Re: [PATCH V3 1/2] mm/mmap: Restrict generic protection_map[] array visibility

[Anshuman Khandual <anshuman.khandual@arm.com>]

On 6/20/22 12:11, Christophe Leroy wrote:
> 
> 
> Le 20/06/2022 à 07:16, Anshuman Khandual a écrit :
>>
>>
>> On 6/16/22 11:05, Christophe Leroy wrote:
>>>
>>> Le 16/06/2022 à 06:09, Anshuman Khandual a écrit :
>>>> Restrict generic protection_map[] array visibility only for platforms which
>>>> do not enable ARCH_HAS_VM_GET_PAGE_PROT. For other platforms that do define
>>>> their own vm_get_page_prot() enabling ARCH_HAS_VM_GET_PAGE_PROT, could have
>>>> their private static protection_map[] still implementing an array look up.
>>>> These private protection_map[] array could do without __PXXX/__SXXX macros,
>>>> making them redundant and dropping them off as well.
>>>>
>>>> But platforms which do not define their custom vm_get_page_prot() enabling
>>>> ARCH_HAS_VM_GET_PAGE_PROT, will still have to provide __PXXX/__SXXX macros.
>>>>
>>>> Cc: Andrew Morton <akpm@linux-foundation.org>
>>>> Cc: linux-mm@kvack.org
>>>> Cc: linux-kernel@vger.kernel.org
>>>> Acked-by: Christoph Hellwig <hch@lst.de>
>>>> Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
>>>> ---
>>>>    arch/arm64/include/asm/pgtable-prot.h | 18 ------------------
>>>>    arch/arm64/mm/mmap.c                  | 21 +++++++++++++++++++++
>>>>    arch/powerpc/include/asm/pgtable.h    |  2 ++
>>>>    arch/powerpc/mm/book3s64/pgtable.c    | 20 ++++++++++++++++++++
>>>>    arch/sparc/include/asm/pgtable_64.h   | 19 -------------------
>>>>    arch/sparc/mm/init_64.c               |  3 +++
>>>>    arch/x86/include/asm/pgtable_types.h  | 19 -------------------
>>>>    arch/x86/mm/pgprot.c                  | 19 +++++++++++++++++++
>>>>    include/linux/mm.h                    |  2 ++
>>>>    mm/mmap.c                             |  2 +-
>>>>    10 files changed, 68 insertions(+), 57 deletions(-)
>>>>
>>>> diff --git a/arch/powerpc/include/asm/pgtable.h b/arch/powerpc/include/asm/pgtable.h
>>>> index d564d0ecd4cd..8ed2a80c896e 100644
>>>> --- a/arch/powerpc/include/asm/pgtable.h
>>>> +++ b/arch/powerpc/include/asm/pgtable.h
>>>> @@ -21,6 +21,7 @@ struct mm_struct;
>>>>    #endif /* !CONFIG_PPC_BOOK3S */
>>>>    
>>>>    /* Note due to the way vm flags are laid out, the bits are XWR */
>>>> +#ifndef CONFIG_ARCH_HAS_VM_GET_PAGE_PROT
>>> This ifdef if not necessary for now, it doesn't matter if __P000 etc
>>> still exist thought not used.
>>>
>>>>    #define __P000	PAGE_NONE
>>>>    #define __P001	PAGE_READONLY
>>>>    #define __P010	PAGE_COPY
>>>> @@ -38,6 +39,7 @@ struct mm_struct;
>>>>    #define __S101	PAGE_READONLY_X
>>>>    #define __S110	PAGE_SHARED_X
>>>>    #define __S111	PAGE_SHARED_X
>>>> +#endif
>>>>    
>>>>    #ifndef __ASSEMBLY__
>>>>    
>>>> diff --git a/arch/powerpc/mm/book3s64/pgtable.c b/arch/powerpc/mm/book3s64/pgtable.c
>>>> index 7b9966402b25..d3b019b95c1d 100644
>>>> --- a/arch/powerpc/mm/book3s64/pgtable.c
>>>> +++ b/arch/powerpc/mm/book3s64/pgtable.c
>>>> @@ -551,6 +551,26 @@ unsigned long memremap_compat_align(void)
>>>>    EXPORT_SYMBOL_GPL(memremap_compat_align);
>>>>    #endif
>>>>    
>>>> +/* Note due to the way vm flags are laid out, the bits are XWR */
>>>> +static const pgprot_t protection_map[16] = {
>>>> +	[VM_NONE]					= PAGE_NONE,
