linux-kernel - Re: [PATCH] kcov: simplify interrupt check

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Message-ID: <CACT4Y+Y90ZJj=FXn-Kdpk6uJ_=qq3NsiOwa1K+xwHDBdHf3MTQ@mail.gmail.com>
Date:   Tue, 21 Mar 2017 10:58:19 +0100
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Hillf Danton <hillf.zj@...baba-inc.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Kefeng Wang <wangkefeng.wang@...wei.com>,
        James Morse <james.morse@....com>,
        Alexander Popov <alex.popov@...ux.com>,
        Andrey Konovalov <andreyknvl@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>,
        syzkaller <syzkaller@...glegroups.com>,
        Quentin Casasnovas <quentin.casasnovas@...cle.com>
Subject: Re: [PATCH] kcov: simplify interrupt check

On Tue, Mar 21, 2017 at 10:28 AM, Hillf Danton <hillf.zj@...baba-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?