linux-kernel - Re: [BUGFIX][PATCH 1/3] fix vread/vwrite to be aware of memory hole

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Message-ID: <20090803091019.GC6016@cr0.nay.redhat.com>
Date:	Mon, 3 Aug 2009 17:10:19 +0800
From:	Amerigo Wang <xiyou.wangcong@...il.com>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc:	Amerigo Wang <xiyou.wangcong@...il.com>,
	Mike Smith <scgtrp@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	bugzilla-daemon@...zilla.kernel.org,
	bugme-daemon@...zilla.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [BUGFIX][PATCH 1/3] fix vread/vwrite to be aware of memory hole

On Fri, Jul 31, 2009 at 07:32:15PM +0900, KAMEZAWA Hiroyuki wrote:
>Amerigo Wang さんは書きました：
>> On Fri, Jul 31, 2009 at 04:11:28PM +0900, KAMEZAWA Hiroyuki wrote:
>>>From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
>>>
>>>vread/vwrite access vmalloc area without checking there is a page or not.
>>>
>>>In old ages, the caller of get_vm_ara() is only IOREMAP and there is no
>>>memory hole within vm_struct's [addr...addr + size - PAGE_SIZE]
>>>( -PAGE_SIZE is for a guard page.)
>>>
>>>After per-cpu-alloc patch, it uses get_vm_area() for reserve continuous
>>>virtual address but remap _later_. There tend to be a hole in valid
>>> vmalloc
>>>area in vm_struct lists.
>>>Then, skip the hole (not mapped page) is necessary.
>>>This patch updates vread/vwrite() for avoiding memory hole.
>>>
>>>Routines which access vmalloc area without knowing for which addr is used
>>>are
>>>  - /proc/kcore
>>>  - /dev/kmem
>>>
>>>kcore checks IOREMAP, /dev/kmem doesn't. After this patch, IOREMAP is
>>>checked and /dev/kmem will avoid to read/write it.
>>>Fixes to /proc/kcore will be in the next patch in series.
>>>
>>>And, this itself fixes the bug as
>>># dd if=/dev/kmem of=/dev/null bs=1024 count=1048576 skip=3145728
>>>can cause panic.
>>
>>
>> What panic? :-) Would you mind to put it here?
>>
>It directly reboot ;( and no log.
>plz try.


I tried it on an x86_64 machine, no panic, just:

 dd: reading `/dev/kmem': Bad address

Only appears on i386? :)

>>>-	return buf - buf_start;
>>>+
>>>+	if (buf == buf_start)
>>>+		return 0;
>>>+	/* zero-fill memory holes */
>>>+	if (buf != buf_start + buflen)
>>>+		memset(buf, 0, buflen - (buf - buf_start));
>>
>>
>> Is this necessary?
>>
>I wrote "filled by zero" and thinks it's sane, then does this.
>Now, /proc/kcore allocates memory by kzalloc(), this is redundant.
>/dev/kmem doesn't do that.

OK.


>ouch..
>
>Thank you for review.
>I'm sorry that new version will not appear until next week.
>I can't access x86-32 in the weekend.

No problem, feel free to do it at any time that is convinient
for you.

Thanks.

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