When the 52-bit virtual addressing was enabled the select like
ARCH_MMAP_RND_BITS_MAX logic was never updated to account for it.
Because of that the rnd max bits would be set to the default value of
18 when ARM64_VA_BITS=52.
Fix this by setting ARCH_MMAP_RND_BITS_MAX to the same value that would
be used if 48-bit addressing was used. That's because the 52-bit
addressing is used only if the caller provides a hint to mmap, with a
fallback to 48-bit addressing.

Fixes: b6d00d47e81a ("arm64: mm: Introduce 52-bit Kernel VAs")
Signed-off-by: Kornel Dulęba <korneld@google.com>
---
 arch/arm64/Kconfig | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index 748c34dc953c..38e0bac567f5 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -332,9 +332,9 @@ config ARCH_MMAP_RND_BITS_MAX
 	default 24 if ARM64_VA_BITS=39
 	default 27 if ARM64_VA_BITS=42
 	default 30 if ARM64_VA_BITS=47
-	default 29 if ARM64_VA_BITS=48 && ARM64_64K_PAGES
-	default 31 if ARM64_VA_BITS=48 && ARM64_16K_PAGES
-	default 33 if ARM64_VA_BITS=48
+	default 29 if (ARM64_VA_BITS=48 || ARM64_VA_BITS=52) && ARM64_64K_PAGES
+	default 31 if (ARM64_VA_BITS=48 || ARM64_VA_BITS=52) && ARM64_16K_PAGES
+	default 33 if (ARM64_VA_BITS=48 || ARM64_VA_BITS=52)
 	default 14 if ARM64_64K_PAGES
 	default 16 if ARM64_16K_PAGES
 	default 18
-- 
2.49.0.504.g3bcea36a83-goog

On 4/4/25 00:06, Kornel Dulęba wrote:
> When the 52-bit virtual addressing was enabled the select like
> ARCH_MMAP_RND_BITS_MAX logic was never updated to account for it.
> Because of that the rnd max bits would be set to the default value of
> 18 when ARM64_VA_BITS=52.
> Fix this by setting ARCH_MMAP_RND_BITS_MAX to the same value that would
> be used if 48-bit addressing was used. That's because the 52-bit
> addressing is used only if the caller provides a hint to mmap, with a
> fallback to 48-bit addressing.

Why should ARCH_MMAP_RND_BITS_MAX value be same for both 48 bits and 52
bits VA in case the user does request for 52 bit VA via mmap() hint and
the HW supports it ?

> 
> Fixes: b6d00d47e81a ("arm64: mm: Introduce 52-bit Kernel VAs")
> Signed-off-by: Kornel Dulęba <korneld@google.com>
> ---
>  arch/arm64/Kconfig | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index 748c34dc953c..38e0bac567f5 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -332,9 +332,9 @@ config ARCH_MMAP_RND_BITS_MAX
>  	default 24 if ARM64_VA_BITS=39
>  	default 27 if ARM64_VA_BITS=42
>  	default 30 if ARM64_VA_BITS=47
> -	default 29 if ARM64_VA_BITS=48 && ARM64_64K_PAGES
> -	default 31 if ARM64_VA_BITS=48 && ARM64_16K_PAGES
> -	default 33 if ARM64_VA_BITS=48
> +	default 29 if (ARM64_VA_BITS=48 || ARM64_VA_BITS=52) && ARM64_64K_PAGES
> +	default 31 if (ARM64_VA_BITS=48 || ARM64_VA_BITS=52) && ARM64_16K_PAGES
> +	default 33 if (ARM64_VA_BITS=48 || ARM64_VA_BITS=52)
>  	default 14 if ARM64_64K_PAGES
>  	default 16 if ARM64_16K_PAGES
>  	default 18

