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[PATCH] kcov: simplify interrupt check

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  [PATCH] kcov: simplify interrupt check Dmitry Vyukov <dvyukov@google.com> - 2017-03-21 10:20 +0100
    Re: [PATCH] kcov: simplify interrupt check "Hillf Danton" <hillf.zj@alibaba-inc.com> - 2017-03-21 10:30 +0100
      Re: [PATCH] kcov: simplify interrupt check Dmitry Vyukov <dvyukov@google.com> - 2017-03-21 11:00 +0100

#1605465 — [PATCH] kcov: simplify interrupt check

in_interrupt() semantics are confusing and wrong for most users
as it also returns true when bh is disabled. Thus we open coded
a proper check for interrupts in __sanitizer_cov_trace_pc()
with a lengthy explanatory comment.

Use the new in_task() predicate instead.

Signed-off-by: Dmitry Vyukov <dvyukov@google.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Kefeng Wang <wangkefeng.wang@huawei.com>
Cc: James Morse <james.morse@arm.com>
Cc: Alexander Popov <alex.popov@linux.com>
Cc: Andrey Konovalov <andreyknvl@google.com>
Cc: linux-mm@kvack.org
Cc: linux-kernel@vger.kernel.org
Cc: syzkaller@googlegroups.com
---
 kernel/kcov.c | 9 +--------
 1 file changed, 1 insertion(+), 8 deletions(-)

diff --git a/kernel/kcov.c b/kernel/kcov.c
index 85e5546cd791..cd771993f96f 100644
--- a/kernel/kcov.c
+++ b/kernel/kcov.c
@@ -60,15 +60,8 @@ 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 || (preempt_count() & (HARDIRQ_MASK | SOFTIRQ_OFFSET
-							| NMI_MASK)))
+	if (!t || !in_task())
 		return;
 	mode = READ_ONCE(t->kcov_mode);
 	if (mode == KCOV_MODE_TRACE) {
-- 
2.12.1.500.gab5fba24ee-goog

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#1605467

On March 21, 2017 5:10 PM Dmitry Vyukov wrote: 
> 
> @@ -60,15 +60,8 @@ 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 || (preempt_count() & (HARDIRQ_MASK | SOFTIRQ_OFFSET
> -							| NMI_MASK)))
> +	if (!t || !in_task())
>  		return;

Nit: can we get the current task check cut off?

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#1605478

On Tue, Mar 21, 2017 at 10:28 AM, Hillf Danton <hillf.zj@alibaba-inc.com> wrote:
>
> On March 21, 2017 5:10 PM Dmitry Vyukov wrote:
>>
>> @@ -60,15 +60,8 @@ 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 || (preempt_count() & (HARDIRQ_MASK | SOFTIRQ_OFFSET
>> -                                                     | NMI_MASK)))
>> +     if (!t || !in_task())
>>               return;
>
> Nit: can we get the current task check cut off?


Humm... good question.
I don't remember why exactly I added it. I guess something was
crashing during boot. Note that this call is inserted into almost all
kernel code. But probably that was before I disabled instrumentation
of some early boot code for other reasons (with KCOV_INSTRUMENT := n
in Makefile), because now I can boot kernel in qemu without this
check. But I am still not sure about real hardware/arm/etc.
Does anybody know if current can ever (including early boot) return
invalid pointer?

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