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Subject: Re: [PATCH] KCOV: Introduced tracing unique covered PCs
To:     Dmitry Vyukov <dvyukov@google.com>
Cc:     Andrey Konovalov <andreyknvl@google.com>,
        Jonathan Corbet <corbet@lwn.net>,
        Andrew Morton <akpm@linux-foundation.org>,
        Wei Yongjun <weiyongjun1@huawei.com>,
        Maciej Grochowski <maciej.grochowski@pm.me>,
        kasan-dev <kasan-dev@googlegroups.com>,
        "open list:DOCUMENTATION" <linux-doc@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        syzkaller <syzkaller@googlegroups.com>
References: <20210211080716.80982-1-info@alexander-lochmann.de>
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 <01a9177f-bfd5-251a-758f-d3c68bafd0cf@alexander-lochmann.de>
 <CACT4Y+ZPX43ihuL0TCiCY-ZNa4RmfwuieLb1XUDJEa4tELsUsQ@mail.gmail.com>
From:   Alexander Lochmann <info@alexander-lochmann.de>
Message-ID: <8841773d-c7d2-73aa-6fa6-fe496952f2ba@alexander-lochmann.de>
Date:   Mon, 15 Mar 2021 22:43:25 +0100
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On 15.03.21 09:02, Dmitry Vyukov wrote:
>>>>  static notrace unsigned long canonicalize_ip(unsigned long ip)
>>>> @@ -191,18 +192,26 @@ void notrace __sanitizer_cov_trace_pc(void)
>>>>         struct task_struct *t;
>>>>         unsigned long *area;
>>>>         unsigned long ip = canonicalize_ip(_RET_IP_);
>>>> -       unsigned long pos;
>>>> +       unsigned long pos, idx;
>>>>
>>>>         t = current;
>>>> -       if (!check_kcov_mode(KCOV_MODE_TRACE_PC, t))
>>>> +       if (!check_kcov_mode(KCOV_MODE_TRACE_PC | KCOV_MODE_UNIQUE_PC, t))
>>>>                 return;
>>>>
>>>>         area = t->kcov_area;
>>>> -       /* The first 64-bit word is the number of subsequent PCs. */
>>>> -       pos = READ_ONCE(area[0]) + 1;
>>>> -       if (likely(pos < t->kcov_size)) {
>>>> -               area[pos] = ip;
>>>> -               WRITE_ONCE(area[0], pos);
>>>> +       if (likely(t->kcov_mode == KCOV_MODE_TRACE_PC)) {
>>>
>>> Does this introduce an additional real of t->kcov_mode?
>>> If yes, please reuse the value read in check_kcov_mode.
>> Okay. How do I get that value from check_kcov_mode() to the caller?
>> Shall I add an additional parameter to check_kcov_mode()?
> 
> Yes, I would try to add an additional pointer parameter for mode. I
> think after inlining the compiler should be able to regestrize it.
> 
Should kcov->mode be written directly to that ptr?
Otherwise, it must be written to the already present variable mode, and
than copied to the ptr (if not NULL).

- Alex

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