linux-kernel - Re: [PATCH] mm: mmap() sometimes succeeds even if the region to map is invalid.

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Date:	Mon, 20 Feb 2012 16:23:56 +0900
From:	Naotaka Hamaguchi <n.hamaguchi@...fujitsu.com>
To:	Venu Byravarasu <vbyravarasu@...dia.com>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	KOSAKI Motohiro <kosaki.motohiro@...il.com>
Subject: Re: [PATCH] mm: mmap() sometimes succeeds even if the region to map
 is invalid.

Hi Venu,

>> 1. mmap() succeeds even if "offset" argument is a negative value, although
>>     it should return EINVAL in such case.
>
>> In such case, it is actually regarded as big positive value
>> because the type of "off" is "unsigned long" in the kernel.
>> For example, off=-4096 (-0x1000) is regarded as
>> off = 0xfffffffffffff000 (x86_64) and as off = 0xfffff000 (x86).
>> It results in mapping too big offset region.
>
> It is not true always.
>
> Considering your example, say if page size is 4k, then PAGE_MASK = 0xFFF
> hence (off&  ~PAGE_MASK) will be true and " -EINVAL" will be returned.

Is PAGE_MASK 0xfffffffffffff000 (x86_64) and 0xfffff000 (x86), isn't it?
Or am I missing something?

arch/x86/include/asm/page_types.h
=================================================
...
#define PAGE_SHIFT      12
#define PAGE_SIZE       (_AC(1,UL) << PAGE_SHIFT)
#define PAGE_MASK       (~(PAGE_SIZE-1))
...
=================================================

Thanks,
Naotaka Hamaguchi

(2012/02/17 18:04), Venu Byravarasu wrote:
>> The detail of these problems is as follows:
>
>> 1. mmap() succeeds even if "offset" argument is a negative value, although
>>     it should return EINVAL in such case.
>
>> In such case, it is actually regarded as big positive value
>> because the type of "off" is "unsigned long" in the kernel.
>> For example, off=-4096 (-0x1000) is regarded as
>> off = 0xfffffffffffff000 (x86_64) and as off = 0xfffff000 (x86).
>> It results in mapping too big offset region.
>
> It is not true always.
>
> Considering your example, say if page size is 4k, then PAGE_MASK = 0xFFF
> hence (off&  ~PAGE_MASK) will be true and " -EINVAL" will be returned.
> --
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