[PATCH] powerpc: Enabling sys_kcmp for CRIU

[PATCH] powerpc: Enabling sys_kcmp for CRIU

[Laurent Dufour]

The commit 8170a83f15ee ("powerpc: Wireup the kcmp syscall to sys_ni") has
disabled the kcmp syscall for powerpc.  This has been done due to the use
of unsigned long parameters which may require a dedicated wrapper to handle
32bit process on top of 64bit kernel.  However in the kcmp() case, the 2
unsigned long parameters are currently only used to carry file descriptors
from user space to the kernel.  Since such a parameter is passed through
register, and file descriptor doesn't need to get extended, there is,
today, no need for a wrapper.

In the case there will be a need to pass address in or out of this system
call, then a wrapper could be required, it will then be to care of it.

As today this is not the case, it is safe to enable kcmp() on powerpc.

Cc: Tony Breeds <tony@bakeyournoodle.com>
Signed-off-by: Laurent Dufour <ldufour@linux.vnet.ibm.com>
---
 arch/powerpc/include/asm/systbl.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/powerpc/include/asm/systbl.h b/arch/powerpc/include/asm/systbl.h
index 91062eef582f..b257f02ab6bf 100644
--- a/arch/powerpc/include/asm/systbl.h
+++ b/arch/powerpc/include/asm/systbl.h
@@ -358,7 +358,7 @@ SYSCALL_SPU(setns)
 COMPAT_SYS(process_vm_readv)
 COMPAT_SYS(process_vm_writev)
 SYSCALL(finit_module)
-SYSCALL(ni_syscall) /* sys_kcmp */
+SYSCALL(kcmp) /* sys_kcmp */
 SYSCALL_SPU(sched_setattr)
 SYSCALL_SPU(sched_getattr)
 SYSCALL_SPU(renameat2)
-- 
1.9.1

Re: powerpc: Enabling sys_kcmp for CRIU

[Michael Ellerman]

On Tue, 2015-05-05 at 15:30:21 UTC, Laurent Dufour wrote:
> The commit 8170a83f15ee ("powerpc: Wireup the kcmp syscall to sys_ni") has
> disabled the kcmp syscall for powerpc.  This has been done due to the use
> of unsigned long parameters which may require a dedicated wrapper to handle
> 32bit process on top of 64bit kernel.  However in the kcmp() case, the 2
> unsigned long parameters are currently only used to carry file descriptors
> from user space to the kernel.  Since such a parameter is passed through
> register, and file descriptor doesn't need to get extended, there is,
> today, no need for a wrapper.
> 
> In the case there will be a need to pass address in or out of this system
> call, then a wrapper could be required, it will then be to care of it.
> 
> As today this is not the case, it is safe to enable kcmp() on powerpc.

That's mostly convincing. Though I see that s390 does have a compat wrapper,
i386 doesn't. So who knows what that means.

But, there's a selftest in tools/testing/selftests/kcmp. So can you run that as
32-bit and confirm it works? Then I'd be 100% convinced :)

cheers

Re: powerpc: Enabling sys_kcmp for CRIU

[Laurent Dufour]

On 11/05/2015 12:31, Michael Ellerman wrote:
> On Tue, 2015-05-05 at 15:30:21 UTC, Laurent Dufour wrote:
>> The commit 8170a83f15ee ("powerpc: Wireup the kcmp syscall to sys_ni") has
>> disabled the kcmp syscall for powerpc.  This has been done due to the use
>> of unsigned long parameters which may require a dedicated wrapper to handle
>> 32bit process on top of 64bit kernel.  However in the kcmp() case, the 2
>> unsigned long parameters are currently only used to carry file descriptors
>> from user space to the kernel.  Since such a parameter is passed through
>> register, and file descriptor doesn't need to get extended, there is,
>> today, no need for a wrapper.
>>
>> In the case there will be a need to pass address in or out of this system
>> call, then a wrapper could be required, it will then be to care of it.
>>
>> As today this is not the case, it is safe to enable kcmp() on powerpc.
> 
> That's mostly convincing. Though I see that s390 does have a compat wrapper,
> i386 doesn't. So who knows what that means.
> 
> But, there's a selftest in tools/testing/selftests/kcmp. So can you run that as
> 32-bit and confirm it works? Then I'd be 100% convinced :)

Hi Michael,

I was myself a bit confused about this system call, so to be sure before
sending the patch I ran the selftest/kcmp series on 64bit, 32bit, and
also build it in 32bit and run it on top of a 64bit kernel without any
trouble...

Cheers,
Laurent.