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Date: Mon, 20 Jun 2022 06:41:16 +0000
From: Christophe Leroy <christophe.leroy@...roup.eu>
To: Anshuman Khandual <anshuman.khandual@....com>,
"linux-mm@...ck.org" <linux-mm@...ck.org>
CC: "hch@...radead.org" <hch@...radead.org>,
Andrew Morton <akpm@...ux-foundation.org>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
Christoph Hellwig <hch@....de>
Subject: Re: [PATCH V3 1/2] mm/mmap: Restrict generic protection_map[] array
visibility
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@...ux-foundation.org>
>>> Cc: linux-mm@...ck.org
>>> Cc: linux-kernel@...r.kernel.org
>>> Acked-by: Christoph Hellwig <hch@....de>
>>> Signed-off-by: Anshuman Khandual <anshuman.khandual@....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.
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