interested in only ignoring coverage from actual interrupts, do a
proper check instead of just calling in_interrupt().

Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
---
Changes in v2:
 - Add a comment explaining why the check is open-coded.

 kernel/kcov.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/kernel/kcov.c b/kernel/kcov.c
index 8d44b3f..30e6d05 100644
--- a/kernel/kcov.c
+++ b/kernel/kcov.c
@@ -53,8 +53,15 @@ void notrace __sanitizer_cov_trace_pc(void)
 	/*
 	 * We are interested in code coverage as a function of a syscall inputs,
 	 * so we ignore code executed in interrupts.
+	 * The checks for whether we are in an interrupt are open-coded, because
+	 * 1. We can't use in_interrupt() here, since it also returns true
+	 *    when we are inside local_bh_disable() section.
+	 * 2. We don't want to use (in_irq() | in_serving_softirq() | in_nmi()),
+	 *    since that leads to slower generated code (three separate tests,
+	 *    one for each of the flags).
 	 */
-	if (!t || in_interrupt())
+	if (!t || (preempt_count() & (HARDIRQ_MASK | SOFTIRQ_OFFSET
+							| NMI_MASK)))
 		return;
 	mode = READ_ONCE(t->kcov_mode);
 	if (mode == KCOV_MODE_TRACE) {
-- 
2.8.0.rc3.226.g39d4020

Re: [PATCH v2] kcov: properly check if we are in an interrupt

[Dmitry Vyukov]

On Mon, Oct 10, 2016 at 6:10 PM, Andrey Konovalov <andreyknvl@google.com> wrote:
> in_interrupt() returns a nonzero value when we are either in an
> interrupt or have bh disabled via local_bh_disable(). Since we are
> interested in only ignoring coverage from actual interrupts, do a
> proper check instead of just calling in_interrupt().
>
> Signed-off-by: Andrey Konovalov <andreyknvl@google.com>

FWIW
Acked-by: Dmitry Vyukov <dvyukov@google.com>

This fixes a very real problem for us.
As per discussion in v1, other solution would involve auditing all
uses of in_interrupt() which needs knowledge about all drivers.

> ---
> Changes in v2:
>  - Add a comment explaining why the check is open-coded.
>
>  kernel/kcov.c | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/kcov.c b/kernel/kcov.c
> index 8d44b3f..30e6d05 100644
> --- a/kernel/kcov.c
> +++ b/kernel/kcov.c
> @@ -53,8 +53,15 @@ void notrace __sanitizer_cov_trace_pc(void)
>         /*
>          * We are interested in code coverage as a function of a syscall inputs,
>          * so we ignore code executed in interrupts.
> +        * The checks for whether we are in an interrupt are open-coded, because
> +        * 1. We can't use in_interrupt() here, since it also returns true
> +        *    when we are inside local_bh_disable() section.
> +        * 2. We don't want to use (in_irq() | in_serving_softirq() | in_nmi()),
> +        *    since that leads to slower generated code (three separate tests,
> +        *    one for each of the flags).
>          */
> -       if (!t || in_interrupt())
> +       if (!t || (preempt_count() & (HARDIRQ_MASK | SOFTIRQ_OFFSET
> +                                                       | NMI_MASK)))
>                 return;
>         mode = READ_ONCE(t->kcov_mode);
>         if (mode == KCOV_MODE_TRACE) {
> --
> 2.8.0.rc3.226.g39d4020
>

Re: [PATCH v2] kcov: properly check if we are in an interrupt

[Andrew Morton]

On Mon, 10 Oct 2016 19:19:11 +0200 Dmitry Vyukov <dvyukov@google.com> wrote:

> On Mon, Oct 10, 2016 at 6:10 PM, Andrey Konovalov <andreyknvl@google.com> wrote:
> > in_interrupt() returns a nonzero value when we are either in an
> > interrupt or have bh disabled via local_bh_disable(). Since we are
> > interested in only ignoring coverage from actual interrupts, do a
> > proper check instead of just calling in_interrupt().
> >
> > Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
> 
> FWIW
> Acked-by: Dmitry Vyukov <dvyukov@google.com>
> 
> This fixes a very real problem for us.
> As per discussion in v1, other solution would involve auditing all
> uses of in_interrupt() which needs knowledge about all drivers.

OK, but I'm not seeing a description of what the problem *is*.  Could
we please have a complete description of the user-visible effects of this
change, for the changelog?

Re: [PATCH v2] kcov: properly check if we are in an interrupt

[Andrey Konovalov]

On Tue, Oct 11, 2016 at 1:32 AM, Andrew Morton
<akpm@linux-foundation.org> wrote:
> On Mon, 10 Oct 2016 19:19:11 +0200 Dmitry Vyukov <dvyukov@google.com> wrote:
>
>> On Mon, Oct 10, 2016 at 6:10 PM, Andrey Konovalov <andreyknvl@google.com> wrote:
>> > in_interrupt() returns a nonzero value when we are either in an
>> > interrupt or have bh disabled via local_bh_disable(). Since we are
>> > interested in only ignoring coverage from actual interrupts, do a
>> > proper check instead of just calling in_interrupt().
>> >
>> > Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
>>
>> FWIW
>> Acked-by: Dmitry Vyukov <dvyukov@google.com>
>>
>> This fixes a very real problem for us.
>> As per discussion in v1, other solution would involve auditing all
>> uses of in_interrupt() which needs knowledge about all drivers.
>
> OK, but I'm not seeing a description of what the problem *is*.  Could
> we please have a complete description of the user-visible effects of this
> change, for the changelog?

Mailed v3 with the description.

>