On Thu, Apr 3, 2025 at 11:46 PM Anshuman Khandual
<anshuman.khandual@arm.com> wrote:
>
>
>
> On 4/4/25 00:06, Kornel Dulęba wrote:
> > When the 52-bit virtual addressing was enabled the select like
> > ARCH_MMAP_RND_BITS_MAX logic was never updated to account for it.
> > Because of that the rnd max bits would be set to the default value of
> > 18 when ARM64_VA_BITS=52.
> > Fix this by setting ARCH_MMAP_RND_BITS_MAX to the same value that would
> > be used if 48-bit addressing was used. That's because the 52-bit
> > addressing is used only if the caller provides a hint to mmap, with a
> > fallback to 48-bit addressing.
>
> Why should ARCH_MMAP_RND_BITS_MAX value be same for both 48 bits and 52
> bits VA in case the user does request for 52 bit VA via mmap() hint and
> the HW supports it ?

Two reasons really.
1. The whole behavior is controlled through a global knob -
/proc/sys/vm/mmap_rnd_bits. ARCH_MMAP_RND_BITS_MAX is used as an upper
bound for the value that can be set to that knob.
So we have a single setting for all processes. Some might want 52 bit
addressing, others will stick with 48.
2. Quoting the documentation for this knob:

"""
mmap_rnd_bits
This value can be used to select the number of bits to use to
determine the random offset to the base address of vma regions
resulting from mmap allocations on architectures which support tuning
address space randomization. This value will be bounded by the
architecture’s minimum and maximum supported values.
"""

I suppose that it's legal for some calls to mmap from the same process
to request a 52 bit VA, while other calls will want only 48 bits.
Because of that the random offset can't be larger than what would work
for the 48 bit case.

>
> >
> > Fixes: b6d00d47e81a ("arm64: mm: Introduce 52-bit Kernel VAs")
> > Signed-off-by: Kornel Dulęba <korneld@google.com>
> > ---
> >  arch/arm64/Kconfig | 6 +++---
> >  1 file changed, 3 insertions(+), 3 deletions(-)
> >
> > diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> > index 748c34dc953c..38e0bac567f5 100644
> > --- a/arch/arm64/Kconfig
> > +++ b/arch/arm64/Kconfig
> > @@ -332,9 +332,9 @@ config ARCH_MMAP_RND_BITS_MAX
> >       default 24 if ARM64_VA_BITS=39
> >       default 27 if ARM64_VA_BITS=42
> >       default 30 if ARM64_VA_BITS=47
> > -     default 29 if ARM64_VA_BITS=48 && ARM64_64K_PAGES
> > -     default 31 if ARM64_VA_BITS=48 && ARM64_16K_PAGES
> > -     default 33 if ARM64_VA_BITS=48
> > +     default 29 if (ARM64_VA_BITS=48 || ARM64_VA_BITS=52) && ARM64_64K_PAGES
> > +     default 31 if (ARM64_VA_BITS=48 || ARM64_VA_BITS=52) && ARM64_16K_PAGES
> > +     default 33 if (ARM64_VA_BITS=48 || ARM64_VA_BITS=52)
> >       default 14 if ARM64_64K_PAGES
> >       default 16 if ARM64_16K_PAGES
> >       default 18

On 4/4/25 22:16, Kornel Dulęba wrote:
> On Thu, Apr 3, 2025 at 11:46 PM Anshuman Khandual
> <anshuman.khandual@arm.com> wrote:
>>
>>
>>
>> On 4/4/25 00:06, Kornel Dulęba wrote:
>>> When the 52-bit virtual addressing was enabled the select like
>>> ARCH_MMAP_RND_BITS_MAX logic was never updated to account for it.
>>> Because of that the rnd max bits would be set to the default value of
>>> 18 when ARM64_VA_BITS=52.
>>> Fix this by setting ARCH_MMAP_RND_BITS_MAX to the same value that would
>>> be used if 48-bit addressing was used. That's because the 52-bit
>>> addressing is used only if the caller provides a hint to mmap, with a
>>> fallback to 48-bit addressing.
>>
>> Why should ARCH_MMAP_RND_BITS_MAX value be same for both 48 bits and 52
>> bits VA in case the user does request for 52 bit VA via mmap() hint and
>> the HW supports it ?
> 
> Two reasons really.
> 1. The whole behavior is controlled through a global knob -
> /proc/sys/vm/mmap_rnd_bits. ARCH_MMAP_RND_BITS_MAX is used as an upper
> bound for the value that can be set to that knob.
> So we have a single setting for all processes. Some might want 52 bit
> addressing, others will stick with 48.
> 2. Quoting the documentation for this knob:
> 
> """
> mmap_rnd_bits
> This value can be used to select the number of bits to use to
> determine the random offset to the base address of vma regions
> resulting from mmap allocations on architectures which support tuning
> address space randomization. This value will be bounded by the
> architecture’s minimum and maximum supported values.
> """
> 
> I suppose that it's legal for some calls to mmap from the same process
> to request a 52 bit VA, while other calls will want only 48 bits.
> Because of that the random offset can't be larger than what would work
> for the 48 bit case.

