LDM/STM alignment fixups on arm64

LDM/STM alignment fixups on arm64

[Srinivas Ramana]

Hi,

While understanding how the alignment are handled on arm and arm64, we 
came across the fixups for LDM/LDRD/STM on arm where as these fixups are 
not present on arm64.

There may be some specific reason why these fixups are not ported to 
arm64. Can you please help us understand this?

With this difference in how kernel handles 32-bit apps on arm and arm64, 
there can be apps which are working without abort on arm, but fail on 
arm64 (SIGBUS). We have tried to get some history on web, but not 
successful.

If this is indeed missing on arm64, do you see any issue if its ported 
(does it fail any guidance)?

Thanks,
-- Srinivas R


-- 
Qualcomm India Private Limited, on behalf of Qualcomm Innovation Center, 
Inc.,
is a member of Code Aurora Forum, a Linux Foundation Collaborative Project.

LDM/STM alignment fixups on arm64

[Russell King - ARM Linux]

On Wed, Apr 19, 2017 at 12:03:58PM +0530, Srinivas Ramana wrote:
> Hi,
> 
> While understanding how the alignment are handled on arm and arm64, we came
> across the fixups for LDM/LDRD/STM on arm where as these fixups are not
> present on arm64.
> 
> There may be some specific reason why these fixups are not ported to arm64.
> Can you please help us understand this?
> 
> With this difference in how kernel handles 32-bit apps on arm and arm64,
> there can be apps which are working without abort on arm, but fail on arm64
> (SIGBUS). We have tried to get some history on web, but not successful.
> 
> If this is indeed missing on arm64, do you see any issue if its ported (does
> it fail any guidance)?

Do you have an application that fails because of this?  Your email makes
it sound very theoretical.

-- 
RMK's Patch system: http://www.armlinux.org.uk/developer/patches/
FTTC broadband for 0.8mile line: currently at 9.6Mbps down 400kbps up
according to speedtest.net.

LDM/STM alignment fixups on arm64

[Srinivas Ramana]

On 04/19/2017 03:28 PM, Russell King - ARM Linux wrote:
> On Wed, Apr 19, 2017 at 12:03:58PM +0530, Srinivas Ramana wrote:
>> Hi,
>>
>> While understanding how the alignment are handled on arm and arm64, we came
>> across the fixups for LDM/LDRD/STM on arm where as these fixups are not
>> present on arm64.
>>
>> There may be some specific reason why these fixups are not ported to arm64.
>> Can you please help us understand this?
>>
>> With this difference in how kernel handles 32-bit apps on arm and arm64,
>> there can be apps which are working without abort on arm, but fail on arm64
>> (SIGBUS). We have tried to get some history on web, but not successful.
>>
>> If this is indeed missing on arm64, do you see any issue if its ported (does
>> it fail any guidance)?
>
> Do you have an application that fails because of this?  Your email makes
> it sound very theoretical.
>

I don't have any application with me right now. But i just tried passing 
an intentional misaligned address in a test program. When i say 
intentional, please note this code is buggy and should be fixed.

So, my question is when arm has such fixups to handle such cases and do 
gracefully, is there any reason why those fixups are not ported to 
arm64? Again, I do agree that apps has to fix these instances, but we do 
have fixups in arch/arm.

I do see that the compiler can detect (if its not intentionally induced) 
such cases and avoiding to generate LDM/STM and generates multiple 
LDR/STR. So, I just want to know if it is safe to assume that the 
compiler would take care of all such misaligned addresses passed to LDM/STM?

------------------------>8---------------------------------------
struct locat {
         int a;
         int b;
         int c;
         int d;
};

int test_func()
{
         struct locat *int_pool1;
         struct locat int_pool2;
         struct locat *int_pool3;
         char *ptr;

         int_pool1 = malloc(sizeof(struct locat) + 16);
         ptr = (char *)int_pool1;
         int_pool3 = (struct locat *)(ptr+1);

         printf("pool1 addr: 0x%08x pool3 addr: 0x%08x \n", &int_pool1, 
int_pool3);
         int_pool2 = *int_pool3;
}

------------------------8<---------------------------------------


Thanks,
-- Srinivas R


-- 
Qualcomm India Private Limited, on behalf of Qualcomm Innovation Center, 
Inc.,
is a member of Code Aurora Forum, a Linux Foundation Collaborative Project.

LDM/STM alignment fixups on arm64

[Russell King - ARM Linux]

On Wed, Apr 19, 2017 at 06:20:10PM +0530, Srinivas Ramana wrote:
> I don't have any application with me right now. But i just tried passing an
> intentional misaligned address in a test program. When i say intentional,
> please note this code is buggy and should be fixed.

Until there's reports of some application that regresses, there is
nothing to fix.

-- 
RMK's Patch system: http://www.armlinux.org.uk/developer/patches/
FTTC broadband for 0.8mile line: currently at 9.6Mbps down 400kbps up
according to speedtest.net.

