Re: [PATCH] mm/kasan/kasan.h: Fix boolean checking issue for kasan_report_enabled()

From: Chen Gang <chengang@emindsoft.com.cn>
To: Alexander Potapenko <glider@google.com>
Cc: Dmitry Vyukov <dvyukov@google.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Andrey Ryabinin <aryabinin@virtuozzo.com>,
	kasan-dev <kasan-dev@googlegroups.com>,
	LKML <linux-kernel@vger.kernel.org>,
	"linux-mm@kvack.org" <linux-mm@kvack.org>,
	Chen Gang <gang.chen.5i5j@gmail.com>
Subject: Re: [PATCH] mm/kasan/kasan.h: Fix boolean checking issue for kasan_report_enabled()
Date: Mon, 02 May 2016 21:51:45 +0800	[thread overview]
Message-ID: <57275B71.8000907@emindsoft.com.cn> (raw)
In-Reply-To: <CAG_fn=VGJAGb71HU4rC9MNboqPqPs4EPgcWBfaiBpcgNQ2qFqA@mail.gmail.com>

On 5/2/16 20:42, Alexander Potapenko wrote:
> On Mon, May 2, 2016 at 2:27 PM, Chen Gang <chengang@emindsoft.com.cn> wrote:
>> On 5/2/16 19:21, Dmitry Vyukov wrote:
>>>
>>> Signed counter looks good to me.
>>
>> Oh, sorry, it seems a little mess (originally, I need let the 2 patches
>> in one patch set).
>>
>> If what Alexander's idea is OK (if I did not misunderstand), I guess,
>> unsigned counter is still necessary.
> I don't think it's critical for us to use an unsigned counter.
> If we increment the counter in kasan_disable_current() and decrement
> it in kasan_enable_current(), as Dmitry suggested, we'll be naturally
> using only positive integers for the counter.
> If the counter drops below zero, or exceeds a certain number (say,
> 20), we can immediately issue a warning.
> 

OK, thanks.

And for "kasan_depth == 1", I guess, its meaning is related with
kasan_depth[++|--] in kasan_[en|dis]able_current():

 - If kasan_depth++ in kasan_enable_current() with preventing overflow/
   underflow, it means "we always want to disable KASAN, if CONFIG_KASAN
   is not under arm64 or x86_64".

 - If kasan_depth-- in kasan_enable_current() with preventing overflow/
   underflow, it means "we can enable KASAN if CONFIG_KASAN, but firstly
   we disable it, if it is not under arm64 or x86_64".

For me, I don't know which one is correct (or my whole 'guess' is
incorrect). Could any members provide your ideas?

>>> We can both issue a WARNING and prevent the actual overflow/underflow.
>>> But I don't think that there is any sane way to handle the bug (other
>>> than just fixing the unmatched disable/enable). If we ignore an
>>> excessive disable, then we can end up with ignores enabled
>>> permanently. If we ignore an excessive enable, then we can end up with
>>> ignores enabled when they should not be enabled. The main point here
>>> is to bark loudly, so that the unmatched annotations are noticed and
>>> fixed.
>>>
>>
>> How about BUG_ON()?
> As noted by Dmitry in an offline discussion, we shouldn't bail out as
> long as it's possible to proceed, otherwise the kernel may become very
> hard to debug.
> A mismatching annotation isn't a case in which we can't proceed with
> the execution.

OK, thanks.

I guess, we are agree with each other: "We can both issue a WARNING and
prevent the actual overflow/underflow.".

Thanks.
-- 
Chen Gang (陈刚)

Managing Natural Environments is the Duty of Human Beings.