Agreed but should not this rationale also be added in the commit
message as well ?

> 
>>
>>>
>>> Fixes: b6d00d47e81a ("arm64: mm: Introduce 52-bit Kernel VAs")

Correct commit to be attributed for this fix.

>>> Signed-off-by: Kornel Dulęba <korneld@google.com>
>>> ---
>>>  arch/arm64/Kconfig | 6 +++---
>>>  1 file changed, 3 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
>>> index 748c34dc953c..38e0bac567f5 100644
>>> --- a/arch/arm64/Kconfig
>>> +++ b/arch/arm64/Kconfig
>>> @@ -332,9 +332,9 @@ config ARCH_MMAP_RND_BITS_MAX
>>>       default 24 if ARM64_VA_BITS=39
>>>       default 27 if ARM64_VA_BITS=42
>>>       default 30 if ARM64_VA_BITS=47
>>> -     default 29 if ARM64_VA_BITS=48 && ARM64_64K_PAGES
>>> -     default 31 if ARM64_VA_BITS=48 && ARM64_16K_PAGES
>>> -     default 33 if ARM64_VA_BITS=48
>>> +     default 29 if (ARM64_VA_BITS=48 || ARM64_VA_BITS=52) && ARM64_64K_PAGES
>>> +     default 31 if (ARM64_VA_BITS=48 || ARM64_VA_BITS=52) && ARM64_16K_PAGES
>>> +     default 33 if (ARM64_VA_BITS=48 || ARM64_VA_BITS=52)
>>>       default 14 if ARM64_64K_PAGES
>>>       default 16 if ARM64_16K_PAGES
>>>       default 18

Otherwise LGTM.

Reviewed-by: Anshuman Khandual <anshuman.khandual@arm.com>

On Thu, Apr 17, 2025 at 7:15 AM Anshuman Khandual
<anshuman.khandual@arm.com> wrote:
>
>
>
> On 4/4/25 22:16, Kornel Dulęba wrote:
> > On Thu, Apr 3, 2025 at 11:46 PM Anshuman Khandual
> > <anshuman.khandual@arm.com> wrote:
> >>
> >>
> >>
> >> On 4/4/25 00:06, Kornel Dulęba wrote:
> >>> When the 52-bit virtual addressing was enabled the select like
> >>> ARCH_MMAP_RND_BITS_MAX logic was never updated to account for it.
> >>> Because of that the rnd max bits would be set to the default value of
> >>> 18 when ARM64_VA_BITS=52.
> >>> Fix this by setting ARCH_MMAP_RND_BITS_MAX to the same value that would
> >>> be used if 48-bit addressing was used. That's because the 52-bit
> >>> addressing is used only if the caller provides a hint to mmap, with a
> >>> fallback to 48-bit addressing.
> >>
> >> Why should ARCH_MMAP_RND_BITS_MAX value be same for both 48 bits and 52
> >> bits VA in case the user does request for 52 bit VA via mmap() hint and
> >> the HW supports it ?
> >
> > Two reasons really.
> > 1. The whole behavior is controlled through a global knob -
> > /proc/sys/vm/mmap_rnd_bits. ARCH_MMAP_RND_BITS_MAX is used as an upper
> > bound for the value that can be set to that knob.
> > So we have a single setting for all processes. Some might want 52 bit
> > addressing, others will stick with 48.
> > 2. Quoting the documentation for this knob:
> >
> > """
> > mmap_rnd_bits
> > This value can be used to select the number of bits to use to
> > determine the random offset to the base address of vma regions
> > resulting from mmap allocations on architectures which support tuning
> > address space randomization. This value will be bounded by the
> > architecture’s minimum and maximum supported values.
> > """
> >
> > I suppose that it's legal for some calls to mmap from the same process
> > to request a 52 bit VA, while other calls will want only 48 bits.
> > Because of that the random offset can't be larger than what would work
> > for the 48 bit case.
>
> Agreed but should not this rationale also be added in the commit
> message as well ?

