linux-kernel - Re: [PATCH] kasan: report only the first error

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Date:   Wed, 22 Mar 2017 18:42:53 +0100
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Alexander Potapenko <glider@...gle.com>
Cc:     Andrey Konovalov <andreyknvl@...gle.com>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mark Rutland <mark.rutland@....com>,
        kasan-dev <kasan-dev@...glegroups.com>,
        Linux Memory Management List <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] kasan: report only the first error

On Wed, Mar 22, 2017 at 6:33 PM, Alexander Potapenko <glider@...gle.com> wrote:
> On Wed, Mar 22, 2017 at 6:07 PM, Andrey Konovalov <andreyknvl@...gle.com> wrote:
>> On Wed, Mar 22, 2017 at 5:54 PM, Andrey Ryabinin
>> <aryabinin@...tuozzo.com> wrote:
>>> On 03/22/2017 07:34 PM, Andrey Konovalov wrote:
>>>> On Wed, Mar 22, 2017 at 5:06 PM, Andrey Ryabinin
>>>> <aryabinin@...tuozzo.com> wrote:
>>>>> Disable kasan after the first report. There are several reasons for this:
>>>>>  * Single bug quite often has multiple invalid memory accesses causing
>>>>>     storm in the dmesg.
>>>>>  * Write OOB access might corrupt metadata so the next report will print
>>>>>     bogus alloc/free stacktraces.
>>>>>  * Reports after the first easily could be not bugs by itself but just side
>>>>>     effects of the first one.
>>>>>
>>>>> Given that multiple reports only do harm, it makes sense to disable
>>>>> kasan after the first one. Except for the tests in lib/test_kasan.c
>>>>> as we obviously want to see all reports from test.
>>>>
>>>> Hi Andrey,
>>>>
>>>> Could you make it configurable via CONFIG_KASAN_SOMETHING (which can
>>>> default to showing only the first report)?
>>>
>>> I'd rather make this boot time configurable, but wouldn't want to without
>>> a good reason.
>>
>> That would work for me.

Also note that KASAN now supports panic_on_warn=1, which achieves more
or less the same. Of course, WARNINGs may be not that bad, but KASAN
reports may be not tool bad as well (e.g. off-by-one reads).


>>>> I sometimes use KASAN to see what bad accesses a particular bug
>>>> causes, and seeing all of them (even knowing that they may be
>>>> corrupt/induced) helps a lot.
>>>
>>> I'm wondering why you need to see all reports?
>>
>> To get a better picture of what are the consequences of a bug. For
>> example whether it leads to some bad or controllable memory
>> corruption. Sometimes it's easier to let KASAN track the memory
>> accesses then do that manually.
> Another case is when you're seeing an OOB read at boot time, which has
> limited impact, and you don't want to wait for the code owner to fix
> it to move forward.
>>>
>>>>
>>>> Thanks!
>>>>
>
>
>
> --
> Alexander Potapenko
> Software Engineer
>
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