LDM/STM alignment fixups on arm64

[Catalin Marinas]

On Wed, Apr 19, 2017 at 06:20:10PM +0530, Srinivas Ramana wrote:
> On 04/19/2017 03:28 PM, Russell King - ARM Linux wrote:
> >On Wed, Apr 19, 2017 at 12:03:58PM +0530, Srinivas Ramana wrote:
> >>While understanding how the alignment are handled on arm and arm64, we came
> >>across the fixups for LDM/LDRD/STM on arm where as these fixups are not
> >>present on arm64.
> >>
> >>There may be some specific reason why these fixups are not ported to arm64.
> >>Can you please help us understand this?
> >>
> >>With this difference in how kernel handles 32-bit apps on arm and arm64,
> >>there can be apps which are working without abort on arm, but fail on arm64
> >>(SIGBUS). We have tried to get some history on web, but not successful.
> >>
> >>If this is indeed missing on arm64, do you see any issue if its ported (does
> >>it fail any guidance)?
> >
> >Do you have an application that fails because of this?  Your email makes
> >it sound very theoretical.
> 
> I don't have any application with me right now. But i just tried passing an
> intentional misaligned address in a test program. When i say intentional,
> please note this code is buggy and should be fixed.
> 
> So, my question is when arm has such fixups to handle such cases and do
> gracefully, is there any reason why those fixups are not ported to arm64?

As Russell said, until we find some application in the wild I wouldn't
rush to provide such emulation. Such code is either broken or relying on
undefined C behaviour so they should rather be fixed. As with the
deprecated/obsolete ARMv7 instructions (SWP, CP15 barriers), we
initially decided not to implement the emulation in the arm64 kernel,
though we eventually accepted it. But in those cases the instructions
were once real and used correctly. The unaligned LDM/STM or LDRD/STRD
have never been supported by the ARM architecture. They were added to
cope with some unaligned accesses in the Linux kernel network stack (in
hindsight, they should have not been provided to user but maybe there
were good reasons, I don't know the full history here).

> Again, I do agree that apps has to fix these instances, but we do have
> fixups in arch/arm.

I also think the default on arch/arm should be SIGBUS for these
instructions on ARMv7 but this was discussed before on the list.

> I do see that the compiler can detect (if its not intentionally induced)
> such cases and avoiding to generate LDM/STM and generates multiple LDR/STR.
> So, I just want to know if it is safe to assume that the compiler would take
> care of all such misaligned addresses passed to LDM/STM?

The compiler won't detect if you break its alignment assumptions (i.e.
in your example pointers to struct locat are 64-bit aligned as per the
EABI/PCS). If you want the compiler to assume unaligned struct pointers,
you'd have to mark the structure with the packed attribute (with the
additional padding if necessary, not in your example though).

-- 
Catalin

LDM/STM alignment fixups on arm64

[Srinivas Ramana]

On 04/19/2017 09:28 PM, Catalin Marinas wrote:
> On Wed, Apr 19, 2017 at 06:20:10PM +0530, Srinivas Ramana wrote:
>> On 04/19/2017 03:28 PM, Russell King - ARM Linux wrote:
>>> On Wed, Apr 19, 2017 at 12:03:58PM +0530, Srinivas Ramana wrote:
>>>> While understanding how the alignment are handled on arm and arm64, we came
>>>> across the fixups for LDM/LDRD/STM on arm where as these fixups are not
>>>> present on arm64.
>>>>
>>>> There may be some specific reason why these fixups are not ported to arm64.
>>>> Can you please help us understand this?
>>>>
>>>> With this difference in how kernel handles 32-bit apps on arm and arm64,
>>>> there can be apps which are working without abort on arm, but fail on arm64
>>>> (SIGBUS). We have tried to get some history on web, but not successful.
>>>>
>>>> If this is indeed missing on arm64, do you see any issue if its ported (does
>>>> it fail any guidance)?
>>>
>>> Do you have an application that fails because of this?  Your email makes
>>> it sound very theoretical.
>>
>> I don't have any application with me right now. But i just tried passing an
>> intentional misaligned address in a test program. When i say intentional,
>> please note this code is buggy and should be fixed.
>>
>> So, my question is when arm has such fixups to handle such cases and do
>> gracefully, is there any reason why those fixups are not ported to arm64?
>
> As Russell said, until we find some application in the wild I wouldn't
> rush to provide such emulation. Such code is either broken or relying on
> undefined C behaviour so they should rather be fixed. As with the
> deprecated/obsolete ARMv7 instructions (SWP, CP15 barriers), we
> initially decided not to implement the emulation in the arm64 kernel,
> though we eventually accepted it. But in those cases the instructions
> were once real and used correctly. The unaligned LDM/STM or LDRD/STRD
> have never been supported by the ARM architecture. They were added to
> cope with some unaligned accesses in the Linux kernel network stack (in
> hindsight, they should have not been provided to user but maybe there
> were good reasons, I don't know the full history here).
>
Sure. Thanks. I think i got some context and guidance now.

>> Again, I do agree that apps has to fix these instances, but we do have
>> fixups in arch/arm.
>
> I also think the default on arch/arm should be SIGBUS for these
> instructions on ARMv7 but this was discussed before on the list.
>
>> I do see that the compiler can detect (if its not intentionally induced)
>> such cases and avoiding to generate LDM/STM and generates multiple LDR/STR.
>> So, I just want to know if it is safe to assume that the compiler would take
>> care of all such misaligned addresses passed to LDM/STM?
>
> The compiler won't detect if you break its alignment assumptions (i.e.
> in your example pointers to struct locat are 64-bit aligned as per the
> EABI/PCS). If you want the compiler to assume unaligned struct pointers,
> you'd have to mark the structure with the packed attribute (with the
> additional padding if necessary, not in your example though).
>
You are right. with __packed__ attribute compiler detects the 
unalignment (added a char to the test structure in my program), I could 
see that compiler generates multiple LDRs instead of LDM.

Thanks for the details.

Thanks,
-- Srinivas R


-- 
Qualcomm India Private Limited, on behalf of Qualcomm Innovation Center, 
Inc.,
is a member of Code Aurora Forum, a Linux Foundation Collaborative Project.