linux-kernel - Re: [PATCH 10/26] x86, pkeys: notify userspace about protection key faults

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Date:	Tue, 22 Sep 2015 13:21:03 -0700
From:	Dave Hansen <dave@...1.net>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	x86@...nel.org, linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH 10/26] x86, pkeys: notify userspace about protection key
 faults

On 09/22/2015 01:03 PM, Thomas Gleixner wrote:
> On Wed, 16 Sep 2015, Dave Hansen wrote:
>>  
>> +static inline u16 vma_pkey(struct vm_area_struct *vma)
>> +{
>> +	u16 pkey = 0;
>> +#ifdef CONFIG_X86_INTEL_MEMORY_PROTECTION_KEYS
>> +	unsigned long f = vma->vm_flags;
>> +	pkey |= (!!(f & VM_HIGH_ARCH_0)) << 0;
>> +	pkey |= (!!(f & VM_HIGH_ARCH_1)) << 1;
>> +	pkey |= (!!(f & VM_HIGH_ARCH_2)) << 2;
>> +	pkey |= (!!(f & VM_HIGH_ARCH_3)) << 3;
> 
> Eew. What's wrong with:
> 
>      pkey = (vma->vm_flags & VM_PKEY_MASK) >> VM_PKEY_SHIFT;

I didn't do that only because we don't have any other need for
VM_PKEY_MASK or VM_PKEY_SHIFT.  We could do:

#define VM_PKEY_MASK (VM_PKEY_BIT0 | VM_PKEY_BIT1 | VM_PKEY_BIT2...)

static inline u16 vma_pkey(struct vm_area_struct *vma)
{
	int vm_pkey_shift = __ffs(VM_PKEY_MASK)
	return (vma->vm_flags & VM_PKEY_MASK) >> vm_pkey_shift;
}

That's probably the same number of lines of code in the end.  The
compiler _probably_ ends up doing the same thing either way.

>> +static u16 fetch_pkey(unsigned long address, struct task_struct *tsk)
> 
> So here we get a u16 and assign it to si_pkey
> 
>> +	if (boot_cpu_has(X86_FEATURE_OSPKE) && si_code == SEGV_PKUERR)
>> +		info.si_pkey = fetch_pkey(address, tsk);
> 
> which is int.
> 
>> +			int _pkey; /* FIXME: protection key value??
> 
> Inconsistent at least.

So I defined all the kernel-internal types as u16 since I *know* the
size of the hardware.

The user-exposed ones should probably be a bit more generic.  I did just
realize that this is an int and my proposed syscall is a long.  That I
definitely need to make consistent.

Does anybody care whether it's an int or a long?
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