Sure, I will update this in v2.
>
>
> >
> >>
> >>>
> >>> Fixes: b6d00d47e81a ("arm64: mm: Introduce 52-bit Kernel VAs")
>
> Correct commit to be attributed for this fix.

What commit would you like me to point at? I selected b6d00d47e81a,
because it introduced "ARM64_VA_BITS=52".
Looking at git blame, ARCH_MMAP_RND_BITS_MAX logic was introduced back
in 2016 in 8f0d3aa9de57 ("arm64: mm: support ARCH_MMAP_RND_BITS"),
which is before "ARM64_VA_BITS=52" was a thing.
I suppose that there's also 3cb7e662a930 ("arm64: Kconfig: Fix
indentation and add comments"), but that's just some whitespace
adjustments.

>
>
> >>> Signed-off-by: Kornel Dulęba <korneld@google.com>
> >>> ---
> >>>  arch/arm64/Kconfig | 6 +++---
> >>>  1 file changed, 3 insertions(+), 3 deletions(-)
> >>>
> >>> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> >>> index 748c34dc953c..38e0bac567f5 100644
> >>> --- a/arch/arm64/Kconfig
> >>> +++ b/arch/arm64/Kconfig
> >>> @@ -332,9 +332,9 @@ config ARCH_MMAP_RND_BITS_MAX
> >>>       default 24 if ARM64_VA_BITS=39
> >>>       default 27 if ARM64_VA_BITS=42
> >>>       default 30 if ARM64_VA_BITS=47
> >>> -     default 29 if ARM64_VA_BITS=48 && ARM64_64K_PAGES
> >>> -     default 31 if ARM64_VA_BITS=48 && ARM64_16K_PAGES
> >>> -     default 33 if ARM64_VA_BITS=48
> >>> +     default 29 if (ARM64_VA_BITS=48 || ARM64_VA_BITS=52) && ARM64_64K_PAGES
> >>> +     default 31 if (ARM64_VA_BITS=48 || ARM64_VA_BITS=52) && ARM64_16K_PAGES
> >>> +     default 33 if (ARM64_VA_BITS=48 || ARM64_VA_BITS=52)
> >>>       default 14 if ARM64_64K_PAGES
> >>>       default 16 if ARM64_16K_PAGES
> >>>       default 18
>
> Otherwise LGTM.
>
> Reviewed-by: Anshuman Khandual <anshuman.khandual@arm.com>

