linux-kernel - Re: [RFC V1 02/31] mm/mmap: Clarify protection

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Date:   Wed, 9 Feb 2022 08:53:44 +0530
From:   Anshuman Khandual <anshuman.khandual@....com>
To:     Firo Yang <firogm@...il.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        hch@...radead.org, akpm@...ux-foundation.org
Subject: Re: [RFC V1 02/31] mm/mmap: Clarify protection_map[] indices



On 2/5/22 2:40 PM, Firo Yang wrote:
> The 01/24/2022 18:26, Anshuman Khandual wrote:
>> protection_map[] maps vm_flags access combinations into page protection
>> value as defined by the platform via __PXXX and __SXXX macros. The array
>> indices in protection_map[], represents vm_flags access combinations but
>> it's not very intuitive to derive. This makes it clear and explicit.
>>
>> Cc: Andrew Morton <akpm@...ux-foundation.org>
>> Cc: linux-mm@...ck.org
>> Cc: linux-kernel@...r.kernel.org
>> Signed-off-by: Anshuman Khandual <anshuman.khandual@....com>
>> ---
>>  mm/mmap.c | 18 ++++++++++++++++--
>>  1 file changed, 16 insertions(+), 2 deletions(-)
>>
>> diff --git a/mm/mmap.c b/mm/mmap.c
>> index 1e8fdb0b51ed..254d716220df 100644
>> --- a/mm/mmap.c
>> +++ b/mm/mmap.c
>> @@ -102,8 +102,22 @@ static void unmap_region(struct mm_struct *mm,
>>   *								x: (yes) yes
>>   */
>>  pgprot_t protection_map[16] __ro_after_init = {
>> -	__P000, __P001, __P010, __P011, __P100, __P101, __P110, __P111,
>> -	__S000, __S001, __S010, __S011, __S100, __S101, __S110, __S111
>> +	[VM_NONE]				= __P000,
>> +	[VM_READ]				= __P001,
>> +	[VM_WRITE]				= __P010,
>> +	[VM_READ|VM_WRITE]			= __P011,
>> +	[VM_EXEC]				= __P100,
>> +	[VM_EXEC|VM_READ]			= __P101,
>> +	[VM_EXEC|VM_WRITE]			= __P110,
>> +	[VM_EXEC|VM_READ|VM_WRITE]		= __P111,
>> +	[VM_SHARED]				= __S000,
>> +	[VM_SHARED|VM_READ]			= __S001,
>> +	[VM_SHARED|VM_WRITE]			= __S010,
>> +	[VM_SHARED|VM_READ|VM_WRITE]		= __S011,
>> +	[VM_SHARED|VM_EXEC]			= __S100,
>> +	[VM_SHARED|VM_READ|VM_EXEC]		= __S101,
>> +	[VM_SHARED|VM_WRITE|VM_EXEC]		= __S110,
>> +	[VM_SHARED|VM_READ|VM_WRITE|VM_EXEC]	= __S111
> 
> Just a little bit picky:)
> Would you mind rearranging vm_flags access commbination in the order as
> the access bits appear in __SXXX or __PXXX? For example, change the following:
> 
> 	[VM_SHARED|VM_READ|VM_WRITE|VM_EXEC]    = __S111
> to
> 	[VM_SHARED | VM_EXEC | VM_WRITE | VM_READ]	= __S111
> 
> I think it's would be more clear for looking.

So the vm_flags combination set here (and like in the platforms)
should be like the following ..

[VM_NONE]
[VM_READ]
[VM_WRITE]
[VM_WRITE | VM_READ]
[VM_EXEC]	
[VM_EXEC|VM_READ]
[VM_EXEC|VM_WRITE]
[VM_EXEC|VM_WRITE | VM_READ]
[VM_SHARED]
[VM_SHARED|VM_READ]
[VM_SHARED|VM_WRITE]
[VM_SHARED|VM_WRITE | VM_READ]
[VM_SHARED|VM_EXEC]
[VM_SHARED|VM_EXEC | VM_READ]
[VM_SHARED|VM_EXEC | VM_WRITE]
[VM_SHARED|VM_EXEC | VM_WRITE | VM_READ]

Implying the relative position for these flags among each other.
 
[VM_SHARED] [VM_EXEC]  [VM_WRITE] [VM_WRITE]

This makes sense, will change the series accordingly.

- Anshuman