>>>> +	[VM_READ]					= PAGE_READONLY,
>>>> +	[VM_WRITE]					= PAGE_COPY,
>>>> +	[VM_WRITE | VM_READ]				= PAGE_COPY,
>>>> +	[VM_EXEC]					= PAGE_READONLY_X,
>>>> +	[VM_EXEC | VM_READ]				= PAGE_READONLY_X,
>>>> +	[VM_EXEC | VM_WRITE]				= PAGE_COPY_X,
>>>> +	[VM_EXEC | VM_WRITE | VM_READ]			= PAGE_COPY_X,
>>>> +	[VM_SHARED]					= PAGE_NONE,
>>>> +	[VM_SHARED | VM_READ]				= PAGE_READONLY,
>>>> +	[VM_SHARED | VM_WRITE]				= PAGE_SHARED,
>>>> +	[VM_SHARED | VM_WRITE | VM_READ]		= PAGE_SHARED,
>>>> +	[VM_SHARED | VM_EXEC]				= PAGE_READONLY_X,
>>>> +	[VM_SHARED | VM_EXEC | VM_READ]			= PAGE_READONLY_X,
>>>> +	[VM_SHARED | VM_EXEC | VM_WRITE]		= PAGE_SHARED_X,
>>>> +	[VM_SHARED | VM_EXEC | VM_WRITE | VM_READ]	= PAGE_SHARED_X
>>>> +};
>>>> +
>>> There is not much point is first additing that here and then move it
>>> elsewhere in the second patch.
>>>
>>> I think with my suggestion to use #ifdef __P000 as a guard, the powerpc
>>> changes could go in a single patch.
>>>
>>>>    pgprot_t vm_get_page_prot(unsigned long vm_flags)
>>>>    {
>>>>    	unsigned long prot = pgprot_val(protection_map[vm_flags &
>>>> diff --git a/mm/mmap.c b/mm/mmap.c
>>>> index 61e6135c54ef..e66920414945 100644
>>>> --- a/mm/mmap.c
>>>> +++ b/mm/mmap.c
>>>> @@ -101,6 +101,7 @@ static void unmap_region(struct mm_struct *mm,
>>>>     *								w: (no) no
>>>>     *								x: (yes) yes
>>>>     */
>>>> +#ifndef CONFIG_ARCH_HAS_VM_GET_PAGE_PROT
>>> You should use #ifdef __P000 instead, that way you could migrate
>>> architectures one by one.
>>
>> If vm_get_page_prot() gets moved into all platforms, wondering what would be
>> the preferred method to organize this patch series ?
>>
>> 1. Move protection_map[] inside platforms with ARCH_HAS_VM_PAGE_PROT (current patch 1)
>> 2. Convert remaining platforms to use ARCH_HAS_VM_PAGE_PROT one after the other
>> 3. Drop ARCH_HAS_VM_PAGE_PROT completely
>>
>> Using "#ifdef __P000" for wrapping protection_map[] will leave two different #ifdefs
>> in flight i.e __P000, ARCH_HAS_VM_PAGE_PROT in the generic mmap code, until both gets
>> dropped eventually. But using "#ifdef __P000" does enable splitting the first patch
>> into multiple changes for each individual platforms.
> 
>  From previous discussions and based on Christoph's suggestion, I guess 
> we now aim at getting vm_get_page_prot() moved into all platforms 
> together with protection_map[]. Therefore the use of #ifdef __P000 could 
> be very temporary at the begining of the series:
> 1. Guard generic protection_map[] with #ifdef ___P000
> 2. Move protection_map[] into architecture and drop __Pxxx/__Sxxx  for arm64
> 3. Same for sparc
> 4. Same for x86
> 5. Convert entire powerpc to ARCH_HAS_VM_PAGE_PROT and move 
> protection_map[] into architecture and drop __Pxxx/__Sxxx
> 6. Replace #ifdef __P000 by #ifdef CONFIG_ARCH_HAS_VM_PAGE_PROT
> 7. Convert all remaining platforms to CONFIG_ARCH_HAS_VM_PAGE_PROT one 
> by one (but keep a protection_map[] table, don't use switch/case)
> 8. Drop ARCH_HAS_VM_PAGE_PROT completely.
> 
> Eventually you can squash step 6 into step 8.

Keeping individual platform changes in a separate patch will make
the series cleaner, and also much easier to review. But the flow
explained above sounds good to me. I will work on these changes.