On 4/17/25 13:38, Kornel Dulęba wrote:
> On Thu, Apr 17, 2025 at 7:15 AM Anshuman Khandual
> <anshuman.khandual@arm.com> wrote:
>>
>>
>>
>> On 4/4/25 22:16, Kornel Dulęba wrote:
>>> On Thu, Apr 3, 2025 at 11:46 PM Anshuman Khandual
>>> <anshuman.khandual@arm.com> wrote:
>>>>
>>>>
>>>>
>>>> On 4/4/25 00:06, Kornel Dulęba wrote:
>>>>> When the 52-bit virtual addressing was enabled the select like
>>>>> ARCH_MMAP_RND_BITS_MAX logic was never updated to account for it.
>>>>> Because of that the rnd max bits would be set to the default value of
>>>>> 18 when ARM64_VA_BITS=52.
>>>>> Fix this by setting ARCH_MMAP_RND_BITS_MAX to the same value that would
>>>>> be used if 48-bit addressing was used. That's because the 52-bit
>>>>> addressing is used only if the caller provides a hint to mmap, with a
>>>>> fallback to 48-bit addressing.
>>>>
>>>> Why should ARCH_MMAP_RND_BITS_MAX value be same for both 48 bits and 52
>>>> bits VA in case the user does request for 52 bit VA via mmap() hint and
>>>> the HW supports it ?
>>>
>>> Two reasons really.
>>> 1. The whole behavior is controlled through a global knob -
>>> /proc/sys/vm/mmap_rnd_bits. ARCH_MMAP_RND_BITS_MAX is used as an upper
>>> bound for the value that can be set to that knob.
>>> So we have a single setting for all processes. Some might want 52 bit
>>> addressing, others will stick with 48.
>>> 2. Quoting the documentation for this knob:
>>>
>>> """
>>> mmap_rnd_bits
>>> This value can be used to select the number of bits to use to
>>> determine the random offset to the base address of vma regions
>>> resulting from mmap allocations on architectures which support tuning
>>> address space randomization. This value will be bounded by the
>>> architecture’s minimum and maximum supported values.
>>> """
>>>
>>> I suppose that it's legal for some calls to mmap from the same process
>>> to request a 52 bit VA, while other calls will want only 48 bits.
>>> Because of that the random offset can't be larger than what would work
>>> for the 48 bit case.
>>
>> Agreed but should not this rationale also be added in the commit
>> message as well ?
> 
> Sure, I will update this in v2.
>>
>>
>>>
>>>>
>>>>>
>>>>> Fixes: b6d00d47e81a ("arm64: mm: Introduce 52-bit Kernel VAs")
>>
>> Correct commit to be attributed for this fix.
> 
> What commit would you like me to point at? I selected b6d00d47e81a,
> because it introduced "ARM64_VA_BITS=52".
> Looking at git blame, ARCH_MMAP_RND_BITS_MAX logic was introduced back
> in 2016 in 8f0d3aa9de57 ("arm64: mm: support ARCH_MMAP_RND_BITS"),
> which is before "ARM64_VA_BITS=52" was a thing.
> I suppose that there's also 3cb7e662a930 ("arm64: Kconfig: Fix
> indentation and add comments"), but that's just some whitespace
> adjustments.

It was just a statement confirming that the commit ID chosen here in
this patch is the right one indeed. Apologies for the confusion.

> 
>>
>>
>>>>> Signed-off-by: Kornel Dulęba <korneld@google.com>
>>>>> ---
>>>>>  arch/arm64/Kconfig | 6 +++---
>>>>>  1 file changed, 3 insertions(+), 3 deletions(-)
>>>>>
>>>>> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
>>>>> index 748c34dc953c..38e0bac567f5 100644
>>>>> --- a/arch/arm64/Kconfig
>>>>> +++ b/arch/arm64/Kconfig
>>>>> @@ -332,9 +332,9 @@ config ARCH_MMAP_RND_BITS_MAX
>>>>>       default 24 if ARM64_VA_BITS=39
>>>>>       default 27 if ARM64_VA_BITS=42
>>>>>       default 30 if ARM64_VA_BITS=47
>>>>> -     default 29 if ARM64_VA_BITS=48 && ARM64_64K_PAGES
>>>>> -     default 31 if ARM64_VA_BITS=48 && ARM64_16K_PAGES
>>>>> -     default 33 if ARM64_VA_BITS=48
>>>>> +     default 29 if (ARM64_VA_BITS=48 || ARM64_VA_BITS=52) && ARM64_64K_PAGES
>>>>> +     default 31 if (ARM64_VA_BITS=48 || ARM64_VA_BITS=52) && ARM64_16K_PAGES
>>>>> +     default 33 if (ARM64_VA_BITS=48 || ARM64_VA_BITS=52)
>>>>>       default 14 if ARM64_64K_PAGES
>>>>>       default 16 if ARM64_16K_PAGES
>>>>>       default 18
>>
>> Otherwise LGTM.
>>
>> Reviewed-by: Anshuman Khandual <anshuman.khandual@arm.com>

