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Subject: bos
To:     Thomas Gleixner <tglx@linutronix.de>
Cc:     Michael Ellerman <mpe@ellerman.id.au>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        LKML <linux-kernel@vger.kernel.org>, rguenther@suse.de,
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        x86@kernel.org, Andrew Morton <akpm@linux-foundation.org>,
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 <alpine.DEB.2.21.1904231533190.9956@nanos.tec.linutronix.de>
From:   Laurent Dufour <ldufour@linux.vnet.ibm.com>
Date:   Tue, 23 Apr 2019 15:36:56 +0200
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Le 23/04/2019 à 15:34, Thomas Gleixner a écrit :
> On Tue, 23 Apr 2019, Laurent Dufour wrote:
>> Le 20/04/2019 à 12:31, Michael Ellerman a écrit :
>>> Thomas Gleixner <tglx@linutronix.de> writes:
>>>> Aside of that the powerpc variant looks suspicious:
>>>>
>>>> static inline void arch_unmap(struct mm_struct *mm,
>>>>                                 unsigned long start, unsigned long end)
>>>> {
>>>>    	if (start <= mm->context.vdso_base && mm->context.vdso_base < end)
>>>>                   mm->context.vdso_base = 0;
>>>> }
>>>>
>>>> Shouldn't that be:
>>>>
>>>>    	if (start >= mm->context.vdso_base && mm->context.vdso_base < end)
>>>>
>>>> Hmm?
>>>
>>> Yeah looks pretty suspicious. I'll follow-up with Laurent who wrote it.
>>> Thanks for spotting it!
>>
>> I've to admit that I had to read that code carefully before answering.
>>
>> There are 2 assumptions here:
>>   1. 'start' and 'end' are page aligned (this is guaranteed by __do_munmap().
>>   2. the VDSO is 1 page (this is guaranteed by the union vdso_data_store on
>> powerpc).
>>
>> The idea is to handle a munmap() call surrounding the VDSO area:
>>        | VDSO |
>>   ^start         ^end
>>
>> This is covered by this test, as the munmap() matching the exact boundaries of
>> the VDSO is handled too.
>>
>> Am I missing something ?
> 
> Well if this is the intention, then you missed to add a comment explaining it :)
> 
> Thanks,
> 
> 	tglx

You're right, and I was thinking the same when I read that code this 
morning ;)

I'll propose a patch to a add an explicit comment.

Thanks,
Laurent.