Re: [PATCH] powerpc/mm: Invalidate subpage_prot() system call on radix platforms

["Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>]

On 12/01/2017 04:01 PM, Michael Ellerman wrote:
> Anshuman Khandual <khandual@linux.vnet.ibm.com> writes:
> 
>> Radix enabled platforms don't support subpage_prot() system calls. But
>> at present the system calls goes through without an error and fails
>> later on while validating expected subpage accesses. Lets not allow
>> the system call on powerpc radix platforms to begin with to prevent
>> this confusion in user space.
>>
>> Signed-off-by: Anshuman Khandual <khandual@linux.vnet.ibm.com>
>> ---
>>   arch/powerpc/mm/subpage-prot.c | 3 +++
>>   1 file changed, 3 insertions(+)
>>
>> diff --git a/arch/powerpc/mm/subpage-prot.c b/arch/powerpc/mm/subpage-prot.c
>> index 781532d..4005468 100644
>> --- a/arch/powerpc/mm/subpage-prot.c
>> +++ b/arch/powerpc/mm/subpage-prot.c
>> @@ -195,6 +195,9 @@ long sys_subpage_prot(unsigned long addr, unsigned long len, u32 __user *map)
>>   	unsigned long next, limit;
>>   	int err;
>>   
>> +	if (radix_enabled())
>> +		return -ENOSYS;
> 
> It's considered poor form for syscalls to return ENOSYS. That is meant
> to mean one thing only, which is that the syscall does not exist.
> 
> See:
>    https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/include/uapi/asm-generic/errno.h#n10
> 
> 
> Now arguably in this case ENOSYS does make some sense, because we'd
> really like it if the syscall didn't exist when radix is enabled.
> 
> But I still don't like it, it means without changing the kernel version,
> the syscall would appear or disappear based on which MMU mode you booted
> into - which is weird.
> 
> So I think ENOENT would be better. It says "the thing you're trying to
> access doesn't exist", where in this case "the thing" is "the subpage
> protection map for this address range". It's better than EINVAL because
> there's no implication that changing the arguments would result in
> different behaviour.
> 

Why not ENOTSUPP?

-aneesh