On Fri, Apr 4, 2025 at 6:46 PM Kornel Dulęba <korneld@google.com> wrote:
>
> On Thu, Apr 3, 2025 at 11:46 PM Anshuman Khandual
> <anshuman.khandual@arm.com> wrote:
> >
> >
> >
> > On 4/4/25 00:06, Kornel Dulęba wrote:
> > > When the 52-bit virtual addressing was enabled the select like
> > > ARCH_MMAP_RND_BITS_MAX logic was never updated to account for it.
> > > Because of that the rnd max bits would be set to the default value of
> > > 18 when ARM64_VA_BITS=52.
> > > Fix this by setting ARCH_MMAP_RND_BITS_MAX to the same value that would
> > > be used if 48-bit addressing was used. That's because the 52-bit
> > > addressing is used only if the caller provides a hint to mmap, with a
> > > fallback to 48-bit addressing.
> >
> > Why should ARCH_MMAP_RND_BITS_MAX value be same for both 48 bits and 52
> > bits VA in case the user does request for 52 bit VA via mmap() hint and
> > the HW supports it ?
>
> Two reasons really.
> 1. The whole behavior is controlled through a global knob -
> /proc/sys/vm/mmap_rnd_bits. ARCH_MMAP_RND_BITS_MAX is used as an upper
> bound for the value that can be set to that knob.
> So we have a single setting for all processes. Some might want 52 bit
> addressing, others will stick with 48.
> 2. Quoting the documentation for this knob:
>
> """
> mmap_rnd_bits
> This value can be used to select the number of bits to use to
> determine the random offset to the base address of vma regions
> resulting from mmap allocations on architectures which support tuning
> address space randomization. This value will be bounded by the
> architecture’s minimum and maximum supported values.
> """
>
> I suppose that it's legal for some calls to mmap from the same process
> to request a 52 bit VA, while other calls will want only 48 bits.
> Because of that the random offset can't be larger than what would work
> for the 48 bit case.

Hi,

Do you have any further comments?

Regards
Kornel
> >
> > >
> > > Fixes: b6d00d47e81a ("arm64: mm: Introduce 52-bit Kernel VAs")
> > > Signed-off-by: Kornel Dulęba <korneld@google.com>
> > > ---
> > >  arch/arm64/Kconfig | 6 +++---
> > >  1 file changed, 3 insertions(+), 3 deletions(-)
> > >
> > > diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> > > index 748c34dc953c..38e0bac567f5 100644
> > > --- a/arch/arm64/Kconfig
> > > +++ b/arch/arm64/Kconfig
> > > @@ -332,9 +332,9 @@ config ARCH_MMAP_RND_BITS_MAX
> > >       default 24 if ARM64_VA_BITS=39
> > >       default 27 if ARM64_VA_BITS=42
> > >       default 30 if ARM64_VA_BITS=47
> > > -     default 29 if ARM64_VA_BITS=48 && ARM64_64K_PAGES
> > > -     default 31 if ARM64_VA_BITS=48 && ARM64_16K_PAGES
> > > -     default 33 if ARM64_VA_BITS=48
> > > +     default 29 if (ARM64_VA_BITS=48 || ARM64_VA_BITS=52) && ARM64_64K_PAGES
> > > +     default 31 if (ARM64_VA_BITS=48 || ARM64_VA_BITS=52) && ARM64_16K_PAGES
> > > +     default 33 if (ARM64_VA_BITS=48 || ARM64_VA_BITS=52)
> > >       default 14 if ARM64_64K_PAGES
> > >       default 16 if ARM64_16K_PAGES
> > >       default 18